IBS with diarrhoea

There was a time when most IBS would be treated using anti-depressants as it was thought to be a psychosomatic condition. Nowadays, as technology has marched forwards, the medical world is gradually coming to realise that in many cases, IBS has an identifiable cause. This is especially true of diarrhoea dominant IBS.

What causes it?

As food materials work their way through our digestive tract they first pass through the stomach, then the 6.5m long, small intestine, finally ending up in the 1.5m long, large intestine (colon). In diarrhoea dominant IBS there is an invasion of the small intestine by bacteria that are normally resident in the colon. This is called “small intestinal bacterial overgrowth” or SIBO for short.

Colonic bacteria in the small intestine generally cause gas, bloating and distension. However, the types of bacteria that invade the small intestine determine whether you get diarrhoea or constipation. Methane producing bacteria lead to constipation and hydrogen producing bacteria produce diarrhoea, or a mix of diarrhoea and constipation.

Is it that simple?

Not quite, there are a few complicating wrinkles.

  • Hydrogen produced by hydrogen producing bacteria (associated with diarrhoea) can feed methane producing bacteria potentially leading to constipation becoming a more significant factor.
  • The walls of the small intestine are responsible for moving food and waste through the system. It is now thought that there are problems with this motility when someone has IBS. Difficulty moving waste through allows bacteria to congregate in certain sections.

How do we confirm it?

Testing is relatively expensive, but nevertheless very important if the correct diagnosis is to be made. Breath tests are cheap, and the presence of methane can accurately predict who has constipation dominant IBS. With diarrhoea dominant IBS other tests are more reliable. There are urine tests available that look for the chemicals produced by the bacteria that can colonise the small intestine during IBS.

How can IBS be treated?


The antibiotic Rifaximin, normally prescribed by a doctor will lead to eradication of the bacteria in more than 70% of patients. However only around 40% of these will remain without symptoms over the long term. In other words, around 30% will be cured permanently. Rifaximin works solely on bacteria in the small intestine as it dissolves in the bile released from the bile ducts at the top end of the small intestine. By the time food waste reaches the colon the Rifaximin is crystalized, as there is not enough bile to keep it dissolved.

A recent study showed that guar gum in combination with Rifaximin was more effective than Rifaximin alone. Initial eradication of bacteria was achieved in 87% of cases with guar gum added and only 62% without the guar gum. There are two possibilities:

  • Guar gum creates an environment in the small intestine that is more conducive to good bacteria getting established there.
  • Guar gum feeds the bad bacteria which makes them more susceptable to the antibiotic drug treatment. Why? A well fed bacterium will reproduce more and Rifaximin attacks fast dividing cells.

Low FODMAP diet

FODMAPs are foods that feed the bacteria that have invaded the small intestine. There is plenty of information on the internet about FODMAPs. Relief from IBS symptoms can often be achieved by reducing intake of these foods. Depending on the particular species of bacteria you have, reducing some of these foods will be more effective than reducing others. For instance free fructose can cause some people problems. It is found in many fruits and so reducing fruit intake could help these people. 

In fact a highly restrictive diet, such as a very low carbohydrate diet is likely to restrict the bacteria as well as bacteria virtually all feed of carbohydrates. Doing this will lead to a period when you will feel better. Unfortunately it is unlikely it will completely eliminate all the bacteria, and a relapse is likely when normal eating patterns are resumed. Staying on a highly restrictive diet is not the answer as it will damage the long term health of anyone who maintains the diet.

There is also an issue with eating no FODMAPs in that they include some of the best nutrients for good bacteria in the colon, and so by restricting them long term you may also damage your health.

The future

Ultimately I believe an effective cure for IBS will be found based on a fuller understanding of the microbiology of the gut, but don't hold your breath as this is a very complex area.

The views expressed in this article are those of the author. All articles published on Nutritionist Resource are reviewed by our editorial team.

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Milton Keynes, Buckinghamshire, MK16
Written by Robin Dowswell, BSc MFNTP
Milton Keynes, Buckinghamshire, MK16

Robin Dowswell is a Nutritional Therapist working just outside Milton Keynes. He specialises in sports nutrition, and has a keen interest in using diet and lifestyle change to enjoy improved health. Check out his nutrition A-Z to find out about over 50 different foods and supplements as well as information on diets for a range of conditions.

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