Bloating; Abnormal Abdominal Physiology or Just a Lot of Gas?

We all think we know what it feels like, but how is it caused?  At one time, doctors believed bloating was due to swallowing too much air. 

At another, it was caused by lordosis, an abnormal posture produced  by arching the spine and pushing the abdomen forwards.  Nowadays, the most popular cause is gaseous distension caused by excessive fermentation of carbohydrate foods.  But, at least half the patients who complain of bloating do not have excessive gas in the abdomen and most do not even have abdominal distension.  So what is going on? How are symptoms of bloating generated?  And how can we treat bloating?  Dr Fernando Azpiroz, a gastroenterologist and physiologist from Barcelona, has been studying bloating for the last twenty years.  We thought we could do not better than to ask him.      

Taken from May’s edition of The Inside Story, the IBS Network’s healthcare professional bulletin.
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Thank you, Dr Azpiroz for speaking to The Inside Story.  Can I start by asking you how you would define bloating? 

FA.  There is no standard definition of bloating and the perception of what it is may vary in different cultures.  When I discussed this with Professor Peter Whorwell recently, the nearest we could come up with was the sensation of increased abdominal pressure.  This gets us away from a definition based on cause or the presence of absence of distension.  But as a clinician, it is  important to ask what the patient means, because that always informs how you manage the condition.   Some patients may talk of being full of wind, others feeling constipated, others may complain of a distended abdomen.   Doctors working in different cultures may see a different spectrum off patients.  In our practice in Barcelona, most patients have a distended abdomen, whereas in Manchester patients do not necessarily have abdominal distension.

Do you make any distinction between upper abdominal bloating or lower abdominal bloating?

FA.  To be honest with you, No!  Though I agree that we can all have a sensation of bloating if we have eaten too much.  Bloating is common in patients with dyspepsia (1) but I suspect that many of them have an in an increase in visceral sensitivity so they sense even normal amounts of food as uncomfortable (2).

What about constipation? Does that cause bloating? 

FA. Yes, I believe it does.  A bowel packed full of faeces may well generate a sensation of bloating (3), but I suspect that visceral hypersensitivity plays an important role here too.

Could the sensation of bloating in constipated patients be due to gastric stasis caused by the effect of rectal stimulation? 

FA. I don’t know, though there was a paper from the Dutch group showing that rectal distension could delay gastric emptying (4).

Your physiological studies have not only shown abnormal visceral responses to infusion of gas in bloaters with pooling in the small intestine (5), but also changes in the function of the diaphragm and the abdominal wall when bloating is induced by infusing gas into the small intestine. How do you explain that? 

FA.  Yes that is very interesting. When we infuse one and a half litres of gas into the small intestine of healthy volunteers, they show a contraction of the internal oblique muscles of the abdomen  and a relaxation of the diaphragm, thus accommodating the gas load with little visible distension.  People with episodic abdominal distension (bloating) have a different  response to gas infusion.  They relax their abdominal muscles (6), and contract and so lower their diaphragm (7).  We have recently shown that they also contract the internal intercostals muscles to spread the thorax.  These physiological responses allow them to get more air into the chest.

So would you see that as an adaptation to the sensation of fullness. After all people with bloating are often depicted in fixed inspiration with pursed lips.

FA.  Exactly, but it’s not a reflex. I believe it is a behavioural response, because it can melt away when the patient relaxes.

Dr Walter Alvarez also observed that distension can disappear during induction of anaesthetic or when patients with bloating go to sleep (8). 

FA. Yes and Professor Peter Whorwell sees the same thing during induction of hypnosis.

So why do you think that bloaters show this abnormal response? 

FA. I suspect it is the increase in visceral sensitivity (2) that makes them feel more full, uncomfortable and distressed after gas infusion following an increase in abdominal content  and that can make them feel they have to make space to get more air in.  This gets worse towards the end of the day, not only because of a cumulative increase in abdominal content, but also because of fatigue.

So this can be like we all feel if we have had a blow out and eaten too much.  We sense there is too much stuff in the abdomen and we can’t get enough breath. 

FA. Exactly.  One other observation makes me think that visceral hypersensitivity is at the root of the sensation of bloating.  In people with intestinal dysmotility, gas will tend to pool in the small intestine but they don’t get distension or a sensation of bloating and they don’t have visceral hypersensitivity (9).

In several of your studies, you infused lipid into the small intestine before you infused gas, and this tended to bring out bloating (10).  Why is that?  

FA. Lipid slows gas transit, causes pooling of gas and leads to increased sensitivity to gut stimuli.  But the curious thing is that rectal distension relieves that in normal subjects, but not in bloaters (11). I don’t know why that should be, perhaps if the rectum is full, it gives a signal that the rest of the abdominal content can move down.

So if this response in bloaters is induced by increased sensitivity to a relatively modest gas infusion,  would you get the same response in normal people if you carried on infusing gas?  Is it just a difference in sensory threshold? 

FA.  It may be, but we would not get ethical approval to conduct such an experiment.

So what is your view of a low FODMAP diet?  Does it help bloating?

FA. Undoubtedly.  But it is not new.  We and others have been using our own low flatogenic diet for years.  I would, however, worry, a little about recent studies showing that a low FODMAP diet can substantially reduce populations of bifidobacteria (16) when these are a major component of most probiotics and we hear so much about how beneficial they are.

So are probiotics useful for bloating? 

FA. Yes, in some of the people, some of the time.  The data is promising.

And what about prebiotics? Some would regard them as FODMAPs.

FA. Well I find them quite useful (17). They may tend to increase gas production to begin with but as the bacteria change, this can disappear.

So given your insight into the physiology of bloating, how do you manage it? 

FA. Since bloating comprises any combination of increased volume of gut contents, segmental pooling, heightened  visceral or abdominal sensitivity, and impaired viscero-somatic reflexes, there is not one optimal treatment for everybody, but different therapeutic strategies for different patients according to what is happening (13).

Finally, back in 1949, Dr Walter Alvarez published a large review entitled, Hysterical, Non-gaseous  Bloating (8),  in which he documented  detailed psychological and physiological observations in over a hundred patients with bloating.  There was psychological disturbance in over 80% of them, and in many of those, bloating was preceded by an episode of trauma or life change.  While this paper was ‘of its time’, what do you think of his observations?

FA. Walter Alvarez was a genius.  He made many of the crucial observations on bloating over 60 years ago.  Emotional tension and distress is so often associated with visceral hypersensitivity that we have to treat his careful observations with attention and respect.  But I am no expert on the psychological influences on gut function, so while some may think psychology is crucial, I could not possibly comment.


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2. Agrawal A., Houghton LA. Lea  P et al (2008) Bloating and distention in irritable Bowel syndrome: the role of visceral sensation, Gastroenterology, 134, 1882–1889. .  
3. Houghton L A, Lea R., Agrawal A. et al,  (2006). Relationship of abdominal bloating to distention in irritable bowel syndrome and effect of bowel habit.  Gastroenterology 131. 1003-1010   
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6. Accarino, A. Perez, F. Azpiroz, F et al (2009)  “Abdominal distention results from caudo-ventral redistribution of contents ,” Gastroenterology, 136,  1544–1551  
8. Alvarez WC, 1949.  Hysterical non-gaseous bloating. Arch.Int.Med. 84: 217-245.
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11. Passos MC, Serra J, Azpiroz F (2005)  Impaired reflex control of intestinal gas transit in patients with abdominal bloating. Gut 54:344-348. 
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13. Azpiroz, F and Malagelada, J (2005) Abdominal Bloating. Gastroenterology 129, 1060-1078 
14. Murray K, Wilkinson-Smith V, Hoad C, Costigan C, Cox E, Lam C, Marciani L, Gowland P, Spiller RC. Differential effects of FODMAPs (fermentable oligo-, di-, mono-saccharides and polyols) on small and large intestinal contents in healthy subjects shown by MRI. Am J Gastroenterol 2014;109:110-119.
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16. Silk DBA, Davis A, Vulevic J, et al (2009).  Clinical trial: the effects of a trans-galactooligosaccharide prebiotic on faecal microbiota and symptoms in irritable bowel syndrome.  Al. Pharm. Ther. 29. 508-518.

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