Study finds dietary fibre is more likely to be cause of, rather than a cure for, constipation and other bowel symptoms

by Dr John Briffa

I get a sense that almost all individuals feel better for having regular, easy, complete bowel motions. Should someone be having problems in this area, the usual first-line approach is to up the intake of fibre. This can come in the form of fruits and vegetables, but many will see ‘healthy wholegrains’ such as wholemeal bread and high bran breakfast cereals as good and convenient options. However, a recent study suggests that if overcoming a sluggish bowel is the aim, one of the last things we should be doing is upping our fibre intake.

The study focused on 63 adult (average age 47) individuals who had persistent constipation for which no medical cause could be identified [1]. Stool (bowel motion) frequency was less than once every three days for at least three months. All participants were on a high-fibre diet and/or were taking fibre supplements.

Study participants were instructed to adopt a low-fibre diet, and specifically to eliminate fruit, vegetables, breakfast cereals, wholemeal bread and brown rice for two weeks. After this, participants were asked to continue eating as little fibre as possible if this helped their symptoms.

6 months after the start of the study, 41 patients had persisted with the ‘no-fibre’ diet, 16 were eating a reduced fibre diet, and 6 were on a high-fibre diet for a variety of reasons (including being vegetarian or religious reasons).

In the 41 patients on the no-fibre diet, average bowel frequency had increased from an average of once every 3.75 days to once every day.

In the 16 patients on the reduced-fibre diet, average bowel frequency had increased from an average of once every 4.19 days to once every 1.9 days.

In the 6 patients who remained on a high-fibre diet, bowel frequency was once a week initially, and it remained the same on the high-fibre diet (as expected).

Symptoms of bloating occurred in 0 and 31 per cent of the low- and reduced-fibre eaters respectively. Of those on the no-fibre diet, no one had to strain to pass a stool. Abdominal pain also improved in this group and any anal bleeding they had resolved completely.

The authors of this study start their discussion of these results with these words:

This study has confirmed that the previous strongly-held belief that the application of dietary fiber to help constipation is but a myth.

They then go on to attempt to explain their findings:

It is well known that increasing dietary fiber increases fecal bulk and volume. Therefore in patients where there is already difficulty in expelling large fecal boluses through the anal sphincter, it is illogical to actually expect that bigger or more feces will ameliorate this problem. More and bulkier fecal matter can only aggravate the difficulty by making the stools even bigger and bulkier. Several reviews and a meta-analysis had already shown that dietary fiber does not improve constipation in patients with irritable bowel diseases.

The authors also provide this handy analogy:

The role of dietary fiber in constipation is analogous to cars in traffic congestion. The only way to alleviate slow traffic would be to decrease the number of cars and to evacuate the remaining cars quickly. Should we add more cars, the congestion would only be worsened. Similarly, in patients with idiopathic constipation [constipation of no known cause] and a colon packed with feces, reduction in dietary fiber would reduce fecal bulk and volume and make evacuation of the smaller and thinner feces easier. Adding dietary fiber would only add to the bulk and volume and thus make evacuation even more difficult.

It’s difficult to argue with the logic of this, nor the results they achieved in their study subjects.

And these results more-or-less mirror my own in practise. What I find tends to work well with individuals suffering from constipation and other ‘irritable bowel’-like symptoms is the elimination of grain, particularly wheat, from the diet. In practice, I’ve found leaving vegetables and some fruit (rich in ‘soluble’ fibre) in the diet allows individuals to attain and maintain good digestive function. My hunch is it’s the much-lauded ‘insoluble’ fibre in wholegrains that does the most damage, here. A review on the role of diet in managing irritable bowel syndrome concluded pretty much the same thing [2].

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