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Bloating And Irritable Bowel Syndrome

Bloating is one of the most common symptoms of irritable bowel syndrome, affecting more than 90% of people with irritable bowel syndrome. For some individuals, the bloating is more disturbing than the disordered bowel habit, and more difficult to live with than the pain. In one survey, 60% of those with irritable bowel syndrome reported bloating as their most bother some symptom. It is more commonly associated with the constipation-predominant type of irritable bowel syndrome, but relieving the constipation does not always relieve the bloating. Bloating and abdominal distension typically occur after meals, become progressively worse during the day and settle after a night’s sleep.

It’s worth remembering that about 30% of people without irritable bowel syndrome will also experience bloating from time to time. It is more common in women and may be worse during their periods.

Most treatments for irritable bowel syndrome are aimed at normalising the bowel habit and reducing the pain, but little is known about their effects onbloating. There are no specific treatments for bloating, and some doctors are not even sure that the abdominal distension really exists. In recent years, though, we have learned a lot more about gas production and transport in the intestine, and hopefully this distressing symptom will not be ignored in the future.



Where does all the gas in the bowel come from?

Gas in the intestine comes mainly from two sources. First, we all inadvertently swallow air, even when we’re not eating. This is called aerophagia. We can’t avoid swallowing some air, but this is usually harmless. Eating slowly and swallowing only small amounts at a time during meals, not sucking hard sweets or chewing gum, and avoiding fizzy drinks is often recommended to reduce aerophagia, but this has not been rigorously tested and is usually disappointing in practice.

The other source of gas is the fermentation of food, especially undigested plant material in the large bowel, by bacteria in the gut. So the volume and constituents of the gas depend partly on the diet and partly on the number and type of bacteria in the bowel – what is called the ‘bacterial flora’. Some carbon dioxide gas is also produced when acid from the stomach is neutralised by the alkali in the duodenum. But carbon dioxide is rapidly absorbed into the bloodstream and removed through the lungs, so this is not a major factor.

What is flatus made up of?

Much of the flatus is air that has been swallowed with the food and passed down the intestine. The carbon dioxide and oxygen in this air are mostly absorbed into the body, leaving the nitrogen behind. Some gas is also produced in the large bowel; this is mainly hydrogen and products of hydrogen such as hydrogen sulphide and methane.

Why does flatulence smell so bad?

The gases that make up flatus are mostly hydrogen, carbon dioxide and methane. Flatus smells obnoxious because of the breakdown of compounds containing sulphur. The main smelly gas in flatus is hydrogen sulphide (as in rotten eggs). Other malodorous gases include methanethiol and dimethyl sulphide.

It is likely that differences in the concentration of these gases are determined partly by the diet and partly by the bacterial flora (the bacteria that live in the gut). This is why the odour varies between individuals and why individuals can recognise the smell of their own flatus. Some foods, such as cabbage, eggs, onions and meat, contain more sulphur than others.

How much gas does the average person have in their intestine?

Not a lot – only about 200–400 ml, which is about 1–2 glassfuls.

And how much gas does an average person pass in a day?

There’s a lot of variation between people, and even in the same person at different times, but about 200–2500 ml (between onehalf and 5 pints) a day is normally produced.

I’m sure men seem to pass more flatus than women, and that it smells worse. Is this true?

This common belief has actually been studied in a small experiment. The flatus from women was found to have a greater concentration of hydrogen sulphide and was judged to have a worse odour. But men tended to pass greater volumes of flatus at a time, so overall there was no significant difference!


Is bloating real? Does my tummy really distend through the day?

Some doctors believe that the bloating and distension of irritable bowel syndrome are perceived rather than real. In other words, individuals feel discomfort in their abdomen, or are unhappy with what feels like a bloated abdomen, and misinterpret these feelings, believing their abdomen to be distended.

Many studies have looked at this question. The girth of the abdomen (the measurement all the way round) has been examined by simple measurement, by X-rays and by CT (computed tomography) scanning. All these studies confirmed a real increase in the abdominal circumference in people with irritable bowel syndrome. Although there were considerable changes in these measurements over a 24-hour period, they usually increased after meals, were lowest during sleep and increased progressively during the day. Interestingly, the measured increase in girth was only poorly related to the severity of the symptoms that the person reported.

Is this bloating caused by gas?

Excess gas in the intestine is probably the main cause of bloating. It was previously thought that most of this gas was produced by the bacterial fermentation of plant material in the large bowel. However, recent studies suggest that it is gas in the small bowel that causes the problem (a question later in this section describes the experiment that was performed).

How does this gas get into the small bowel?

The small bowel is normally sterile, meaning that there are no bacteria, and the gas in it comes from swallowed air.

What happens if bacteria do get into the small bowel?

The small bowel leads into the large bowel (see Figure 1.1), but the two are separated by a valve called the ileocaecal valve. This valve acts to stop the contents of the large bowel passing back into the small bowel. This is important because the large bowel is full of bacteria, whereas the small bowel is normally sterile. The valve is not fully effective, but bacteria entering the small bowel are normally rapidly flushed back into the large bowel.

However, bacteria from the large bowel sometimes manage to stay in the small bowel. This is called small bowel bacterial overgrowth. The infection does not usually cause fever or pain as the bacteria do not actually invade the wall of the bowel. They do, however, interfere with digestion. In severe cases, this results in diarrhoea and weight loss. It has been suggested that, in milder cases, the main symptom might be bloating due to the gas produced by these bacteria. So it is possible that small bowel bacterial overgrowth may be an important part of what causes irritable bowel syndrome.

What happens to air in the small bowel?

This air is normally rapidly propelled down the small bowel. Gas is propelled much more rapidly than food and can pass through the entire length of the intestine in less than half an hour. But not all the air we swallow is passed out. The carbon dioxide is absorbed into the bloodstream and removed from the body via the lungs. Most of the oxygen is also absorbed, but some of it is used by the bacteria in the large bowel. The nitrogen, which forms 78% of the air, passes into the large bowel and then out of the body as flatus.

I’ve got irritable bowel syndrome. Does this mean that I’m producing more gas?

Probably not, but no one really knows yet. Some studies show no increase in gas production, whereas others show a small increase. One study published in the medical journal The Lancet in 1998 showed that more hydrogen was produced in people with irritable bowel syndrome compared with people without irritable bowel syndrome eating the same diet. But the difference was small – 332 ml compared with 162 ml over 24 hours for the two groups. In terms of volume, this is just a glass of water and cannot account for any bloating. Moreover, when total gas production, including methane, was measured, there was no significant difference between those with and those without irritable bowel syndrome.

But if people with irritable bowel syndrome don’t produce a lot more gas, why do they bloat?

The problem here is impaired transport of gas down the small intestine rather than too much gas being produced. Until recently, bloating and excess wind were thought to be a large bowel problem. But an elegant series of experiments on people with and without irritable bowel syndrome by a group of doctors in Barcelona changed our understanding of how gas is handled in the intestine, and the causes of bloating.

These experiments involved gas (a mixture of oxygen, nitrogen and carbon dioxide) being continuously blown into the small intestine through a small tube passed down through the mouth. Within half an hour, the gas started to come out through the anus, and it was then collected and measured. The amount of gas retained (‘trapped’) in the intestine was then calculated as the difference between the volume of gas going into the tube in the mouth and the volume of gas collected from the anus. The people with irritable bowel syndrome retained significantly more gas – over 400 ml after 2 hours. As a result, their abdominal girth increased by 7 mm, compared with just 1 mm in those without irritable bowel syndrome. Moreover, over half of those with irritable bowel syndrome experienced significant discomfort, whereas none of the other group did.

Further studies have shown that gas infused into the small bowel causes significantly more symptoms than a similar volume infused into the large bowel. Interestingly, there was the same degree of abdominal distension each time, but it was gas in the small bowel that proved uncomfortable.

It now seems clear that the uncomfortable bloating of irritable bowel syndrome results from problems in the movement of gas down the small intestine, causing gas to be retained in the small intestine. This is in contrast to the previous, and still widely held, belief that bloating is caused by too much gas in the large bowel. The small intestine is only 2–3 centimetres wide, compared with 5 centimetres for the large intestine, so relatively smaller volumes of gas held in localised parts of the small intestine may be responsible for the uncomfortable feeling of ‘trapped gas’ that is so common in irritable bowel syndrome.

Do we know why people with abdominal bloating have this problem with transport of intestinal gas?

There are a number of reflexes that help to control the intestine. Glucose (a type of sugar) entering the small bowel tends to speed up transport along the small intestine, whereas fat in the small intestine slows it down. This makes sense as fat is more difficult to digest than sugar so needs to stay in the intestine for longer. In one experiment, the researchers put gas along with fat into the small intestine. In all the subjects, some of the gas was retained because propulsion down the small intestine was slower. But significantly more gas was retained in individuals with irritable bowel syndrome, suggesting that they had a stronger response to fat.

Another reflex that normally helps to keep things moving is triggered from the rectum: if the rectum is distended, gas moves more quickly down the small intestine. This reflex can be strong enough to counteract the slowing effect of fat. In one study, researchers put air bags into the rectums of people with and without irritable bowel syndrome and inflated these so that they could just be felt. In people without irritable bowel syndrome, inflating the air bag caused the gas to move more quickly down the small intestine. It also prevented a fat infusion delaying gas transport. But in people with irritable bowel syndrome, rectal distension did not stop gas being retained. Unfortunately, we don’t yet know why irritable bowel syndrome impairs the reflex control of the intestine in this way.


Why do I feel bloated when I eat rich or fatty foods?

Fat is more difficult to digest than other foods. This may explain why there are normal reflexes that aim to slow the emptying of the stomach after a fatty meal, and to slow movement through the small intestine – this allows more time for digestion. It is possible that these reflexes are exaggerated in people with irritable bowel syndrome.

Which foods are most likely to make gas problems and bloating worse?

In a survey of 330 people with irritable bowel syndrome, the foods most commonly reported to produce abdominal distension or gas problems were pizza, cabbage, onion, peas/beans, raw vegetables and deep-fried food.

When we look at the previous questions, we can see why fatty foods such pizza and fried foods might produce bloating. The common experience that high-fibre foods also cause bloating suggests that fermentation of fibre in the large bowel does indeed have a role.

Why could fibre in the diet make bloating worse?

The traditional view is that fibre passes largely undigested from the small intestine into the large intestine. There it is fermented by bacteria to produce gases such as hydrogen, hydrogen sulphide and methane. So the more fibre ingested, the more gas will be produced. But recent research strongly suggests that uncomfortable bloating is a consequence of gas retention in the small intestine (see earlier in the section), implying that fermentation of fibre can’t fully explain the problem although it probably contributes.

Does eating fibre affect how quickly gas goes through the small intestine?

A high-fibre diet may slow the movement of gas, leading to gas retention and bloating.

The effect of a high-fibre diet on gas transit has only recently been examined. In a study from the USA involving ten people without irritable bowel syndrome, gas was infused into the small intestine via a tube passed through the mouth. The gas was collected by another tube placed in the rectum. Each person was studied twice: once after a standard diet for 7 days, and once after the same diet had been supplemented by 30 g of a substance called psyllium each day. Psyllium (Ispaghula husk) is a form of fibre frequently recommended for irritable bowel syndrome, usually in sachets containing 3.5 g, and the dose prescribed in the study was larger than the 10 g or so a day usually recommended for irritable bowel syndrome.

Once the gas had begun to be infused in the experiment, there was a delay before gas appeared at the rectum. This delay showed the time taken for the gas to pass all the way down the intestine. After a normal diet, gas reached the rectum in an average of 19 minutes, but after the high-fibre diet it took 38 minutes. Moreover, after the high-fibre diet, almost 400 ml of the gas was trapped in the intestine, whereas after the standard diet there was no gas retention.

It is not clear why fibre in the form of psyllium slows gas transit through the intestine. It is possible that the increased bulk of the fibre forms a mechanical obstruction to the flow of gas. Or the fibre may hold onto the gas in the same way in which it retains water in the stool.

Another intriguing possibility involves the gas methane. About 70% of us have bacteria in our bowel that can ferment fibre to methane. Some of this methane is absorbed into the bloodstream and widely distributed throughout the body, finally being excreted by the lungs. A surprising finding is that methane seems to slow the intestine down, an effect studied so far only in dogs. Intriguingly, in humans, excess methane measured in the breath has been associated with constipation.

If I eat more fibre, will it make my bloating better?

Bloating is most frequently associated with the constipation-predominant form of irritable bowel syndrome. Some people find taking extra fibre a great help in regulating their bowel and relieving their constipation. And often, but not always, relief of the constipation helps with the bloating. It may be worth trying to increase the fibre content of your diet for a few weeks. Bear in mind that extra fibre may initially make the bloating worse, before relief of the constipation improves it.

Could anything else cause the bloating?

Another explanation for bloating has been termed ‘abdominal proptosis’. In this, the abdominal muscles fail to tighten up in response to a full intestine after a meal. This can combine with an excess curvature of the lumbar spine (the lower back). The abdomen therefore gets pushed out, making it look distended. Little evidence has been published to support this explanation, but there is no reason why it should not to be a contributory cause for bloating in some people. It’s possible that strengthening your abdominal muscles with exercise such as sit-ups may help.


I know that some people with cancer or cirrhosis of the liver can get very bloated because their abdomen fills with fluid. How can I know if my bloating is caused by fluid?

There is normally very little if any fluid between the internal organs and the abdominal wall. When fluid does accumulate here, it is called free fluid or ascites. This fluid is shifted by gravity, and the distension caused by ascites can be seen to affect the lower part of the abdomen when the individual is standing, and the sides when he or she is lying down. Ascites comes and goes very slowly over days and weeks. By contrast, the bloating of irritable bowel syndrome comes on rapidly after meals or develops through the day. It also resolves after a night’s sleep. So if you are not bloated when you wake up, you can be sure that fluid is not accumulating in your abdomen.

Can constipation make me bloated?

Alarge bowel heavily loaded with stool can certainly cause abdominal distension. Such distension will not resolve with a night’s sleep. The diagnosis of constipation is usually obvious to everyone, but occasionally an abdominal X-ray is necessary to assess it.

What can I do to stop passing all this wind?

Passing wind is normal and unavoidable! Healthy people pass flatus an average of 14 times a day, with a volume of 25–100 ml (up to half a glassful) each time. You should not, in fact, try to hold it in as there is some evidence that voluntarily holding onto the wind slows down gas transit in the intestine. And this may increase bloating. It’s healthy just to let the wind pass.

In that case, can I do anything to reduce the volume of wind that I pass?

Eat slowly and chew well so as to swallow less air. Eat less fibre, especially less of the highly fermentable foods such as beans, brussel sprouts, cabbage, onions, fruit juice in some people, and cereals. If you suspect that a particular food is causing your flatulence or bloating, you can test your suspicion by avoiding it for a few days to a week. If avoiding a food works, it’s worth confirming the link by eating the food again. Many people make associations between certain foods and symptoms that are not borne out on retesting, and it would be a shame to limit your diet for nothing. The popular laxative lactulose is also a common cause of wind.

Is there anything I can do about the smell I feel I’m making?

Medicinal (‘activated’) charcoal taken with meals will absorb aromatic gases such as hydrogen sulphide, causing the smell of the flatus to diminish or even go away all together. The charcoal does not work straight away, as it has to pass through the large bowel to have an effect. You will see it turn your stool dark. Some people take charcoal on a regular basis. Others take it for a few days before a social event or if they are about to have a meal they feel will cause them trouble. Charcoal is available without prescription from chemists and health food shops as tablets or capsules. The indigestion remedy bismuth subsalicylate (Pepto-Bismol) can also reduce the smell by absorbing any hydrogen sulphide. It’s available from the chemist without prescription. Some people find that taking peppermint oil capsules helps to make the smell of their flatulence less unpleasant. Charcoal-lined cushions, underwear made from carbon fibre, and charcoal panty liners to absorb the gases are available but are expensive.

What is the best way of masking the smell?

Striking a match is the simplest method. Some of the gas will burn (not visibly) and the whiff of smoke will effectively mask any remaining smell; it’s easy to keep a small box of matches with you. Perfume or deodorant sprays are a more expensive alternative to mask the smell.

What is simethicone?

Simethicone is a non-prescription drug that acts on the surface of bubbles by reducing the surface tension, causing the bubbles to break up. It causes gas bubbles to break and join together, forming larger bubbles. Why this should improve symptoms is unclear, but it is widely promoted for ‘gas’ problems in irritable bowel syndrome. It is available without prescription but tends to be more expensive than charcoal. In my experience, charcoal works better but simethicone is more palatable.

Is there any truth in the traditional advice that taking a walk after meals helps the food to digest? Interestingly, there is evidence that gas moves more quickly down the small intestine if you are standing up rather than lying down. This comes from a small study on volunteers without irritable bowel syndrome, but it does suggest that bloating may be diminished by keeping upright after meals. Whether walking itself has an effect is not known, but do try it as it might help you.

Are there any medicines that can help with bloating?

There are no medications specifically recommended for bloating. Many of the medicines that help with pain, constipation or diarrhoea do not help the bloating, but there are a few drugs you can try.

Domperidone (Motilium) is often tried to help bloating. It is actually marketed as an antiemetic (a medicine to stop you being sick) and is useful in nausea from any cause. It works by increasing peristalsis (the co-ordinated contractions of the gut that propel the contents forwards), mainly in the stomach and small intestine. Domperidone does not cross into the brain and is very safe. It therefore sounds ideal for disorders caused by slow movement through the stomach and small intestine. But studies of domperidone in irritable bowel syndrome have produced disappointing results, with no great benefit compared with a placebo (dummy) treatment.

In practice, I feel that about a third of patients who use it get significant benefit. It is available without prescription in the UK as Motilium but is fairly expensive to buy over the counter. If fullness after meals is a problem for you, it may be worth asking your doctor for a prescription.

I sometimes feel like sticking a needle into my belly to let all the air out. I know that’s silly, but what would actually happen?

It’s certainly tempting to burst the balloon, but that’s not what would happen! Even if the needle did penetrate a part of the bowel containing gas, it is not under such high pressure that it would vent to the outside. Moreover, the contraction of the gut muscles effectively separates the bowel into different compartments, so that even if you drained one part of gas, the rest would remain. Doctors often insert needles through the abdominal wall to drain fluid from within the abdominal cavity. Occasionally, we get it wrong and there is no fluid actually there. In such circumstances, the needle may penetrate the bowel wall – then we may be able to aspirate a little air, but otherwise nothing very much happens.

What can I do to prevent or reduce the bloating I feel?

Although our understanding of bloating has improved, our treatments still have a long way to go. All we can do is infer from our current knowledge that some changes in diet and lifestyle might help. I usually suggest the following:

  1. Eat slowly and carefully, and chew well to avoid swallowing air.
  2. Eat low-fat, high-carbohydrate foods. We saw in an earlier question that fat in the small bowel slows the transit of gas, whereas sugar speeds it up.
  3. Eat less fibre. Fibre slows the movement of gas and is fermented to produce yet more gas.
  4. If you become constipated, use ‘stimulant’ laxatives like senna rather than those called ‘osmotic’ laxatives, such as lactulose.
  5. Exercise to tighten your abdominal muscles and lose weight.
  6. Try medicinal (‘activated’) charcoal.
  7. Try domperidone.
  8. Buy loose-fitting clothes to make you feel more comfortable.
  9. Try a combination of things as different things suit different people.



  • irritable bowel syndrome really does cause bloating.
  • The bloating gets worse during the day and settles at night.
  • It is mostly due to air retained in the small bowel.
  • Air in the small bowel comes mostly from air swallowed with food.
  • Fibre in the diet usually makes bloating worse.
  • The bloating is occasionally due to constipation.
  • A fatty meal may make the bloating worse.
  • Foods reported to produce most gas problems include pizza, cabbage, onion, peas/beans, raw vegetables and deep-fried food.
  • Medicinal (activated) charcoal is probably the best treatment to reduce the smell of flatulence. Peppermint oil can improve the aroma.
  • Bloating remains one of the most difficult symptoms to treat, but our increasing understanding of its underlying mechanisms will hopefully lead to better treatments in the future.