Irritable Bowel Syndrome

IBS is very misunderstood, and most explanations for the condition are very simplistic - simply putting the symptoms down to stress or a neurotic personality. While stress will certainly make digestive symptoms worse, many if not most IBS patients are not under particularly great stress, and most are not neurotic.

Recent research shows that the HPA axis - the body's energy regulation and stress tolerance system - has a large infleunce over colon motility and absorption, and IBS patients show altered HPA axis function, which is likely to be the cause of the symptoms.

CRH, the hormone which initiates HPA axis activation in the hypothalamus, modulates the motility of the colon, while cortisol, the end-product of HPA axis activation, causes increased absorption of water and sodium by the colon and rectum. Too much CRH or too little cortisol will therefore result in diarrhea, while too little CRH or too much cortisol will result in constipation.

Studies show that patients suffering from diarrhea-predominant IBS tend to have a greater circadian rhythm of cortisol - higher in the morning and lower at night than controls - as well as having higher levels of cortisol after meals and during stress. This points to a greater HPA axis activation, or a greater stress tolerance, rather than being simply due to too much stress.

There is a large overlap between CFS and IBS, with many CFS patients suffering from IBS symptoms, and many IBS patients suffering from symptoms such as fatigue, depression and anxiety. In both cases it is likely that the HPA axis is the cause of the symptoms, and that this HPA axis imbalance is ultimately caused by similar factors.

Many IBS patients follow a pattern of alternating constipation and diarrhea, which suggests altering HPA axis activity. In many cases this pattern follows the patient's lifestyle - for example, diarrhea during the week or during periods of high activity, and constipation at weekends or when relaxing. Rather than simply being due to stress, it is more likely that IBS patients simply have a greater control over HPA axis activity. Many IBS (as well as CFS) patients have what could be described as a over-achiever/burnout-prone personality - they are able to achieve high levels of activity or workload (probably due to being able to sustain higher-than-normal HPA axis activation), but they suffer from "burnout" after these periods of high workload or when they lose motivation.

Gut flora

The colon is home to a variety of bacteria which are beneficial to digestion. These "good bacteria" break down undigested food and fibre without causing unpleasant smelling gas or irritating the colon. The action of these bacteria is also important in generating vitamins as a by-product of the breakdown of indigestible fibre.

Although the immune system tries to keep a balance of good bacteria in the gut, sometimes other, more harmful types of bacteria can take up residence, giving rise to symptoms such as diarrhea, bloating and flatulence. Some factors which are known to upset the balance of bacteria in the gut include the use of broad-spectrum antibiotics, excessive alcohol consumption, stress and poor diet.

An upset balance in gut bacteria is sometimes postulated as a cause of IBS-D. However it is more likely that any bacterial imbalance is caused by the underlying HPA axis abnormality. This results in an excessively short or long transit time of food through the gut and colon, either causing the bacteria to be flushed out of the colon too quickly or else letting them build up to excessively high levels. In addition, if undigested food is pushed too quickly into the colon, this can promote the growth of undesirable bacteria which thrive on undigested protein and fat. These bacteria tend to cause problems such as diarrhea and foul smelling gas when they become too prolific.

Interaction of Factors

It is not clear how the mental factors, such as stress, burnout and mental attitude, are linked to physical factors such as lactose intolerance in causing IBS symptoms. It could be that these are two separate issues, and that people with lactose intolerance have a different type of IBS to those who have abnormal HPA axis responses. More research clearly needs to be done in order to determine exactly what is happening in the different groups of IBS patients, what is causing their symptoms, and whether it is one condition or a number of different conditions.

Most IBS patients find that their symptoms are exacerbated both by lifestyle (or psychological) factors, as well as by diet. It is likely that certain foods cause problems due to them either being more difficult to digest, or because they are more likely to cause the "bad" bacteria to proliferate in the gut when they are not fully digested. It may be the combination of these foods with an abnormal HPA axis rhythm which then causes the symptoms of IBS.

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