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Abdominal Bloating

  • Fernando Azpiroz
    Correspondence
    Address requests for reprints to: Fernando Azpiroz, MD, Digestive System Research Unit, Hospital General Vall d’Hebron, 08035 Barcelona, Spain. fax: (34) 93-489-44-56.
    Affiliations
    Digestive System Research Unit, Hospital General Vall d’Hebron, Autonomous University of Barcelona, Barcelona, Spain
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  • Juan–R. Malagelada
    Affiliations
    Digestive System Research Unit, Hospital General Vall d’Hebron, Autonomous University of Barcelona, Barcelona, Spain
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      Abdominal bloating is a common and significant clinical problem that remains to be scientifically addressed. Bloating is one of the most bothersome complaints in patients with various functional gut disorders. However, in the current standard classification, abdominal bloating is merely regarded as a secondary descriptor, which masks its real clinical effect. Four factors are involved in the pathophysiology of bloating: a subjective sensation of abdominal bloating, objective abdominal distention, volume of intra-abdominal contents, and muscular activity of the abdominal wall. The primer to elicit subjective bloating may be any of the other 3 factors, or the sensation may be related to distorted perception. All of these mechanisms may play an independent role or may be interrelated. Gas transit studies have evidenced that patients with bloating have impaired reflex control of gut handling of contents. Segmental pooling, either of gas or of solid/liquid components, may induce a bloating sensation, particularly in patients with altered gut perception. Furthermore, altered viscerosomatic reflexes may contribute to abdominal wall protrusion and objective distention, even without major intra-abdominal volume increment. Bloating probably is a heterogeneous condition produced by a combination of pathophysiological mechanisms that differ among individual patients and that in most cases are subtle and undetectable by conventional methods. Further advances in the pathophysiology and clinical forms of bloating are warranted to develop mechanistic strategies rather than the current empiric treatment strategies for comprehensive and effective management of this problem.

      Abbreviation used in this paper:

      IBS (irritable bowel syndrome)
      Bloating, like some other descriptors for abdominal sensations, is an ambiguous term that alludes both to the subjective sensation and to the objective abdominal distention. The ambiguity of the English term is shared by other languages. Furthermore, bloating means different things to different patients (and to their doctors). Some use the term bloating to refer to the sensation of a swollen/distended abdomen, and others use it to refer to the sensation of a full belly, the feeling of abdominal pressure or wall tension, or the sensation of excess gas. Yet others use it for various apparently unrelated sensations, such as needing to burp; nausea; crampy, gurgling, or rumbling stomach; or needing to go to the bathroom. Many patients with bloating, approximately 24%, report no visible abdominal distention.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      As clinicians, when inquiring about bloating, do we really know what we are specifically asking the patient? In this review, we primarily focus on bloating, meaning the subjective sensation of abdominal distention, and, among other aspects, we will also address whether the subjective sensation of the patient corresponds with objective distention of the abdomen.
      In the current gold standard classification of functional gut disorders, abdominal bloating and distention are merely regarded as secondary descriptors.
      • Talley N.
      • Stanghellini V.
      • Heading R.C.
      • Koch K.L.
      • Malagelada J.-R.
      • Tytgat G.N.
      Functional gastroduodenal disorders.
      • Thompson W.G.
      • Longstreth G.
      • Drossman D.A.
      • Heaton K.
      • Irvine E.J.
      • Muller-Lissner S.C.
      Functional bowel disorders in functional abdominal pain.
      Thus, patients with bloating as their predominant complaint are dispersed among different categories, such as functional dyspepsia and irritable bowel syndrome (IBS), and lumped together with nonbloating patients.
      • Talley N.
      • Stanghellini V.
      • Heading R.C.
      • Koch K.L.
      • Malagelada J.-R.
      • Tytgat G.N.
      Functional gastroduodenal disorders.
      • Thompson W.G.
      • Longstreth G.
      • Drossman D.A.
      • Heaton K.
      • Irvine E.J.
      • Muller-Lissner S.C.
      Functional bowel disorders in functional abdominal pain.
      Only patients who do not meet the criteria that define these categories, ie, by exclusion, are categorized as having functional bloating.
      • Thompson W.G.
      • Longstreth G.
      • Drossman D.A.
      • Heaton K.
      • Irvine E.J.
      • Muller-Lissner S.C.
      Functional bowel disorders in functional abdominal pain.
      In the past few years, various clinical studies have raised the significance of abdominal bloating as an important, troublesome, and poorly understood clinical problem. It is also highly prevalent. The sensation of abdominal bloating may affect 10%–30% of the population in community-based studies.
      • Thompson W.G.
      • Longstreth G.
      • Drossman D.A.
      • Heaton K.
      • Irvine E.J.
      • Muller-Lissner S.C.
      Functional bowel disorders in functional abdominal pain.
      In a survey of US householders, 15.9% of the adult population reported abdominal bloating or distention within the month before the interview, and in more than 75% of them, the symptom was moderate or severe.
      • Sandler R.S.
      • Stewart W.F.
      • Liberman J.N.
      • Ricci J.A.
      • Zorich N.L.
      Abdominal pain, bloating, and diarrhea in the United States prevalence and impact.
      Bloating is one of the most common and bothersome complaints in a large proportion of patients with various functional gut disorders, such as functional dyspepsia
      • Knill-Jones R.P.
      A formal approach to symptoms of dyspepsia.
      • Talley N.J.
      • Phillips S.F.
      • Melton L.J.
      • et al.
      A patient questionnaire to identify bowel disease.
      and IBS,
      • Lembo T.
      • Naliboff B.
      • Munakata J.
      • Fullerton S.
      • Saba L.
      • Tung S.
      • Schmulson M.
      • Mayer E.A.
      Symptoms and visceral perception in patients with pain-predominant irritable bowel syndrome.
      • Sach J.
      • Bolus R.
      • Fitzgerald L.
      • Naliboff B.
      • Chang L.
      • Mayer E.
      Is there a difference between abdominal pain and discomfort in moderate to severe IBS patients?.
      • Wiklund I.K.
      • Fullerton S.
      • Hawkey C.J.
      • Jones R.H.
      • Longstreth G.F.
      • Mayer E.A.
      • Peacock R.A.
      • Wilson I.K.
      • Naesdal J.
      An irritable bowel syndrome-specific symptom questionnaire development and validation.
      and it is frequently associated with constipation
      • Marcus S.N.
      • Heaton K.W.
      Irritable bowel-type symptoms in spontaneous and induced constipation.
      • Mertz H.
      • Naliboff B.
      • Mayer E.A.
      Symptoms and physiology in severe chronic constipation.
      and diarrhea.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      The importance of bloating is placed in perspective by considering the enormous economic burden imposed by this type of functional gut disorder.
      • Longstreth G.F.
      • Wilson A.
      • Knight K.
      • Wong J.
      • Chiou C.F.
      • Barghout V.
      • et al.
      Irritable bowel syndrome, health care use, and costs a US managed care perspective.
      Despite its clinical, social, and economic importance, bloating remains substantially ignored, without a proper clinical classification, a known pathophysiology, or an effective treatment.
      • Azpiroz F.
      • Serra J.
      Treatment of excessive intestinal gas.
      It is not even clear to what extent individual patients’ complaints of bloating correlate with objective evidence of abdominal distention, and the uncertainty regarding the subjective or the objective origin of the complaints further adds to the confusion. In summary, bloating is a common and highly significant clinical problem that remains to be scientifically addressed.

      Aims and Methods

      Our aim was to clarify the concept, clinical importance, and pathophysiology of abdominal bloating and, thereby, to pave the way for the comprehensive management of this problem. Specifically, we performed a critical analysis of the following topics: the pathophysiological mechanisms involved in bloating, the various forms of presentation of bloating in clinical practice, and the current treatment options for this heterogeneous condition.
      To this aim, a literature review was performed based on a PUBMED search over January 1989 to September 2004 on the following terms: “abdominal bloating,” “IBS and intestinal gas,” and “IBS and flatulence.” A total of 473 articles were identified. These articles, as well as chapters on intestinal gas in standard textbooks,
      • Suarez F.L.
      • Levitt M.D.
      Intestinal gas.
      • Calloway D.H.
      Gas in the alimentary canal.
      • Levitt M.D.
      • Bond J.H.
      • Levitt D.G.
      Gastrointestinal gas.
      served as literature sources for articles published before 1989. However, only articles relevant to the areas of controversy have been quoted.
      The experimental evidence available on this subject is scarce; hence, this review is necessarily, to some extent, based on theoretical analysis and the authors’ interpretation of existing data. References indicate the original source of information, but the referenced article may not necessarily support the concepts expressed.

      Pathophysiology of Bloating: Putative Factors and Mechanisms

      Four factors are included in the pathophysiology of bloating: subjective sensation, objective girth changes, volume of intra-abdominal contents, and muscular activity of the abdominal walls (Figure 1). The primer to elicit subjective bloating may be any of the other 3 factors, or the sensation may be related to distorted perception. These mechanisms, ie, abnormal perception, objective distention, intra-abdominal volume increment, and abdominal wall dystony, may play an independent role or may be interrelated. Indeed, objective abdominal distention, whether accompanied or not by a subjective sensation of bloating, may be due to changes in abdominal wall activity produced either by a real volume increment of abdominal contents or just by intra-abdominal content redistribution. Intra-abdominal content may increase at the expense of either intraluminal volume (ie, gas or liquid/solid gut content) or extraluminal volume (ie, tissue water increment due to edema or vascular congestion). Changes in abdominal wall activity, which are potentially related to viscerosomatic reflexes, may increase the tension of abdominal muscles, which may be subjectively interpreted as a bloating sensation but may also produce objective abdominal distention due to redistribution of intra-abdominal contents even in the absence of net changes in intra-abdominal volume. The experimental evidence, in terms of the amount of data, supporting each one of these possible mechanisms is frankly uneven and will be analyzed below. Most of the information available relates to the role of intestinal gas in bloating, because this has been the primary focus of experimental studies on this topic. Bloating, like many other abdominal symptoms, is probably a heterogeneous condition produced by a combination of pathophysiological mechanisms that differ among individual patients.
      Figure thumbnail gr1
      Figure 1Factors involved in bloating. Subjective bloating may be related to objective abdominal distention or to distorted perception. Objective abdominal distention may be due to increased abdominal contents or to intra-abdominal content redistribution. Intra-abdominal content may increase at the expense of either intraluminal or extraluminal volume. Changes in abdominal wall activity may induce a subjective sensation of bloating and/or objective abdominal distention due to content redistribution, even without a net increment in intra-abdominal volume.
      The pathophysiology of bloating will be reviewed below by analyzing the relation of bloating to objective distention, abdominal wall dystony, abnormal perception, and intra-abdominal contents. The latter issue is by far the best documented, including aspects on the role of intestinal gas, impaired handling of gut contents, the responsible area of the gut, and the intra-abdominal component that gives rise to bloating.

       Does the Subjective Sensation of Bloating Really Correspond With Objective Distention?

      This is a deceptively simple and key question, yet it is not easy to answer. Most patients indicate that their abdomen is or becomes episodically distended, and their claim is often corroborated by a proxy. However, the examining doctor may be mystified or uncertain and, in any case, unable to either prove or disprove the assertion. There are indeed patients whose abdomens appear to be truly distended. However, to show even the seemingly obvious may be difficult, because there are no practical office or bedside devices to reliably measure abdominal volume. Tape measures of abdominal girth have been shown to reproducibly detect even small changes in abdominal circumference induced experimentally by intestinal gas infusion.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Intestinal gas dynamics and tolerance in humans.
      • Caldarella M.P.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Prokinetic effects of neostigmine in patients with intestinal gas retention.
      • Serra J.
      • Salvioli B.
      • Azpiroz F.
      • Malagelada J.R.
      Lipid-induced intestinal gas retention in the irritable bowel syndrome.
      However, tape measurement requires an immobilized patient fitted with a belt-type measuring device that remains in place during the entire experiment, and each individual serves as his or her own control. Measurements performed in the context of clinical research studies have not yielded uniform results (Table 1). A multicenter study evaluating patients with functional gut disorders (IBS and functional dyspepsia) failed to detect differences in abdominal circumference between those who reported visible abdominal distention and those who did not.
      • Poynard T.
      • Hernandez M.
      • Xu P.
      • Couturier D.
      • Frexinos J.
      • Bommelaer G.
      • Benand-Agostini H.
      • Chaput J.C.
      • Rheims N.
      Visible abdominal distention and gas surface description of an automatic method of evaluation and application to patients with irritable bowel syndrome and dyspepsia.
      Using a tape measure, Maxton et al
      • Maxton D.G.
      • Martin D.F.
      • Whorwell P.
      • Godfrey M.
      Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.
      showed that in women with IBS, girth significantly increased during the day. Furthermore, the anteroposterior diameter of the abdomen measured by computed tomography was also shown to increase during the day.
      • Maxton D.G.
      • Martin D.F.
      • Whorwell P.
      • Godfrey M.
      Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.
      Another study reported that in patients with bloating, girth, but not weight, increased during episodes of visible abdominal distention. However, in this study, measurements were performed by the patients themselves, and the variability of the changes reported (5.3 ± 3.0 cm in girth and 1.0 ± 0.9 kg in weight; mean ± SD) was quite large.
      • Sullivan S.N.
      A prospective study of unexplained visible abdominal bloating.
      Table 1Do Patients With a Bloating Sensation Have Objective Abdominal Distention?
      StudyMethodResult
      Poynard
      • Poynard T.
      • Hernandez M.
      • Xu P.
      • Couturier D.
      • Frexinos J.
      • Bommelaer G.
      • Benand-Agostini H.
      • Chaput J.C.
      • Rheims N.
      Visible abdominal distention and gas surface description of an automatic method of evaluation and application to patients with irritable bowel syndrome and dyspepsia.
      Tape measureNo
      Maxton
      • Maxton D.G.
      • Martin D.F.
      • Whorwell P.
      • Godfrey M.
      Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.
      Tape measureYes
      Maxton
      • Maxton D.G.
      • Martin D.F.
      • Whorwell P.
      • Godfrey M.
      Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.
      Computed tomographic scanYes
      Sullivan
      • Sullivan S.N.
      A prospective study of unexplained visible abdominal bloating.
      Tape measureYes
      Lea
      • Lea R.
      • Whorwell P.
      • Reilly B.
      • Houghton L.
      Abdominal distension in irritable bowel syndrome (IBS) diurnal variation and its relationship to abdominal bloating.
      PlethysmographyYes
      Lea
      • Lea R.
      • Houghton L.A.
      • Whorwell P.J.
      • Reilly B.
      Relationship of abdominal bloating to physical distension in irritable bowel syndrome (IBS) effect of bowel habit.
      PlethysmographyVariable
      Constipation-predominant yes; diarrhea-predominate no.
      a Constipation-predominant yes; diarrhea-predominate no.
      More recently, automated methods have been developed to measure girth changes.
      • Marino B.
      • Ogliari C.
      • Basilisco G.
      Effect of rectal distension on abdominal girth.
      • Lewis M.
      • Reilly B.
      • Houghton L.
      • Whorwell P.
      Ambulatory abdominal inductance plethysmography towards objective assessment of abdominal distension in irritable bowel syndrome.
      Preliminary results with an ambulatory technique using inductance plethysmography indicate that clinical variations in girth are significantly greater in IBS patients complaining of bloating than in healthy subjects.
      • Lea R.
      • Whorwell P.
      • Reilly B.
      • Houghton L.
      Abdominal distension in irritable bowel syndrome (IBS) diurnal variation and its relationship to abdominal bloating.
      However, the relationship of distention to subjective bloating is variable, showing a good correlation in constipation-predominant IBS patients, but not in diarrhea-predominant IBS.
      • Lea R.
      • Houghton L.A.
      • Whorwell P.J.
      • Reilly B.
      Relationship of abdominal bloating to physical distension in irritable bowel syndrome (IBS) effect of bowel habit.
      Hence, it seems that subjective claims of distention represent true perceptions of a real event in a significant proportion of patients, but this may not be the case in many others. For one thing, some patients complain of bloating but readily acknowledge no physical evidence of abdominal distention. Others point toward a “distended” abdomen that the examining physician appreciates as normal. Finally, other patients, usually with persistent bloating, show a prominent, fatty abdomen associated with IBS-like symptoms that they interpret as secondary to gut distention. A prospective study reported that patients with bloating were more likely to have experienced recent weight gain than healthy controls, despite similar age, sex, and body mass index between groups.
      • Sullivan S.N.
      A prospective study of unexplained visible abdominal bloating.
      Thus, fat accumulation in the abdomen may favor the development or awareness of bloating as a symptom.

       Abdominal wall activity

      The shape of the abdomen is determined by the disposition of the walls of the abdominal cavity, specifically, the vertebral column, which determines the configuration of the posterior abdominal wall, the diaphragm, and the anterolateral musculature. The influence of the pelvic floor, with its limited mobility, is probably insignificant. Even without increments in intra-abdominal volume, a change in the relative position of the walls may produce visible, objective distention. Furthermore, signals arising from the abdominal wall—for instance, because of a muscular dystony—may induce a subjective sensation of abdominal bloating, even in the absence of true abdominal distention, and this could explain some cases of apparently imaginary bloating.
      Is a classic article, Alvarez
      • Alvarez W.
      Hysterical type of nongaseous abdominal bloating.
      described in great detail a series of patients in whom pronounced abdominal distention was, in his view, related to the muscular activity of the abdominal wall. This hypothesis is substantiated by the fact that in some patients, visible abdominal distention has a very rapid onset
      • Alvarez W.
      Hysterical type of nongaseous abdominal bloating.
      • Maxton D.G.
      • Whorwell P.J.
      Abdominal distension in irritable bowel syndrome the patient’s perception.
      and resolves instantaneously by gentle abdominal palpation while asking the patient to relax, by anesthesia,
      • Alvarez W.
      Hysterical type of nongaseous abdominal bloating.
      or by hypnotic induction (Whorwell, personal communication, November 1995). Furthermore, distention may affect only part of the abdomen, and rapid resolution is not associated with gas evacuation.
      • Alvarez W.
      Hysterical type of nongaseous abdominal bloating.
      However, experimental evidence of these abdominal wall hypotheses has been difficult to obtain. Computed tomography has failed to identify differences in lumbar lordosis and diaphragmatic position in patients with bloating.
      • Maxton D.G.
      • Martin D.F.
      • Whorwell P.
      • Godfrey M.
      Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.
      It has also been reported that patients with bloating have weak abdominal muscles as compared with healthy controls.
      • Sullivan S.N.
      A prospective study of unexplained visible abdominal bloating.
      In contrast, McManis et al
      • McManis P.G.
      • Newall D.
      • Talley N.J.
      Abdominal wall muscle activity in irritable bowel syndrome with bloating.
      studied a group of patients with IBS and abdominal distention by means of surface electromyography and found that patients and healthy subjects alike increased electromyographic activity in the lower abdomen while standing compared with lying supine, but there were no differences between groups. Using a more elaborate technique and simultaneously recording the muscular activity at 8 different sites, we recently reproduced these results.
      • Tremolaterra F.
      • Serra J.
      • Azpiroz F.
      • Villoria A.
      Bloating and abdominal wall dystony.
      However, we were also able to show a dystonic response of the abdominal wall to intra-abdominal volume increments in patients with bloating. Intestinal gas retention modeled by rectal gas infusion during anal blockade increased the muscular tone of the abdominal muscles in healthy subjects in the upright position. Similar volumes of gas retention produced significantly greater objective abdominal distention and subjective symptoms in patients with bloating than in healthy subjects. Exaggerated abdominal distention in patients was associated with failed contraction of the abdominal musculature and even paradoxical relaxation of the internal oblique.
      • Tremolaterra F.
      • Serra J.
      • Azpiroz F.
      • Villoria A.
      Bloating and abdominal wall dystony.
      Animal studies have shown the relevance of viscerosomatic reflexes. For instance, chemical irritation of the colon in rats induces abdominal wall contractions that are inhibited by colonic distention.
      • Martinez V.
      • Thakur S.
      • Mogil J.
      Differential effects of chemical and mechanical colonic irritation on behavioral pain response to intraperitoneal acetic acid in mice.
      Abnormal viscerosomatic reflex activity may also participate in the mechanism of abdominal distention and muscular wall dystony in patients with bloating.

       Abnormal perception

      Abnormal perception related to cognitive interpretation, abdominal wall sensations, or visceral sensitivity is probably a key contributing factor to the sensation of abdominal bloating. Some patients with a normal or simply fatty abdomen, but with a distorted interpretation of reality, may believe, sometimes to the point of the obsession, that their abdomen is distended. Suarez et al
      • Suarez F.L.
      • Dennis A.
      • Savalano D.
      • Levitt M.
      A comparison of symptoms after the consumption of milk or lactose-hydrolyzed milk by people with self-reported severe lactose intolerance.
      elegantly showed that people who regarded themselves as severely lactose intolerant and complained of bloating after consumption of even small amounts of dairy products, when specifically tested in a double-blind fashion, recorded negligible symptoms when consuming 250 mL of milk, whether it was lactose hydrolyzed or not. Hence, despite their conviction, the experimental evidence proved that customary milk-related symptoms in these patients had an imaginary origin.
      However, in other cases, altered perception seems to be due to genuine hypersensitivity with impaired modulation of sensory signals.
      • Azpiroz F.
      Gastrointestinal perception pathophysiological implications.
      Hence, the bloating sensation could plausibly arise from a hypersensitive abdominal wall (for instance, in case of abdominal wall trauma, injuries, or scars) and may give the patient the sensation of increased abdominal wall tension, which is interpreted as bloating. This mechanism may contribute to postoperative bloating after laparotomy. Alternatively, the sensation may arise from abdominal viscera. Indeed, visceral hyperalgesia has been well characterized in patients with functional gut disorders, such as functional dyspepsia and IBS.
      • Azpiroz F.
      Gastrointestinal perception pathophysiological implications.
      In these patients, physiological stimuli in the gut, normally unperceived, may induce abdominal symptoms—specifically, bloating. Furthermore, probing stimuli in the laboratory, such as gut distention, tend to reproduce the customary symptoms, depending on the area on the gut stimulated, and in some patients they induce the sensation of bloating.
      • Azpiroz F.
      Gastrointestinal perception pathophysiological implications.
      The area of the gut affected by the sensory dysfunction depends on the clinical syndrome.
      • Bouin M.
      • Lupien F.
      • Riberdy M.
      • Boibin M.
      • Plourde V.
      • Poitras P.
      Intolerance to visceral distension in functional dyspepsia or irritable bowel syndrome an organ specific defect or a pan intestinal dysregulation?.
      It has been shown that patients with IBS have increased sensitivity in the large and the small bowel.
      • Thompson W.G.
      • Longstreth G.
      • Drossman D.A.
      • Heaton K.
      • Irvine E.J.
      • Muller-Lissner S.C.
      Functional bowel disorders in functional abdominal pain.
      • Accarino A.M.
      • Azpiroz F.
      • Malagelada J.-R.
      Selective dysfunction of mechanosensitive intestinal afferents in the irritable bowel syndrome.
      By contrast, in patients with functional dyspepsia, the stomach seems predominantly affected. Specifically, in a group of patients with postprandial bloating, gastric, but not duodenal, distentions induced an exaggerated perception and reproduced the customary bloating symptom.
      • Coffin B.
      • Azpiroz F.
      • Guarner F.
      • Malagelada J.-R.
      Selective gastric hypersensitivity and reflex hyporeactivity in functional dyspepsia.
      It has been recently shown that in dyspeptic patients with postprandial symptoms (most of them with dysmotility-like dyspepsia and bloating-related symptoms), fundic distention reproduced the customary symptoms better than antral distention, whereas in dyspeptic patients with ulcer-like pain during fasting, antral distention replicated their usual symptoms more closely.
      • Caldarella M.
      • Azpiroz F.
      • Malagelada J.-R.
      Antro-fundic dysfunctions in functional dyspepsia.
      In other studies, using different selection criteria, intestinal hypersensitivity has been also shown in dyspeptic patients.
      • Talley N.
      • Stanghellini V.
      • Heading R.C.
      • Koch K.L.
      • Malagelada J.-R.
      • Tytgat G.N.
      Functional gastroduodenal disorders.
      In patients with functional gut disorders, altered sensitivity combines with impaired control of gut motility, and both dysfunctions may interact to produce their symptoms.
      • Azpiroz F.
      Gastrointestinal perception pathophysiological implications.
      This also applies to bloating. Basically, intraluminal trapping of contents causing focal distention in a hypersensitive area would have a synergistic effect in inducing the symptoms. Indeed, minor motility disturbances that do not compromise function may become clinically relevant and produce symptoms only in the presence of altered gut perception.

       Intra-abdominal contents

      Abdominal distention and bloating may be due to a pooling of intraluminal contents, tissue–water increment in viscera, or even free intraperitoneal content, such as ascites. Of all elements within the abdominal cavity, intraluminal gas seems to be the most versatile. Given the characteristic rapid fluctuations of nonorganic bloating, intestinal gas has been, and is still considered, the most likely candidate to explain bloating, whereas other putative causes have received much less attention.

       The gas hypothesis

      Little is known about intestinal gas homeostasis, but it seems to be a finely regulated process. Many studies on different populations using different methods have shown that despite the very large capacity of the entire gastrointestinal tract, the total volume of intraluminal gas amounts only to 100–200 mL.
      • Caldarella M.P.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Prokinetic effects of neostigmine in patients with intestinal gas retention.
      • Levitt M.D.
      Volume and composition of human intestinal gas determined by means of an intestinal washout technic.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Impaired transit and tolerance of intestinal gas in the irritable bowel syndrome.
      • Bedell G.N.
      • Marshall R.
      • Dubois A.B.
      • Harris J.H.
      Measurement of the volume of gas in the gastrointestinal tract values in normal subjects and ambulatory patients.
      This fairly constant and relatively low volume is astonishing if one considers the diverse and complicated processes of gas input and gas output.
      • Suarez F.L.
      • Levitt M.D.
      Intestinal gas.

       Physiological background

      Gas input results from swallowing, chemical reactions, diffusion from blood, and bacterial fermentation. Gas output is achieved by eructation, absorption, bacterial consumption, and anal evacuation (Figure 2).
      Figure thumbnail gr2
      Figure 2Intestinal gas metabolism. Gas input results from swallowing, chemical reactions, diffusion from blood, and bacterial fermentation. Gas output is achieved by eructation, absorption, bacterial consumption, and anal evacuation. Gas transit determines the time of exposure for diffusion of gases across the gut–blood barrier, as well as for bacterial consumption and, hence, may influence intestinal gas volume, composition, and tolerance.
      Swallowing introduces a small amount of air into the stomach that is obviously much greater with gaseous beverages. Air in excess is eliminated from the stomach by belching, absorption, or emptying into the intestine. Chemical reactions—specifically, neutralization of acids and alkalis in the upper gut—produce enormous quantities of carbon dioxide,
      • Fordtran J.S.
      • Morawski S.G.
      • Santa Ana C.A.
      • Rector F.C.
      Gas production after reaction of sodium bicarbonate and hypochloric acid.
      which is absorbed in the small bowel.
      In the colon, intraluminal bacteria play a major role in gas metabolism by both producing and consuming large amounts of intraluminal gases, and their activity constitutes the determinant of anal gas output. The net effect of colonic flora on gas metabolism depends on the balance between gas-producing and gas-consuming microorganisms, which varies considerably among individuals.
      • Suarez F.L.
      • Levitt M.D.
      Intestinal gas.
      The composition of the colonic microflora seems to be determined by early environmental conditions, but there may also be an adaptive component to alimentary habits later in life.
      • Scheppach W.
      • Fabian C.
      • Ahrens F.
      • Spengler M.
      • Kasper H.
      Effect of starch malabsorption on colonic function and metabolism in humans.
      • Stephen A.M.
      • Cummings J.H.
      Mechanism of action of dietary fibre in the human colon.
      In approximately 90% of the general Western population, the colon harbors hydrogen-producing bacteria that ferment undigested substrates, particularly carbohydrates, and release hydrogen and carbon dioxide.
      • Levitt M.D.
      • Bond J.H.
      Volume, composition, and source of intestinal gas.
      • Levitt M.D.
      Intestinal gas production—recent advances in flatology.
      • Suarez F.
      • Furne J.
      • Springfield J.
      • Levitt M.
      Insights into human colonic physiology obtained from the study of flatus composition.
      Part of the colonic flora consumes intraluminal gases, and this may account for a considerable proportion of intraluminal gas disposal.
      • Suarez F.
      • Furne J.
      • Springfield J.
      • Levitt M.
      Insights into human colonic physiology obtained from the study of flatus composition.
      • Strocchi A.
      • Levitt M.D.
      Factors affecting hydrogen production and consumption by human fecal flora. The critical roles of hydrogen tension and methanogenesis.
      • Gibson G.R.
      • Cummings J.H.
      • Macfarlane G.T.
      • Allison C.
      • Segal I.
      • Vorster H.H.
      • Walker A.R.P.
      Alternative pathways for hydrogen disposal during fermentation in the human colon.
      Both hydrogen and carbon dioxide are consumed in large quantities, and oxygen that reaches the colon is partly consumed by aerobic bacteria, thereby reducing the intraluminal content of oxygen. Most individuals have a pool of sulfate-reducing bacteria that may be present throughout the colon.
      • Suarez F.
      • Furne J.
      • Springfield J.
      • Levitt M.
      Insights into human colonic physiology obtained from the study of flatus composition.
      These bacteria consume hydrogen and release very small amounts of sulfur-containing gases (hydrogen sulfide and methanethiol) that are highly odoriferous. Approximately 30% of the population also has a pool of methanogenic bacteria in the left colon that consume large quantities of hydrogen and release methane.
      • Suarez F.
      • Furne J.
      • Springfield J.
      • Levitt M.
      Insights into human colonic physiology obtained from the study of flatus composition.
      • Strocchi A.
      • Levitt M.D.
      Factors affecting hydrogen production and consumption by human fecal flora. The critical roles of hydrogen tension and methanogenesis.
      • Flourie B.
      • Pellier P.
      • Florent C.
      • Marteau P.
      • Pochart P.
      • Rambaud J.C.
      Site and substrates for methane production in human colon.
      • Kajs T.M.
      • Fitzgerald J.A.
      • Buckner R.Y.
      • Coyle G.A.
      • Stinson B.S.
      • Morel J.G.
      • Levitt M.D.
      Influence of a methanogenic flora on the breath H2 and symptom response to ingestion of sorbitol or oat fiber.
      • Strocchi A.
      • Furne J.
      • Ellis C.
      • Levitt M.D.
      Methanogens outcompete sulphate reducing bacteria for H2 in the human colon.
      Because of the competition for hydrogen of both bacterial pools, in these subjects sulfate-reducing bacteria are circumscribed to the right colon.
      • Suarez F.
      • Furne J.
      • Springfield J.
      • Levitt M.
      Insights into human colonic physiology obtained from the study of flatus composition.
      Intraluminal gases tend to equilibrate with the gases in venous blood, depending on 3 factors: the partial pressure of each gas at both sides of the gut-blood barrier, its diffusibility, and the time of exposure of the gas to the diffusible surface, that is, the speed of gas transit.
      • Suarez F.L.
      • Levitt M.D.
      Intestinal gas.
      • Foster R.E.
      Physiological basis of gas exchange in the gut.
      • Pogrund R.S.
      • Steggerda F.R.
      Influence of gaseous transfer between the colon and blood stream on percentage gas compositions of intestinal flatus in man.
      Hence, highly diffusible gases present in large quantities within the gut, such as carbon dioxide in the small bowel, are readily absorbed. Oxygen, coming from swallowed air, is also absorbed from the small intestine to equilibrate with its partial pressure in blood. The diffusibility of nitrogen is much lower, but still, a large proportion of nitrogen in flatus may derive from blood. Part of the gases produced by colonic bacteria diffuse into the blood and are excreted by breath, where they can be detected by gas chromatography.
      • Suarez F.L.
      • Levitt M.D.
      Intestinal gas.
      Indeed, approximately half of the hydrogen evacuated follows this pathway.
      • King T.S.
      • Elia M.
      • Hunter J.O.
      Abnormal colonic fermentation in irritable bowel syndrome.
      • Christl S.U.
      • Gibson G.R.
      • Murgatroyd P.R.
      • Scheppach W.
      • Cummings J.H.
      Impaired hydrogen metabolism in pneumatosis cystoides intestinalis.
      This is the basis for the hydrogen and methane breath tests. The remaining gases are eliminated by the anus. The composition of intraluminal gas varies greatly along the gut, and the composition of the anal outflow reflects the net balance of multiple processes within the gut lumen.
      • Suarez F.
      • Furne J.
      • Springfield J.
      • Levitt M.
      Insights into human colonic physiology obtained from the study of flatus composition.
      During basal conditions, nitrogen accounts for most of the volume in flatus, but after consumption of meals rich in fermentable carbohydrates, hydrogen, carbon dioxide, and methane (in producers) markedly rise and predominate. The amount of oxygen, the other major component in flatus, is fairly low (approximately ≤10%). Various other gases are also present in trace quantities.
      • Suarez F.L.
      • Levitt M.D.
      Intestinal gas.
      It is important to remember that gas transit determines the time of exposure for diffusion of intraluminal gases across the gut–blood barrier, as well as for bacterial consumption, and, hence, may influence not only the volume of gas in the gut, but also the final composition of gas in flatus.
      • El Oufir L.
      • Flourie B.
      • des Varannes S.B.
      • Barry J.L.
      • Cloarec D.
      • Bornet F.
      • Galmiche J.P.
      Relations between transit time, fermentation products, and hydrogen consuming flora in healthy humans.
      Intestinal gas transit and tolerance have been measured by using a gas challenge test. The test consists of an infusion of a mixture of gases, in various proportions to minimize absorption, into the jejunum while anal gas output is quantified (Figure 3). A dose–response study showed that most healthy subjects propel and evacuate as much gas as is infused, up to 30 mL/min, without discomfort.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Intestinal gas dynamics and tolerance in humans.
      Hence, gas transit is finely adapted to a very broad range of intestinal gas loads, but the type of motor activity that determines gas transit is not known. Gas infusion does not induce detectable changes in small-bowel intestinal motility recorded by manometry.
      • Galati J.S.
      • McKee D.P.
      • Quigley E.M.
      Response to intraluminal gas in irritable bowel syndrome. Motility versus perception.
      In contrast, preliminary experiments with the barostat suggest that gas infusion induces tonic changes: a contraction orad to the infusion site and a relaxation distal to the collection site.
      • Harder H.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Reflex control of intestinal gas dynamics and tolerance.
      Conceivably, movement and displacement of large masses of low-resistance gas are produced by changes in tonic activity and capacitance of the gut.
      • Tremolaterra F.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Intestinal tone and gas motion.
      Gas boluses infused into the left colon have been shown to elicit forceful peristaltic contractions preceding small gas expulsion,
      • Bassotti G.
      • Germani U.
      • Morelli A.
      Flatus-related colorectal and anal motor events.
      but this type of phasic event has not been recorded during continuous gas infusion with a barostat located inside the rectum.
      • Harder H.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Reflex control of intestinal gas dynamics and tolerance.
      Hence, these phasic events could be a response to focal distention produced by abrupt intraluminal gas delivery.
      Figure thumbnail gr3
      Figure 3The gas challenge test. To evaluate gut propulsion, gas is infused at a constant rate in the jejunum while anal gas outflow is measured. Objective abdominal distention and subjective sensations are simultaneously measured.

       Do Patients With Bloating Produce More Intestinal Gas?

      As described previously, gas production is determined by 2 main factors: the amount of fermentable foodstuffs that escape small-bowel absorption and enter the colon and the composition of the colonic flora. Thus, bloating may be a clinical feature of malabsorption disorders in which excessive amounts of unabsorbed substrates are fermented in the colon. Under special circumstances, such as bacterial overgrowth, abnormal fermentation of foodstuffs may take place within the small bowel. In both of these situations, intestinal malabsorption and bacterial overgrowth, bloating, and other gas symptoms usually constitute a minor part of the clinical presentation and, hence, are of limited clinical relevance. The real challenge in clinical practice is individuals who complain of gas symptoms without detectable abnormalities by conventional testing, that is, patients with functional gastrointestinal disorders or, more precisely, with disorders of gastrointestinal function.
      • Thompson W.G.
      • Longstreth G.
      • Drossman D.A.
      • Heaton K.
      • Irvine E.J.
      • Muller-Lissner S.C.
      Functional bowel disorders in functional abdominal pain.
      Whether some degree of nutrient malabsorption plays a role in patients with functional bloating remains doubtful. Some studies suggest that the absorption capacity of certain substrates in the small bowel is reduced in IBS patients.
      • Rumessen J.J.
      • Gudmand-Hoyer E.
      Functional bowel disease malabsorption and abdominal distress after ingestion of fructose, sorbitol, and fructose-sorbitol mixtures.
      • Fernandez-Banares F.
      • Esteve-Pardo M.
      • de Leon R.
      • Humbert P.
      • Cabre E.
      • Llovet J.M.
      • Gassull M.A.
      Sugar malabsorption in functional bowel disease clinical implications.
      • Symons P.
      • Jones M.P.
      • Kellow J.E.
      Symptom provocation in irritable bowel syndrome. Effects of differing doses of fructose-sorbitol.
      However, other studies did not replicate these results.
      • Afdhal N.H.
      • Piggott C.
      • Long A.A.
      • O’Donoghue D.P.
      Carbohydrate handling by colonic flora—is it pathogenic in the irritable bowel syndrome?.
      • Nelis G.F.
      • Vermeeren M.A.
      • Jansen W.
      Role of fructose-sorbitol malabsorption in the irritable bowel syndrome.
      Furthermore, it has been shown that people with proven lactose malabsorption tolerate 250 mL of milk without symptoms because the colon is able to deal with a certain amount of unabsorbed substrates via fermentation and absorption of short-chain fatty acids and absorption/consumption of gases.
      • Suarez F.L.
      • Dennis A.
      • Savalano D.
      • Levitt M.
      A comparison of symptoms after the consumption of milk or lactose-hydrolyzed milk by people with self-reported severe lactose intolerance.
      Because the definition of malabsorption is somewhat ambiguous, some investigators have explored the effect of exclusion diets on bloating. Again, the results have been inconclusive.
      • McKee A.M.
      • Prior A.
      • Whorwell P.J.
      Exclusion diets in irritable bowel syndrome are they worthwhile?.
      Furthermore, potential beneficial effects of excluding offending foodstuffs may not necessarily imply excess fermentation, because other possible pathophysiological mechanisms, such as allergic or atopic reactions, may be involved.
      In a brilliant series of studies in the 1970s, Levitt
      • Levitt M.D.
      Volume and composition of human intestinal gas determined by means of an intestinal washout technic.
      measured intestinal gas production by using a washout technique. In these studies, gas was measured by infusing argon into the intestine at a relatively high flow rate (40 mL/min) and recovering rectal gas. During fasting, with no nutrients arriving into the colon, gas production was similar in healthy subjects and IBS patients
      • Lasser R.B.
      • Bond J.H.
      • Levitt M.D.
      The role of intestinal gas in functional abdominal pain.
      (Table 2). A single study, published only in abstract form, reported on postprandial gas production and showed no differences between patients with bloating and healthy controls.
      • Lasser R.B.
      • Levitt M.D.
      • Bond J.H.
      Studies of intestinal gas after ingestion of a standard meal.
      Table 2Do Patients With Bloating Produce More Intestinal Gas?
      StudyMethodResult
      Lasser
      • Lasser R.B.
      • Bond J.H.
      • Levitt M.D.
      The role of intestinal gas in functional abdominal pain.
      • Lasser R.B.
      • Levitt M.D.
      • Bond J.H.
      Studies of intestinal gas after ingestion of a standard meal.
      WashoutNo
      Haderstorfer
      • Haderstorfer B.
      • Whitehead W.E.
      • Schuster M.M.
      Intestinal gas production from bacterial fermentation of undigested carbohydrate in irritable bowel syndrome.
      Breath testNo
      King
      • King T.S.
      • Elia M.
      • Hunter J.O.
      Abnormal colonic fermentation in irritable bowel syndrome.
      CalorimetryYes
      Hydrogen production was larger, but total gas (hydrogen plus methane) production was not different from that in controls.
      Pimentel
      • Pimentel M.
      • Chow E.J.
      • Lin H.C.
      Normalization of lactulose breath testing correlates with symptom improvement in irritable bowel syndrome. A double-blind, randomized, placebo-controlled study.
      Breath testYes
      a Hydrogen production was larger, but total gas (hydrogen plus methane) production was not different from that in controls.
      Evaluation of intestinal gas production by breath tests has yielded conflicting results (Table 2). In one study, intestinal hydrogen production was measured during waking hours for 7 days in IBS patients with bloating and in a group of healthy controls. Breath hydrogen concentration was found to be similar in both groups, despite significantly higher rates of bloating in patients during the test period.
      • Haderstorfer B.
      • Whitehead W.E.
      • Schuster M.M.
      Intestinal gas production from bacterial fermentation of undigested carbohydrate in irritable bowel syndrome.
      Furthermore, breath hydrogen concentrations did not correlate with symptoms of bloating. In contrast, another study measuring total excretion (breath plus anal) by indirect calorimetry showed that on a standard diet, IBS patients excreted more hydrogen than healthy subjects. This suggests that patients might have a hyperactive gas-producing colonic flora.
      • King T.S.
      • Elia M.
      • Hunter J.O.
      Abnormal colonic fermentation in irritable bowel syndrome.
      However, some subjects produced methane as well as hydrogen, and when the total volume excreted (hydrogen plus methane) was measured, no difference was found between both groups. A third research line showed that the proportion of abnormal lactulose breath tests in IBS patients was higher than that in healthy controls; this was attributed to small-bowel bacterial overgrowth because oral neomycin normalized the breath test and reduced IBS symptoms more than did placebo.
      • Pimentel M.
      • Chow E.J.
      • Lin H.C.
      Normalization of lactulose breath testing correlates with symptom improvement in irritable bowel syndrome. A double-blind, randomized, placebo-controlled study.
      This interpretation has been questioned: it has been argued that the reliability of the lactulose breath test to diagnose small-bowel bacterial overgrowth is uncertain, that the reported symptom response to antibiotic therapy was relatively poor, and that the placebo response was unusually low compared with other studies.
      • Hasler W.
      Lactulose breath testing, bacterial overgrowth, and IBS just a lot of hot air?.
      In any event, it is important to point out that increased intestinal gas production (or reduced consumption), be it due to malabsorption, hyperactive colonic flora, or bacterial overgrowth, would likely result in flatulence, but not necessarily bloating, because most healthy subjects are able to propel and evacuate very large gas loads without perception of abdominal distention.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Intestinal gas dynamics and tolerance in humans.
      • Suarez F.L.
      • Dennis A.
      • Savalano D.
      • Levitt M.
      A comparison of symptoms after the consumption of milk or lactose-hydrolyzed milk by people with self-reported severe lactose intolerance.
      Hence, other factors should be also operating to produce bloating.

       Intraluminal gas volume

      In their early studies with the washout technique, Lasser et al
      • Lasser R.B.
      • Bond J.H.
      • Levitt M.D.
      The role of intestinal gas in functional abdominal pain.
      • Lasser R.B.
      • Levitt M.D.
      • Bond J.H.
      Studies of intestinal gas after ingestion of a standard meal.
      showed that the amount and composition of intestinal gas in IBS patients was similar to that of healthy subjects (Table 3). Using a similar technique, we did not find significant differences in intestinal gas volume among patients complaining of abdominal bloating and healthy controls.
      • Caldarella M.P.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Prokinetic effects of neostigmine in patients with intestinal gas retention.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Impaired transit and tolerance of intestinal gas in the irritable bowel syndrome.
      Table 3Do Patients With Bloating Have More Gas Volume Within the Gut?
      StudyMethodResult
      Lasser
      • Lasser R.B.
      • Bond J.H.
      • Levitt M.D.
      The role of intestinal gas in functional abdominal pain.
      • Lasser R.B.
      • Levitt M.D.
      • Bond J.H.
      Studies of intestinal gas after ingestion of a standard meal.
      WashoutNo
      Serra
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Impaired transit and tolerance of intestinal gas in the irritable bowel syndrome.
      WashoutNo
      Caldarella
      • Caldarella M.P.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Prokinetic effects of neostigmine in patients with intestinal gas retention.
      WashoutNo
      Chami
      • Chami T.N.
      • Schuster M.M.
      • Bohlman M.E.
      • Pulliam T.J.
      • Kamal N.
      • Whitehead W.E.
      A simple radiologic method to estimate the quantity of bowel gas.
      X-ray filmYes
      Koide
      • Koide A.
      • Yamaguchi T.
      • Odaka T.
      • Koyama H.
      • Tsuyuguchi T.
      • Kitahara H.
      • Ohto M.
      • Saisho H.
      Quantitative analysis of bowel gas using plain abdominal radiograph in patients with irritable bowel syndrome.
      X-ray filmYes
      Poynard
      • Poynard T.
      • Hernandez M.
      • Xu P.
      • Couturier D.
      • Frexinos J.
      • Bommelaer G.
      • Benand-Agostini H.
      • Chaput J.C.
      • Rheims N.
      Visible abdominal distention and gas surface description of an automatic method of evaluation and application to patients with irritable bowel syndrome and dyspepsia.
      X-ray filmYes
      Maxton
      • Maxton D.G.
      • Martin D.F.
      • Whorwell P.
      • Godfrey M.
      Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.
      Computed tomographic scanNo
      Two separate studies using plain abdominal radiographs
      • Chami T.N.
      • Schuster M.M.
      • Bohlman M.E.
      • Pulliam T.J.
      • Kamal N.
      • Whitehead W.E.
      A simple radiologic method to estimate the quantity of bowel gas.
      • Koide A.
      • Yamaguchi T.
      • Odaka T.
      • Koyama H.
      • Tsuyuguchi T.
      • Kitahara H.
      • Ohto M.
      • Saisho H.
      Quantitative analysis of bowel gas using plain abdominal radiograph in patients with irritable bowel syndrome.
      concluded that intra-abdominal gas content was larger in IBS patients than in healthy subjects (by 54% and 118%, respectively), but no significant correlation between gas content and symptoms was found. The method was shown to be technically reproducible, but changes in body position from upright to supine increased the estimated gas content by 67%.
      • Chami T.N.
      • Schuster M.M.
      • Bohlman M.E.
      • Pulliam T.J.
      • Kamal N.
      • Whitehead W.E.
      A simple radiologic method to estimate the quantity of bowel gas.
      A multicenter study using abdominal plain x-ray films taken 1 to 2 hours after breakfast or lunch found a larger (approximately 28%) total gas area in patients with IBS complaining of visible abdominal distention as opposed to patients who did not.
      • Poynard T.
      • Hernandez M.
      • Xu P.
      • Couturier D.
      • Frexinos J.
      • Bommelaer G.
      • Benand-Agostini H.
      • Chaput J.C.
      • Rheims N.
      Visible abdominal distention and gas surface description of an automatic method of evaluation and application to patients with irritable bowel syndrome and dyspepsia.
      However, the correlation between the abdominal gas content and the usual intensity of distention was poor. It is interesting to note that no difference was found between patients with and without abdominal distention when their symptoms were more typical of functional dyspepsia than of bloating. Because the total intraluminal gas volume in IBS, measured by independent studies, is only 100–300 mL,
      • Caldarella M.P.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Prokinetic effects of neostigmine in patients with intestinal gas retention.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Impaired transit and tolerance of intestinal gas in the irritable bowel syndrome.
      • Lasser R.B.
      • Bond J.H.
      • Levitt M.D.
      The role of intestinal gas in functional abdominal pain.
      • Lasser R.B.
      • Levitt M.D.
      • Bond J.H.
      Studies of intestinal gas after ingestion of a standard meal.
      the percentage differences reported by these 3 studies would account for a relatively small difference in absolute volume. In fact, a computed tomography study of intestinal gas could not detect significant differences between controls and patients with significant daytime girth increments and bloating.
      • Maxton D.G.
      • Martin D.F.
      • Whorwell P.
      • Godfrey M.
      Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.
      These data suggest that clinical bloating may not be simply the result of too much intestinal gas, but rather that other factors, ie, abnormal gas handling and distribution within the gut, may be involved.

       Impaired gut handling

      Conventional studies on gut motility and transit of chyme have failed to detect consistent abnormalities that could explain bloating. However, studies on intestinal gas transit and tolerance suggest that patients with bloating have impaired intestinal handling of gas loads.
      • Caldarella M.P.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Prokinetic effects of neostigmine in patients with intestinal gas retention.
      • Serra J.
      • Salvioli B.
      • Azpiroz F.
      • Malagelada J.R.
      Lipid-induced intestinal gas retention in the irritable bowel syndrome.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Impaired transit and tolerance of intestinal gas in the irritable bowel syndrome.
      • Lasser R.B.
      • Bond J.H.
      • Levitt M.D.
      The role of intestinal gas in functional abdominal pain.
      These gas transit studies may be of particular relevance by disclosing gut propulsive abnormalities that, in conjunction with sensory abnormalities, probably constitute the pathophysiological basis of bloating.
      Only a very small proportion of healthy individuals (approximately 15%) show impaired handling and increased perception of intestinal gas.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Intestinal gas dynamics and tolerance in humans.
      If this fraction of the otherwise healthy population were to be challenged with a gas overload, they would experience abdominal symptoms.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Intestinal gas dynamics and tolerance in humans.
      Likewise, increased colonic gas formation, experimentally induced by direct infusion of starch into the colon, produces abdominal symptoms only in a small fraction of healthy subjects.
      • Flourie B.
      • Florent C.
      • Jouany J.P.
      • Thivend P.
      • Etanchaud F.
      • Rambaud J.C.
      Colonic metabolism of wheat starch in healthy humans. Effects on fecal outputs and clinical symptoms.
      A similar effect can be observed in the case of experimental malabsorption induced by amylase inhibitors.
      • Boibin M.
      • Flourié B.
      • Rizza R.A.
      • Go V.L.W.
      • DiMagno E.P.
      Gastrointestinal and metabolic effects of amylase inhibition in diabetics.
      These findings suggest that some people are prone to bloating, but this symptom may develop only under special circumstances.
      In their seminal work with the washout technique, Lasser et al
      • Lasser R.B.
      • Bond J.H.
      • Levitt M.D.
      The role of intestinal gas in functional abdominal pain.
      noted that a proportion of patients with bloating and related symptoms did not tolerate the procedure (Figure 4), and in some of them the infused gas refluxed into the stomach. This was interpreted as an indication of some kind of gut motor dysfunction. Using a gas challenge test, we later showed that bloating patients who met Rome II criteria for either IBS or functional bloating diagnoses had impaired transit and tolerance of intestinal gas.
      • Caldarella M.P.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Prokinetic effects of neostigmine in patients with intestinal gas retention.
      • Serra J.
      • Salvioli B.
      • Azpiroz F.
      • Malagelada J.R.
      Lipid-induced intestinal gas retention in the irritable bowel syndrome.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Impaired transit and tolerance of intestinal gas in the irritable bowel syndrome.
      These patients retained gas and/or experienced abdominal discomfort in response to intestinal gas loads that are well tolerated by most healthy individuals (Figure 5). Furthermore, the gas challenge reproduced their customary symptoms. In these experiments, patients also developed objective abdominal distention that correlated with the volume of gas retained in their guts.
      Figure thumbnail gr4
      Figure 4Severity of symptoms induced by infusion of gas into the jejunum of healthy subjects and patients complaining of excessive gas, abdominal pain, and bloating (reprinted with permission
      • Lasser R.B.
      • Bond J.H.
      • Levitt M.D.
      The role of intestinal gas in functional abdominal pain.
      ).
      Figure thumbnail gr5
      Figure 5Individual responses (perception and gas retention) to a gas challenge test (12 mL/min jejunal gas plus 0.5 kcal/min duodenal lipids for 120 minutes). Healthy subjects tolerated the challenge test with low perception and retention. In contrast, patients who complained of bloating had impaired gut handling and fell outside the normal ranges. Reprinted with permission.
      • Caldarella M.P.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Prokinetic effects of neostigmine in patients with intestinal gas retention.

       Mechanisms of impaired gas handling

      The mechanism of gas retention in such patients remains unknown. However, in healthy subjects, 2 mechanisms have been experimentally shown to be involved in intestinal gas retention: increased resistance to gas flow, modeled by self-restraint anal gas evacuation; and impaired intestinal propulsion, produced by glucagon-induced motor inhibition
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Mechanisms of intestinal gas retention in humans impaired propulsion versus obstructed evacuation.
      (Figure 6). By using the gas challenge test, it was shown that gas retention produced by impaired propulsion induced objective abdominal distention, which was well tolerated. However, when gas retention and objective distention were produced by increased resistance to flow, subjective abdominal complaints were much higher than when gas retention was induced by pharmacological inhibition of propulsion. Hence, abdominal distention depends largely on the volume of gas retained, but abdominal discomfort likely relates more directly to uncoordinated intestinal motility rather than to weak propulsion. Conceivably, small gas bubbles pushed against high-resistance barriers may increase intestinal wall tension and produce symptoms. Furthermore, if several bubbles are trapped at different levels, perception would increase by spatial summation phenomena.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Modulation of gut perception in humans by spatial summation phenomena.
      Figure thumbnail gr6
      Figure 6Mechanisms of gut retention and intraluminal pooling. Retention can be due to a high-resistance barrier obstructing flow or to impaired propulsion. Symptom perception depends on the mechanism of retention: at the same volumes, flow obstruction produces more symptoms than impaired propulsion, but objective abdominal distention is similar.
      In healthy subjects, voluntary anal contraction has been shown to effectively retain gas.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Mechanisms of intestinal gas retention in humans impaired propulsion versus obstructed evacuation.
      In most bloated patients, gas retention and symptoms during the gas challenge test were similar whether gas was collected by an external cannula or by an intrarectal cannula that bypassed the anal gate.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Impaired transit and tolerance of intestinal gas in the irritable bowel syndrome.
      Nevertheless, in clinical practice, there are patients in whom the anus probably contributes to gas retention, either because of poor anal relaxation (functional outlet obstruction; see below) or voluntary inhibition of anal flatus as a result of social or psychological interferences.

       Reflex control

      Further data suggest that impaired gas transit in patients with unexplained bloating is the result of abnormal reflex control. Gas transit is regulated by viscerovisceral reflexes that operate along the gastrointestinal tract.
      • Harder H.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Reflex control of intestinal gas dynamics and tolerance.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Gastric distension and duodenal lipid infusion modulate intestinal gas transit and tolerance in humans.
      Intraluminal lipids dose-dependently delay gas transit and induce retention of exogenous gas loads.
      • Serra J.
      • Salvioli B.
      • Azpiroz F.
      • Malagelada J.R.
      Lipid-induced intestinal gas retention in the irritable bowel syndrome.
      Lipids seem more effective when infused into the ileum than into the duodenum, in consonance with the activation of the ileal brake mechanism that regulates the transit of solid/liquid chyme.
      • Hernando-Harder A.C.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Sites of symptomatic gas retention during intestinal lipid perfusion in healthy subjects.
      By contrast, other reflexes speed gas transit. For instance, gastric distention produces an immediate evacuation of the endogenous gas present in the gut and accelerates the transit of exogenous gas loads, thus suggesting the release of gastrocolic reflexes.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Gastric distension and duodenal lipid infusion modulate intestinal gas transit and tolerance in humans.
      However, the gastrocolic reflex seems to be part of a more generalized reflex phenomenon, because distention performed at various levels of the gut, such as the duodenum or the rectum, produces the same stimulatory effect.
      • Harder H.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Reflex control of intestinal gas dynamics and tolerance.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Gastric distension and duodenal lipid infusion modulate intestinal gas transit and tolerance in humans.
      Furthermore, focal gut distention antagonizes the inhibitory effect of lipids.
      • Harder H.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Reflex control of intestinal gas dynamics and tolerance.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Gastric distension and duodenal lipid infusion modulate intestinal gas transit and tolerance in humans.
      It has been shown that intestinal gas clearance is enhanced in the erect position
      • Dainese R.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.-R.
      Influence of body posture on intestinal transit of gas.
      and during mild physical activity
      • Dainese R.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.-R.
      Effect of physical activity on intestinal gas transit and evacuation in healthy subject.
      ; this suggests that somatovisceral reflexes may also participate in the regulation of intestinal gas transit. Conceivably, under physiological conditions, different types of reflexes interact to produce a net final effect. It has been shown that the slowing effect of lipids is up-regulated in patients with bloating,
      • Serra J.
      • Salvioli B.
      • Azpiroz F.
      • Malagelada J.R.
      Lipid-induced intestinal gas retention in the irritable bowel syndrome.
      whereas the accelerating effect of distention is markedly impaired.
      • Passos M.C.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Impaired reflex control of intestinal gas propulsion in patients with abdominal bloating.
      Indeed, the gas challenge test, in conjunction with intraluminal lipid infusion, provides a clear-cut distinction between IBS/bloated patients and healthy subjects: patients retain large volumes of gas and or report symptoms, thus indicating that gas propulsion may be ineffective or symptomatic.
      • Serra J.
      • Salvioli B.
      • Azpiroz F.
      • Malagelada J.R.
      Lipid-induced intestinal gas retention in the irritable bowel syndrome.

       Which Area of the Gut Gives Rise to Bloating?

      In healthy subjects, gas tolerance depends on the site of retention. In a model of gas retention modeled by infusion during blocked anal gas outflow, it was shown that similar volumes of gas retention produced significantly more abdominal symptoms with jejunal than with rectal gas infusion, whereas abdominal distention was similar.
      • Harder H.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Intestinal gas distribution determines abdominal symptoms.
      Scintigraphic imaging using xenon 133 to label gas showed that gas infused into the jejunum predominantly accumulated in the small bowel and proximal colon, whereas gas infused per rectum largely accumulated in the distal colon.
      • Harder H.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Intestinal gas distribution determines abdominal symptoms.
      These data indicate that gas-related symptoms are determined by intestinal gas distribution, whereas abdominal distention depends on the total volume of gas retained in the gut.
      Xenon-133 scintigraphy has been also used to investigate segmental gas transit. In healthy individuals, total gut transit of gas was expeditious, and, interestingly, small-intestinal transit time was similar to colonic transit time. Hence, in contrast to solids and liquids, the speed of intestinal gas clearance is equally determined by the passage time through the small-intestinal and colonic compartments. Using the gas challenge technique, we have shown that in patients with bloating, intestinal gas clearance is delayed because of impaired small-intestinal propulsion, whereas colonic transit is normal.
      • Salvioli B.
      • Serra J.
      • Azpiroz F.
      • Lorenzo C.
      • Aguade S.
      • Castell J.
      • Malagelada J.-R.
      Origin of gas retention in patients with bloating.
      These data were confirmed by measuring the responses to gas directly infused at different levels of the gut via an intraluminal catheter. These responses showed that patients retained gas infused into the jejunum, but not infused into the ileum or cecum.
      • Salvioli B.
      • Serra J.
      • Azpiroz F.
      • Lorenzo C.
      • Aguade S.
      • Castell J.
      • Malagelada J.-R.
      Origin of gas retention in patients with bloating.
      Hence, the ileocolonic junction does not seem to be responsible for gas retention, and, altogether, these data point toward the proximal small bowel, rather than the colon, as the source of symptoms in patients complaining of abdominal bloating. Indeed, large amounts of carbon dioxide are physiologically produced from neutralization of acids in the proximal small bowel.
      • Suarez F.L.
      • Levitt M.D.
      Intestinal gas.
      • Fordtran J.S.
      • Morawski S.G.
      • Santa Ana C.A.
      • Rector F.C.
      Gas production after reaction of sodium bicarbonate and hypochloric acid.
      Carbon dioxide is rapidly absorbed into the bloodstream, but the luminal fraction may still overflow an incompetent intestine and produce symptoms.
      At the same time, it would seem reasonable to contemplate other possibilities. For instance, patients with increased gas production who are unwilling or unable to expel excess gas may accumulate gas in the colon. Conceivably, the area of the gut in which bloating originates also depends on the clinical features. Indeed, postprandial bloating in dyspeptic patients may originate in the stomach.
      • Coffin B.
      • Azpiroz F.
      • Guarner F.
      • Malagelada J.-R.
      Selective gastric hypersensitivity and reflex hyporeactivity in functional dyspepsia.
      • Caldarella M.
      • Azpiroz F.
      • Malagelada J.-R.
      Antro-fundic dysfunctions in functional dyspepsia.
      Normally, a meal largely accommodates in the proximal stomach, partly because antral filling induces a fundic relaxatory reflex. Additionally, when gastric emptying starts, intestinal nutrients induce enterofundic relaxatory reflexes that control the accommodation process over the postprandial period. Dyspeptic patients show impaired enterofundic and antrofundic relaxatory reflexes, and this may result in defective fundic accommodation, with distal displacement of intragastric contents and antral overload.
      • Caldarella M.
      • Azpiroz F.
      • Malagelada J.-R.
      Antro-fundic dysfunctions in functional dyspepsia.
      Furthermore, these patients show increased gastric perception, which also affects the antrum.
      • Caldarella M.
      • Azpiroz F.
      • Malagelada J.-R.
      Antro-fundic dysfunctions in functional dyspepsia.
      The mixed sensory motor dysfunction, leading to overdistention of a hypersensitive antrum, may explain the genesis of dyspeptic bloating independently of gas transit in the bowel.

       Gas or No Gas: Which Is the Key Offending Element?

      Gas transit studies in patients with unexplained bloating have evidenced a failure of gut propulsive motility associated with increased perception, which, in some way, seems to play a pathophysiological role in their abdominal symptoms. However, the triggering factor of bloating may not be invariably gas, but other element(s) of gut content.
      Bloating may be a prominent feature in the absence of increased gas production. Levitt et al
      • Levitt M.D.
      • Furne J.
      • Olsson S.
      The relation of passage of gas and abdominal bloating to colonic gas production.
      nicely showed the disparities between bloating and flatulence by observing the responses of healthy subjects to oral loads of either lactulose or 2 types of fiber (psyllium and methylcellulose). Lactulose, which is not absorbed in the small intestine and is fermented in the colon, releasing hydrogen, induced flatulence, a sensation of rectal gas, and bloating. It is interesting to note that neither fermentable (psyllium) nor nonfermentable (methylcellulose) fiber produced hydrogen release detectable by the breath test and did not induce flatulence or rectal gas sensation but still induced the sensation of abdominal bloating. The authors concluded that bloating associated with experimental fiber overload presumably reflects an increased intraluminal mass.
      Increased intestinal fluid content may also be an important cause of bloating under some circumstances, for instance, in acute diarrheal conditions and perhaps in some instances of postprandial bloating. However, data on the transit of chyme in the small bowel in relation to bloating are inconsistent and show both accelerated
      • Hebden J.M.
      • Blackshaw E.
      • D’Amato M.
      • Perkins A.C.
      • Spiller R.C.
      Abnormalities of GI transit in bloated irritable bowel syndrome effect of bran on transit and symptoms.
      and delayed
      • Cann P.A.
      • Read N.W.
      • Brown C.
      Irritable bowel syndrome relationship of disorders in the transit of a single solid meal to symptom patterns.
      transit. It has been hypothesized that the terminal ileum and the ileocolonic junction play a special role in the regulation of chyme transit. However, specific studies on ileocecal transit have also yielded contradictory results,
      • Trotman I.F.
      • Price C.C.
      Bloated irritable bowel syndrome defined by dynamic 99mTc bran scan.
      • Hutchinson R.
      • Notgui A.
      • Smith N.B.
      • et al.
      Scintigraphic measurement of ileocaecal transit in irritable bowel syndrome and chronic idiopathic constipation.
      although chyme arrival to the cecum seems temporarily related to symptoms.
      • Cann P.A.
      • Read N.W.
      • Brown C.
      Irritable bowel syndrome relationship of disorders in the transit of a single solid meal to symptom patterns.
      In some patients, intestinal infusion of nutrients reproduces customary postprandial bloating and visible abdominal distention. They may constitute a specific subset of patients, different from typical dyspepsia and IBS, but data available on this condition are too scanty to allow even speculative conclusions.
      Abdominal bloating and distention may also derive from an expansion in extraluminal water content in the abdominal cavity. Vascular ingurgitation and visceral edema, conceivably under neurohormonal influences, could be involved in some forms (for instance, menstrual bloating), but this possibility has not been explored and stands only on a theoretical basis.

      An Integrative Overview

      Bloating may be produced by different mechanisms. The pathophysiology of bloating may be evident, for instance, in case of intra-abdominal volume increment, but in most patients, the dysfunction that leads to bloating is more subtle and difficult to detect by conventional methods. Gas transit studies have evidenced that patients with bloating have impaired reflex control of gut handling of contents (Figure 7). Segmental pooling, either of gas or of solid/liquid components, may induce a bloating sensation, particularly in patients with altered gut perception. Gut stimuli induce viscerosomatic reflexes to adapt the abdominal wall to its content, and some data indicate that patients with bloating also have affected viscerosomatic reflexes that lead to abdominal wall dystony and abnormal protrusion (Figure 7).
      Figure thumbnail gr7
      Figure 7Pathophysiology of bloating. Bloating may be produced by intra-abdominal volume increment, but in most cases the dysfunction is more subtle and multifactorial. Impaired reflex control of gut handling of contents may result in segmental pooling and bloating sensations, particularly in patients with altered gut perception. Impaired viscerosomatic reflexes may lead to abdominal wall dystony and distention, even without major increments in intra-abdominal volume.

      Clinical Presentation

       General Clinical Features

      Abdominal bloating presents some characteristic and well-recognized features, but given the scarcity of systematic investigations, data are mostly based on physicians’ impressions. Recently, Whitehead’s group (personal communication, December 2003) has initiated a large-scale study to characterize the diverse clinical presentations of bloating; this study may yield useful information.
      Bloating, as with most functional gastrointestinal symptoms, is much more frequent in women than in men.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      • Sandler R.S.
      • Stewart W.F.
      • Liberman J.N.
      • Ricci J.A.
      • Zorich N.L.
      Abdominal pain, bloating, and diarrhea in the United States prevalence and impact.
      The severity of bloating may vary from very mild to severe and uncomfortable.
      • Sandler R.S.
      • Stewart W.F.
      • Liberman J.N.
      • Ricci J.A.
      • Zorich N.L.
      Abdominal pain, bloating, and diarrhea in the United States prevalence and impact.
      An important aspect of the anamnesis is to discriminate whether bloating is uncomfortable/painful or whether the patient has associated but separate abdominal discomfort or pain.
      • Lembo T.
      • Naliboff B.
      • Munakata J.
      • Fullerton S.
      • Saba L.
      • Tung S.
      • Schmulson M.
      • Mayer E.A.
      Symptoms and visceral perception in patients with pain-predominant irritable bowel syndrome.
      • Sach J.
      • Bolus R.
      • Fitzgerald L.
      • Naliboff B.
      • Chang L.
      • Mayer E.
      Is there a difference between abdominal pain and discomfort in moderate to severe IBS patients?.
      It is also important to record the patient’s own impression about the presence and severity of objective abdominal distention.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      Bloating may be localized in the upper abdomen (sometimes associated with dyspeptic symptoms) or in the lower abdomen, as part of IBS or related syndromes. Of course, a large overlap exists, and many patients describe bloating of the entire abdomen.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      • Maxton D.G.
      • Whorwell P.J.
      Abdominal distension in irritable bowel syndrome the patient’s perception.
      Bloating may be related to food intake. A substantial number of patients with bloating, up to 82%, describe that it develops or worsens in the early postprandial period.
      • Maxton D.G.
      • Whorwell P.J.
      Abdominal distension in irritable bowel syndrome the patient’s perception.
      Some patients complain of even being unable to complete a full meal because of bloating. This effect is more pronounced when eating large and specially fatty meals. Postprandial bloating is a characteristic feature in specific subgroups of patients, such as those with dyspepsia and binge eaters (see below). Some patients claim specific food intolerances in connection with bloating,
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      but the putative mechanism of the intolerance, or even whether this is real or imaginary, remains unclear. High-fiber foods or fiber supplements are frequently reported to worsen bloating, in consonance with the experimental findings of Levitt et al
      • Hebden J.M.
      • Blackshaw E.
      • D’Amato M.
      • Perkins A.C.
      • Spiller R.C.
      Abnormalities of GI transit in bloated irritable bowel syndrome effect of bran on transit and symptoms.
      described previously. Dairy products are frequently reported as deleterious, but only an undetermined fraction of these cases are due to genuine lactose intolerance, particularly when intolerance occurs in response to low quantities of dietary lactose.
      • Suarez F.L.
      • Dennis A.
      • Savalano D.
      • Levitt M.
      A comparison of symptoms after the consumption of milk or lactose-hydrolyzed milk by people with self-reported severe lactose intolerance.
      Fatty foods and carbonated drinks are also frequently reported as offending. Both of these possibilities are supported by some experimental evidence, as described previously, but in some patients the relation may actually be based on imaginary assumptions. Food intolerances may be related to atopic phenomena, but most likely this is an uncommon occurrence.
      Circadian variations are a common feature of bloating.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      • Maxton D.G.
      • Martin D.F.
      • Whorwell P.
      • Godfrey M.
      Abdominal distension in female patients with irritable bowel syndrome exploration of possible mechanisms.
      • Maxton D.G.
      • Whorwell P.J.
      Abdominal distension in irritable bowel syndrome the patient’s perception.
      In most patients, bloating progressively develops during daily activity and tends to diminish or disappear after night rest. This probably has a physiological basis, because continuous measurement of girth in healthy subjects has shown that girth progressively increases during the day and returns to basal at night.
      • Lewis M.
      • Reilly B.
      • Houghton L.
      • Whorwell P.
      Ambulatory abdominal inductance plethysmography towards objective assessment of abdominal distension in irritable bowel syndrome.
      • Lea R.
      • Whorwell P.
      • Reilly B.
      • Houghton L.
      Abdominal distension in irritable bowel syndrome (IBS) diurnal variation and its relationship to abdominal bloating.
      Meals are another factor that has been shown to increase girth in healthy subjects.
      • Lewis M.
      • Reilly B.
      • Houghton L.
      • Whorwell P.
      Ambulatory abdominal inductance plethysmography towards objective assessment of abdominal distension in irritable bowel syndrome.
      Girth is also larger in the upright position than in the supine position,
      • Lewis M.
      • Reilly B.
      • Houghton L.
      • Whorwell P.
      Ambulatory abdominal inductance plethysmography towards objective assessment of abdominal distension in irritable bowel syndrome.
      and likewise, bloating tends to be alleviated by lying down,
      • Maxton D.G.
      • Whorwell P.J.
      Abdominal distension in irritable bowel syndrome the patient’s perception.
      which may point to a contribution from the abdominal musculature. Stress is reported by some patients (approximately one third in one study) to worsen bloating,
      • Maxton D.G.
      • Whorwell P.J.
      Abdominal distension in irritable bowel syndrome the patient’s perception.
      and patients tend to feel better when relaxed.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      In some patients, bloating is associated with tiredness and difficulties sleeping, and these symptoms altogether impair quality of life. No clear mechanism has been elucidated. Some patients characteristically describe a rapid onset of bloating after a precipitating event: <60 seconds in 35% and <10 minutes in another 26%.
      • Maxton D.G.
      • Whorwell P.J.
      Abdominal distension in irritable bowel syndrome the patient’s perception.
      Such abrupt development would fit with either a vascular or muscular mechanism of bloating, but this remains purely speculative. Abdominal rumbling is a frequent associated feature, but passing flatus or stool does not necessarily alleviate bloating.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      • Maxton D.G.
      • Whorwell P.J.
      Abdominal distension in irritable bowel syndrome the patient’s perception.
      In up to 40% of women, bloating gets worse before and during the menstrual period.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      • Heitkemper M.M.
      • Cain K.C.
      • Jarrett M.E.
      • Burr R.L.
      • Crowell M.D.
      • Woods N.F.
      Relationship of bloating to other GI and menstrual symptoms in women with irritable bowel syndrome.
      Furthermore, bloating is one of the most frequent menstrual symptoms.
      • Huerta-Franco M.R.
      • Malacara J.M.
      Association of physical and emotional symptoms with the menstrual cycle and life-style.
      Neurohormonal factors have been implicated, but the prevalence of bloating is the same in premenopausal and postmenopausal women.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      Furthermore, some recent data indicate that in healthy women during the menstrual period, bloating may progressively increase during the day but is not paralleled by objective girth increments.
      • Lea R.
      • Walker B.
      • Reilly B.
      • Whorwell P.J.
      • Houghton L.
      Abdominal bloating experienced at menses is not related to abdominal distension in healthy women.

       Clinical Conditions Associated With Bloating

      Abdominal bloating in patients has various forms of clinical presentation, and conversely, in different clinical syndromes, bloating may be a prominent feature or even the primary complaint.

       Constipation

      A considerable proportion of patients who complain of bloating acknowledge that their abdominal symptom is related to bowel habit: bloating develops as days go by without a bowel movement and resolves with evacuation. Conversely, the incidence of bloating in constipated patients is very high—up to 80% in some studies.
      • Marcus S.N.
      • Heaton K.W.
      Irritable bowel-type symptoms in spontaneous and induced constipation.
      • Mertz H.
      • Naliboff B.
      • Mayer E.A.
      Symptoms and physiology in severe chronic constipation.
      Furthermore, when 12 healthy subjects were made constipated with loperamide, there was a significant increase in abdominal bloating, similar to that reported by constipated patients.
      • Marcus S.N.
      • Heaton K.W.
      Irritable bowel-type symptoms in spontaneous and induced constipation.
      Reciprocally, effective laxative treatment in 24 constipated subjects markedly alleviated the prevalence and severity of their bloating.
      • Marcus S.N.
      • Heaton K.W.
      Irritable bowel-type symptoms in spontaneous and induced constipation.
      In contrast to these symptomatic patients, in other cases, such as encopresis, fecal retention is well tolerated even for prolonged periods of time without any abdominal complaints. Plausibly, constipated patients with significant abdominal symptoms, including bloating, have an irritable bowel with poor tolerance of the overload. Most of these constipation-predominant IBS patients with bloating have visible abdominal distention.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.

       Diarrhea

      In some patients, bloating is associated with loose stools, increased frequency of stools, or urgency. This subgroup would fall within the definition of diarrhea-predominant IBS, and in approximately one third of them, bloating is not associated with visible abdominal distention.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      Patients with both bloating and diarrhea should be evaluated for possible lactose or fructose intolerance. In this particular group, weight loss or nutritional deficiency should alert physicians to the possibility of malabsorption or maldigestion. Indeed, abdominal bloating is a common clinical feature of organic diarrhea, such as malabsorptive diarrhea, infectious diarrhea, and other types. Mechanisms involved—ie, inflammation, fluid or gas pooling, motor/sensory disturbances, carbohydrate fermentation, and so on—may not be singularly involved in each condition but, rather, are mixed in various proportions. Unfortunately, clinicopathophysiological relations have not been deeply investigated, and only conjectural conclusions are possible.

       Irritable bowel syndrome

      A series of studies in the past few years have clearly shown the importance of bloating among the symptom spectrum of IBS.
      • Chang L.
      • Lee O.Y.
      • Naliboff B.
      • Schmulson M.
      • Mayer E.A.
      Sensation of bloating and visible abdominal distention in patients with irritable bowel syndrome.
      • Lembo T.
      • Naliboff B.
      • Munakata J.
      • Fullerton S.
      • Saba L.
      • Tung S.
      • Schmulson M.
      • Mayer E.A.
      Symptoms and visceral perception in patients with pain-predominant irritable bowel syndrome.
      • Mertz H.
      • Naliboff B.
      • Mayer E.A.
      Symptoms and physiology in severe chronic constipation.
      Indeed, IBS is the clinical group in which the incidence and implications of bloating have been best established. Bloating-type symptoms are the most common abdominal complaint in patients with IBS and are reported by up to 96% of them.
      • Lembo T.
      • Naliboff B.
      • Munakata J.
      • Fullerton S.
      • Saba L.
      • Tung S.
      • Schmulson M.
      • Mayer E.A.
      Symptoms and visceral perception in patients with pain-predominant irritable bowel syndrome.
      Furthermore, 60% of IBS patients consider bloating their most bothersome abdominal complaint, even more so than abdominal pain, which is listed by only 29% of the patients.
      • Lembo T.
      • Naliboff B.
      • Munakata J.
      • Fullerton S.
      • Saba L.
      • Tung S.
      • Schmulson M.
      • Mayer E.A.
      Symptoms and visceral perception in patients with pain-predominant irritable bowel syndrome.
      Bloating also has a major effect on quality of life and impairs it more than does abdominal pain.
      • Sach J.
      • Bolus R.
      • Fitzgerald L.
      • Naliboff B.
      • Chang L.
      • Mayer E.
      Is there a difference between abdominal pain and discomfort in moderate to severe IBS patients?.
      • Wiklund I.K.
      • Fullerton S.
      • Hawkey C.J.
      • Jones R.H.
      • Longstreth G.F.
      • Mayer E.A.
      • Peacock R.A.
      • Wilson I.K.
      • Naesdal J.
      An irritable bowel syndrome-specific symptom questionnaire development and validation.
      From a different perspective, it can be stated that a considerable proportion of patients with bloating fulfill IBS criteria. That is, bloating is a part (even the most apparent and bothersome part) of IBS, which by definition is associated with disturbed bowel habits. Hence, in these patients, bloating may be associated with constipation, diarrhea, or both, thus sharing the characteristics of the groups described previously.

       Dyspepsia

      Bloating is one of the integral symptoms of dysmotility-like dyspepsia, and, indeed, a very large proportion of dyspeptic patients (54%–57%) reported they frequently felt “blown up.”
      • Knill-Jones R.P.
      A formal approach to symptoms of dyspepsia.
      Visible abdominal distention has also been reported by 36% of patients with dyspepsia, vs 7% of healthy subjects.
      • Talley N.J.
      • Phillips S.F.
      • Melton L.J.
      • et al.
      A patient questionnaire to identify bowel disease.
      In dyspepsia, bloating is often located in the upper abdomen, although, not surprisingly, given the common association between dyspepsia and IBS, it may also be diffuse. Stomach rumbling may be an associated symptom. Bloating in patients with dyspepsia tends to be precipitated by meals,
      • Caldarella M.
      • Azpiroz F.
      • Malagelada J.-R.
      Antro-fundic dysfunctions in functional dyspepsia.
      and some patients may even occasionally refrain from eating to prevent bloating. It is unclear whether these patients constitute a distinctive subgroup of dyspepsia or whether they just use the term bloating to mean an imprecise sensation that could also be described as abdominal fullness, tension, pressure, or another similar term.

       Eating disorders and obesity

      Bloating is a frequent clinical feature of eating disorders, such as binge eating
      • Crowell M.D.
      • Cheskin L.J.
      • Musial F.
      Prevalence of gastrointestinal symptoms in obese and normal weight binge eaters.
      • Chami T.N.
      • Andersen A.E.
      • Crowell M.D.
      • Schuster M.M.
      • Whitehead W.E.
      Gastrointestinal symptoms in bulimia nervosa effects of treatment.
      and anorexia,
      • Waldholtz B.D.
      • Andersen A.E.
      Gastrointestinal symptoms in anorexia nervosa. A prospective study.
      and it is also related to body mass index and obesity.
      • Crowell M.D.
      • Cheskin L.J.
      • Musial F.
      Prevalence of gastrointestinal symptoms in obese and normal weight binge eaters.
      • Talley N.J.
      • Quan C.
      • Jones M.P.
      • Horowitz M.
      Association of upper and lower gastrointestinal tract symptoms with body mass index in an Australian cohort.
      • Delgado-Aros S.
      • Locke III, G.R.
      • Camilleri M.
      • Talley N.J.
      • Fett S.
      • Zinsmeister A.R.
      • Melton III, L.J.
      Obesity is associated with increased risk of gastrointestinal symptoms a population-based study.
      Although healthy individuals may get bloated on occasion after overindulging in a large meal or in association with an overload of fermentable foodstuffs, such bloating tends to be relatively short, lasting a maximum of a few hours. Self-induced bloating is rarely a cause for concern or medical consultation, because the individual easily connects the bloating sensation to excess feeding and subsequent spontaneous relief. Important exceptions, however, are individuals who are so annoyed by postprandial bloating or who have such unrealistic expectations of overload tolerance that they may consult, not out of true concern about the significance of their symptom, but to obtain some form of preventive relief to allow them to overindulge without feeling uncomfortable.

       Aerophagia

      Some patients complain of upper abdominal bloating that is alleviated by belching. The problem here is that during repetitive attempted belching, the patient swallows air, thereby increasing the discomfort. When belching finally occurs, the sensation of bloating diminishes, leading the patient to erroneously conclude that there is excess gas production in the stomach.
      • Suarez F.L.
      • Levitt M.D.
      Intestinal gas.
      • Bond J.H.
      • Levitt M.D.
      Gaseousness and intestinal gas.
      Frequently the underlying sensation of abdominal bloating/fullness in these patients is just a dyspeptic symptom that they misinterpret as gas, thus triggering the aerophagia/belching vicious circle.

       Flatulence

      Some patients complain of excessive and/or odoriferous gas evacuation, which may become socially disabling. However, flatulence is neither necessarily nor frequently associated with bloating. The latter may occur only in response to increased gas production in susceptible individuals, who, as explained earlier in this review, manifest either impaired gas handling or increased visceral sensation, because the normal gut is extremely efficient in disposing of even extremely large gas loads.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Intestinal gas dynamics and tolerance in humans.
      An increased volume of flatus may be exceptionally related to air swallowing
      • Levitt M.D.
      • Furne J.
      • Aeolus M.R.
      • Suarez F.L.
      Evaluation of an extremely flatulent patient case report and proposed diagnostic and therapeutic approach.
      and could theoretically also be related to impaired gas absorption or even diffusion from blood. However, by and large, both excessive and odoriferous gas evacuation depend on the fermentation of undigested substrates by microflora in the colon. Odor is produced by trace elements, such as sulfur-containing gases and other yet-unidentified components.
      • Suarez F.
      • Furne J.
      • Springfield J.
      • Levitt M.
      Insights into human colonic physiology obtained from the study of flatus composition.
      • Suarez F.L.
      • Springfield J.
      • Levitt M.D.
      Identification of gases responsible for the odour of human flatus and evaluation of a device purported to reduce this odour.
      Excessive volume may result from either increased production or impaired consumption by colonic bacteria.
      • Suarez F.L.
      • Levitt M.D.
      Intestinal gas.
      It has been shown that some food components in the normal diet are incompletely absorbed in the small bowel and enter the colon, where they undergo fermentation by colonic bacteria and release gas.
      • Anderson I.H.
      • Levine A.S.
      • Levitt M.D.
      Incomplete absorption of the carbohydrate in all-purpose wheat flour.
      • Levitt M.D.
      • Hirsh P.
      • Fetzer C.A.
      • Sheahan M.
      • Levine A.S.
      H2 excretion after ingestion of complex carbohydrates.
      • Flourie B.
      • Leblond A.
      • Florent C.
      • Rautureau M.
      • Bisalli A.
      • Rambaud J.C.
      Starch malabsorption and breath gas excretion in healthy humans consuming low- and high-starch diets.
      Incompletely absorbed gas-producing food components include fermentable dietary fiber, such as xylan and pectin
      • Wolever T.M.
      • Robb P.A.
      Effect of guar, pectin, psyllium, soy polysaccharide, and cellulose on breath hydrogen and methane in healthy subjects.
      • Grimble G.
      Fibre, fermentation, flora, and flatus.
      ; some types of dietary starch, for instance, in macaroni and white wheat bread
      • Anderson I.H.
      • Levine A.S.
      • Levitt M.D.
      Incomplete absorption of the carbohydrate in all-purpose wheat flour.
      ; some oligosaccharides, such as raffinose and stachyose
      • Steggerda F.R.
      Gastrointestinal gas following food consumption.
      • Wagner J.R.
      • Carson J.F.
      • Becker R.
      • Gumbmann M.R.
      • Danhof I.E.
      Comparative flatulence activity of beans and bean fractions for man and the rat.
      • Steggerda F.R.
      • Dimmick J.F.
      Effects of bean diets on concentration of carbon dioxide in flatus.
      ; and some sugars, such as sorbitol and fructose.
      • Stone-Dorshow T.
      • Levitt M.D.
      Gaseous response to ingestion of a poorly absorbed fructo-oligosaccharide sweetener.
      • Wursch P.
      • Koellreutter B.
      • Schweizer T.F.
      Hydrogen excretion after ingestion of five different sugar alcohols and lactulose.
      Other components of normal meals interfere with absorption of nutrients: for instance, fiber increases starch malabsorption,
      • Hamberg O.
      • Rumessen J.J.
      • Gudmand-Hoyer E.
      Inhibition of starch absorption by dietary fibre. A comparative study of wheat bran, sugar-beet fibre, and pea fibre.
      and a pancreatic amylase inhibitor in beans antagonizes carbohydrate digestion and absorption.
      • Boibin M.
      • Flourié B.
      • Rizza R.A.
      • Go V.L.W.
      • DiMagno E.P.
      Gastrointestinal and metabolic effects of amylase inhibition in diabetics.
      • Brugge W.R.
      • Rosenfeld M.S.
      Impairment of starch absorption by a potent amylase inhibitor.
      • Taylor R.H.
      • Barker H.M.
      • Bowey E.A.
      • Canfield J.E.
      Regulation of the absorption of dietary carbohydrate in man by two new glycosidase inhibitors.
      • Layer P.
      • Zinsmeister A.R.
      • DiMagno E.P.
      Effects of decreasing intraluminal amylase activity on starch digestion and postprandial gastrointestinal function in humans.
      Mucoproteins of endogenous origin may also be fermented, and this would account for exaggerated gas evacuation during fasting in some patients.
      • Perman J.A.
      • Modler S.
      Glycoproteins as substrates for production of hydrogen and methane by colonic bacterial flora.
      In some patients, flatulence may be the consequence of a malabsorption disorder with an excess load of undigested substrates into the colon, but in such cases other manifestations of the primary disorder are usually apparent. Isolated flatulence is a more challenging clinical problem. It is known that gas evacuation in healthy subjects maintained on a similar diet may differ substantially depending on the composition of their colonic microflora. However, within subjects, gas output varies in relation to the diet and the amount of undigested residues arriving into the colon that serve as a gas-producing substrate to the colonic flora.
      • Steggerda F.R.
      Gastrointestinal gas following food consumption.
      • Kirk E.
      The quantity and composition of human colonic flatus.
      It has been shown that healthy subjects on a normal diet containing 200 g of beans evacuated 705 mL of gas per 24 hours, whereas with a fiber-free diet, gas evacuation was 214 mL.
      • Tomlin J.
      • Lowis C.
      • Read N.W.
      Investigation of normal flatus production in healthy volunteers.
      In healthy subjects, the frequency of gas evacuation is variable (usually approximately 10 evacuations per day).
      • Furne J.K.
      • Levitt M.D.
      Factors influencing frequency of flatus emission by healthy subjects.
      Flatulence is manifested by an increased number of evacuations, and objective recording in a diary may help to clarify the diagnosis in some cases.

       Organic diseases

      Acute diarrheal diseases caused by salmonella and other enteropathogenic infections may be associated with severe bloating in the early stages of the clinical picture, even before the onset of diarrhea. Under these circumstances, the patient may experience sudden bloating, but the origin of the problem becomes quickly evident. Malabsorptive conditions, chiefly celiac disease and other mucosal small-bowel enteropathies, may produce significant bloating, with or without associated flatulence, and in that instance, the diagnosis may not be as readily apparent. Acute or subacute bowel ischemia from low-output cardiac failure or mesenteric insufficiency is a clinically important cause of bloating that is often associated with visible abdominal distention caused by dilated air-filled bowel loops. However, ischemic bloating tends to be seen in hospitalized patients or in the context of major cardiovascular deficits. Bloating may also be a clinical complaint in patients with ascitis. Rarely, episodic abdominal bloating, pain, and distention may be a feature of angioedema affecting the gut.

      Management

      As described previously, abdominal bloating is a multifactorial and multimorphic clinical condition. The management of these patients is mostly unclear, but in some subgroups, specific recommendations can be made.

       Aerophagia

      Some patients with aerophagia can be retrained to control air swallowing, and even a clear pathophysiological explanation may suffice. However, when aerophagia and bloating are part of a dyspeptic syndrome, the therapeutic outcome may be poor, because the associated dyspeptic symptoms may not resolve after retraining.

       Flatulence

      Patients complaining of odoriferous and/or excessive gas evacuation usually benefit from a regimen that excludes gas-producing foodstuffs. Unfortunately, reliable scientific information about diet-related gas metabolism and evacuation is surprisingly scarce. Based on systematic and methodical observations of a patient who passed large amounts of flatus, a classification of foodstuffs depending on their gas-producing capacity was elaborated.
      • Sutalf L.O.
      • Levitt M.D.
      Follow-up of a flatulent patient.
      Very few controlled studies on specific foodstuffs have been later added to complement this information.
      • Wolever T.M.
      • Robb P.A.
      Effect of guar, pectin, psyllium, soy polysaccharide, and cellulose on breath hydrogen and methane in healthy subjects.
      • Wagner J.R.
      • Carson J.F.
      • Becker R.
      • Gumbmann M.R.
      • Danhof I.E.
      Comparative flatulence activity of beans and bean fractions for man and the rat.
      • Hickey C.
      • Calloway D.
      • Murphy E.
      Intestinal gas production following ingestion of fruits and fruit juice.
      Extremely flatulogenic foodstuffs include beans, brussels sprouts, onions, celery, carrots, raisins, bananas, prune juice, apricots, wheat germ, and bagels. Moderately flatulogenic foodstuffs include potatoes, eggplant, citrus fruit, apples, pastries, and bread. Low flatulogenic foodstuffs include meat, fowl, fish, eggs, some vegetables (lettuce, tomato, avocado, broccoli, cauliflower, and asparagus), some fruits (cherries, grapes, and cantaloupe), rice, corn chips, popcorn, nuts, and chocolate.
      After 1 week on a “gas-free” diet, patients usually experience frank symptom relief. Orderly reintroduction of other foodstuffs helps the patient learn to identify the offending meal components and avoid them to prevent recurrence of the flatulence. Basically, these patients need to decide the “flatus price” they have to pay for each type of meal and whether it is worth it.
      Theoretically, a change in the composition of the colonic bacteria, reducing the activity of gas-producing flora and/or increasing the gas-consuming pool, should also help to reduce excess gas production. In practice, however, this approach has been generally unsuccessful. Recent attempts with antibiotics, prebiotics, and probiotics have not produced satisfactory clinical results so far, particularly with respect to bloating.
      • Madden J.A.
      • Hunter J.O.
      A review of the role of the gut microflora in irritable bowel syndrome and the effects of probiotics.
      • Hamilton-Miller J.M.T.
      Probiotics in the management of irritable bowel syndrome a review of clinical trials.

       Difficult Gas Evacuation With or Without Constipation

      Rectal evacuation is physiologically achieved by a mild abdominal compression coupled with anal relaxation. In case of abdominoperineal dyssynergia, inadequate anal relaxation during straining is associated with impaired evacuation.
      • Azpiroz F.
      • Enck P.
      • Whitehead W.E.
      Anorectal functional testing. Review of a collective experience.
      • Whitehead W.E.
      • Wald A.
      • Diamant N.
      • Enck P.
      • Pemberton J.H.
      • Rao S.S.
      Functional disorders of the anus and rectum.
      Functional outlet obstruction may produce a sensation of difficult gas evacuation, constipation, or both, but some patients also complain of bloating and other IBS-type abdominal symptoms. Probable pathophysiological mechanisms of bloating include fecal retention, increased time for fermentation, and possibly impaired gas evacuation. Anal uncoordination can be resolved with biofeedback treatment,
      • Azpiroz F.
      • Enck P.
      • Whitehead W.E.
      Anorectal functional testing. Review of a collective experience.
      • Whitehead W.E.
      • Wald A.
      • Diamant N.
      • Enck P.
      • Pemberton J.H.
      • Rao S.S.
      Functional disorders of the anus and rectum.
      and in some patients the abdominal symptoms fade when colonic retention is corrected. In some patients, constipation is associated with upper abdominal bloating, which is aggravated by meals, thus mimicking a dyspeptic syndrome, and these symptoms may also improve if constipation resolves.

       Pseudoobstruction With or Without Abdominal Pain

      In some patients, bloating is very severe and mimics the clinical presentation of chronic or subacute intestinal pseudoobstruction when distention is prominent or of functional abdominal pain when the bloating sensation is intense and becomes painful. These rare patients require further evaluation, including intestinal manometry, to rule out intestinal neuropathy.
      • Barbara G.
      • Stanghellini V.
      • De Giorgio R.
      • Cremon C.
      • Cottrell G.S.
      • Santini D.
      • Pasquinelli G.
      • Morselli-Labate A.M.
      • Grady E.F.
      • Bunnett N.W.
      • Collins S.M.
      • Corinaldesi R.
      Activated mast cells in proximity to colonic nerves correlate with abdominal pain in irritable bowel syndrome.

       General Measures

      The role of diet when bloating is not associated with flatulence remains unsettled. Fiber overload is nowadays a common occurrence in bloated patients. Fiber has been shown to worsen IBS symptoms; this phenomenon may be directly related to gas production or to the intraluminal overload.
      • Francis C.Y.
      • Whorwell P.J.
      Bran and irritable bowel syndrome time for reappraisal.
      Such patients usually benefit from removal of the extra fiber load. Except in documented cases of malabsorption, there is no strong evidence to support the use of exclusion diets. Even patients with lactose malabsorption may tolerate normal amounts of dairy products in their diet.
      • Suarez F.L.
      • Dennis A.
      • Savalano D.
      • Levitt M.
      A comparison of symptoms after the consumption of milk or lactose-hydrolyzed milk by people with self-reported severe lactose intolerance.
      Artificial sweeteners are incompletely absorbed in the small bowel and subsequently fermented in the colon. Hence, excessive consumption of sugar-free candies and diet soft drinks could contribute to bloating, but this possibility has not been experimentally tested. In patients with a fatty, prominent abdomen, particularly if associated with recent weight gain, a low-calorie diet and weight reduction may improve the symptoms, although some patients are reluctant to accept this solution. It is important to be aware that bloating may be due not only to gas, but also to solid and fluid intraluminal content, as well as to extraluminal fluid, for instance, in case of ascitis.
      Some experimental data have shown that exercise improves intestinal gas clearance in healthy subjects,
      • Dainese R.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.-R.
      Effect of physical activity on intestinal gas transit and evacuation in healthy subject.
      but its therapeutic value in patients with bloating has not been tested yet. Likewise, intestinal gas transit is more rapid in the upright position than in the supine position,
      • Dainese R.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.-R.
      Influence of body posture on intestinal transit of gas.
      and this could play a potential role in the pathophysiology and management of immobilized patients.
      Hypnosis has been shown to improve bloating in patients with IBS.
      • Lea R.
      • Houghton L.A.
      • Calvert E.L.
      • Larder S.
      • Gonsalkorale W.M.
      • Whelan V.
      • Randles J.
      • Cooper P.
      • Cruickshanks P.
      • Miller V.
      • Whorwell P.J.
      Gut-focused hypnotherapy normalizes disordered rectal sensitivity in patients with irritable bowel syndrome.
      The rapid effect of hypnotic induction on objective abdominal distention suggests that this option may be particularly effective in patients with bloating related to abdominal wall dystony. Nevertheless, hypnosis has been also shown to exert sensory motor influences on the gut.
      • Lea R.
      • Houghton L.A.
      • Calvert E.L.
      • Larder S.
      • Gonsalkorale W.M.
      • Whelan V.
      • Randles J.
      • Cooper P.
      • Cruickshanks P.
      • Miller V.
      • Whorwell P.J.
      Gut-focused hypnotherapy normalizes disordered rectal sensitivity in patients with irritable bowel syndrome.
      • Whorwell P.J.
      • Hoghton L.A.
      • Taylor E.E.
      • Maxton D.G.
      Physiological effects of emotion a study via hypnosis.

       Putative Pharmacological Therapies

      Current treatment options have been recently reviewed elsewhere.
      • Azpiroz F.
      • Serra J.
      Treatment of excessive intestinal gas.
      In some patients, bloating is just a feature of specific functional gastrointestinal disorders, particularly IBS and dyspepsia, and the treatment recommendations for these conditions would also apply here. Antidepressants seem effective in these disorders and may also improve bloating.
      • Masand P.S.
      • Gupta S.
      • Schwartz T.L.
      • Kaplan D.
      • Virk S.
      • Hameed A.
      • Lockwood K.
      Does a preexisting anxiety disorder predict response to paroxetine in irritable bowel syndrome?.

       Prokinetics Versus Spasmolytics

      Prokinetics and spasmolytics have been traditionally used in the treatment of bloating. Both modify the gut accommodation/propulsion balance, and it is not clear to what extent by enhancing one factor the other may be hampered, that is, whether the effects on gas clearance and abdominal symptoms may antagonize. Experimental data in healthy subjects using the gas challenge test showed that intestinal gas retention produced fewer abdominal symptoms when glucagon, a potent relaxatory drug, was concomitantly administered, but the same objective abdominal distention remained.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Mechanisms of intestinal gas retention in humans impaired propulsion versus obstructed evacuation.
      Conversely, the administration of neostigmine, a potent prokinetic drug, produced gas clearance in patients with intestinal gas retention, and this was followed by improvement of both abdominal symptoms and objective distention
      • Caldarella M.P.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Prokinetic effects of neostigmine in patients with intestinal gas retention.
      (Figure 8). The results with these drugs, used as a proof of concept, indicate that prokinetic and spasmolytic activity could have a role in the treatment of abdominal bloating. However, the experience with currently available drugs is not definitive.
      • Poynard T.
      • Regimbeau C.
      • Benhamou Y.
      Meta-analysis of smooth muscle relaxants in the treatment of irritable bowel syndrome.
      Nevertheless, these data open the possibility that new compounds, particularly those with prokinetic activity, could be an effective treatment option.
      • Muller-Lissner S.A.
      • Fumagalli I.
      • Bardhan K.D.
      • Pace F.
      • Pecher E.
      • Nault B.
      • Ruegg P.
      Tegaserod, a 5-HT(4) receptor partial agonist, relieves symptoms in irritable bowel syndrome patients with abdominal pain, bloating and constipation.
      Peppermint oil has been proposed to antagonize the bad odor of flatus, but its active principal ingredient, menthol, seems to act as a spasmolytic rather than a deodorant because of calcium channel blocker activity, with controversial effects on bloating and flatulence.
      • Hills J.M.
      • Aaronson P.
      The mechanism of action of peppermint oil on gastrointestinal smooth muscle.
      • Pittler M.H.
      • Ernst E.
      Peppermint oil for irritable bowel syndrome a critical review and metaanalysis.
      Figure thumbnail gr8
      Figure 8Prokinetic effects on intestinal gas retention, abdominal symptoms, and distention. Neostigmine administration during intestinal gas infusion in patients significantly reduced gas retention, abdominal symptoms, and distention as compared with saline administration (*P < .05 for all). Reprinted with permission.
      • Caldarella M.P.
      • Serra J.
      • Azpiroz F.
      • Malagelada J.R.
      Prokinetic effects of neostigmine in patients with intestinal gas retention.

       Gas-Reducing Substances

      Silicone derivates with surfactant properties have been postulated as antifoaming agents to improve gas absorption and evacuation from the gut, but their efficacy seems questionable.
      • Metcalf T.J.
      • Irons T.G.
      • Sher L.D.
      • Young P.C.
      Simethicone in the treatment of infant colic a randomized, placebo-controlled, multicenter trial.
      • Holtmann G.
      • Gschossmann J.
      • Mayr P.
      • Talley N.J.
      A randomized placebo-controlled trial of simethicone and cisapride for the treatment of patients with functional dyspepsia.
      Oral activated charcoal can adsorb and inactivate poisons in the gut, but it seems ineffective in reducing either the total gas volume or the fraction of odoriferous trace elements in flatus.
      • Suarez F.L.
      • Furne J.
      • Springfield J.
      • Levitt M.D.
      Failure of activated charcoal to reduce the release of gases produced by the colonic flora.
      Enzymatic preparations are a popular therapy for gas symptoms and bloating, but effectivity data are scarce.
      • Suarez F.
      • Levitt M.D.
      • Adsheat J.
      • Barkin J.S.
      Pancreatic supplements reduce symptomatic response of healthy subjects to a high fat meal.
      • Ganiats T.G.
      • Norcross W.A.
      • Halverson A.L.
      • Burford P.A.
      • Palinkas L.A.
      Does Beano prevent gas? A double-blind crossover study of oral alpha-galactosidase to treat dietary oligosaccharide intolerance.

      Conclusion

      Bloating is being increasingly recognized as an important clinical problem. Predictably, in the next few years this condition will elicit a concerted effort to understand its pathogenetic mechanisms and to develop effective therapy. In this review, we have proposed a framework for the investigation of bloating, taking into account both the clinical and the laboratory perspectives. Given the paucity of prior solid information on bloating, rather than providing answers, we aimed at formulating a comprehensive series of questions pertinent to its pathophysiology and polymorphic clinical presentation. We hope that this approach will stimulate the development of specific studies and, ultimately, make bloating a treatable condition.

      Acknowledgment

      The authors thank Gloria Santaliestra for secretarial assistance.

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