The Trouble with Fibre... - British Society of Lifestyle Medicine

A patient told me today that the reason she doesn’t eat fruit is because it “doesn’t suit her gut”. On closer questioning it turns out that fruit caused her to move her bowels more frequently – once or twice a day – which was much less convenient for her than the once or twice a week bowel motions she was used to. She described having to move her bowels at work as “incredibly embarrassing” and was really reluctant to improve her diet.

This is not the first time I have heard this. Other patients have complained that pulses or too many vegetables do the same. In actual fact moving your bowels several times a day, which is normal with a high fibre diet, seems to be quite unacceptable to a lot of people. Is this because our low fibre western diet has been around for a few generations and people have got used to the sluggish, hard bowel movements associated with it? Or is it just that going to the toilet is another inconvenience in our busy lives? Either way, I have noticed for a long time now that it’s rare for patients to report moving their bowels more than once a day unless they have irritable bowel syndrome or another bowel disorder.

The British dietetic society reports that we should eat 30g of dietary fibre per day but in actual fact most adults only consume 18g which is 60% of the recommended intake. This is a great frustration for me as a GP and lifestyle medic, but why should we be so bothered about fibre intake?

There are several types of fibre, soluble fibre found in fruit and oats; insoluble fibre found in bran and nuts, and resistant starch, found in lots of root vegetables, pulses and some fruits. Resistant starch escapes digestion in the small bowel and instead is fermented by gut microbes in the large bowel.

Recently interest has grown into the health benefits of fibre – in particular, prebiotics – which is a term for types of non-soluble fibre present in lots of fibrous fruit and vegetables. This type of fibre feed the beneficial micro-organisms in our gut microbiome. the micro-organisms break down the fibre by fermentation and the by-products of this process are short chain fatty acids (SCFA’s). These substances play a number of important roles in our gut, such as improving gut motility, production of anti-inflammatory cytokines, immune regulation, glucose and lipid metabolism, appetite suppression and maintaining the structure and integrity of the gut lining.

Soluble fibre attracts water to form a gel like substance in the stomach and small bowel. It clings on to sugars and slows their metabolism and it binds to some fats to carry them out of the body aiding cholesterol lowering.

In simple terms, as well as keeping your stools soft and frequent, fibre plays a really important role in controlling blood sugar, cholesterol, and lowering inflammation alongside helping our immune system and maintaining a really healthy gut.

Lots of diseases have been linked to low fibre intake e.g., constipation, inflammatory bowel disease, bowel cancer, diabetes and ischaemic heart disease. Now with advances in microbiome science we can also see a link with low dietary fibre and an unhealthy gut microbiome which has been associated with many more disorders including cancers, mental health disorders, immune disorders as well as allergies and dementia.

If we eat enough fibre we are likely to move our bowels at least 2-3 times a day with soft large stools, in fact the size of a fibrous stool is much larger and heavier than an equivalent western diet stool – which may come as a shock to many! If we change from a western diet to a whole food plant based diet rich in fibre there will be a dramatic change in our bowel habit and it may take a while for our bowels and the micro-organisms in our guts to adapt. This is why sudden diet changes are often unsuccessful as they can cause bloating and abdominal discomfort with urgency and increasing bowel frequency. It’s much better to gradually increase the fibre in your diet over a period of weeks to months to allow your micro-organisms to get used to the change.

Ultimately, I’m worried that our convenience lifestyle has gone too far and we now find that one of our most basic functions – moving our bowels – has become inconvenient. In our attempt to make this bodily function fit into our busy and social lives we have neglected the health of our gut and not realised the considerable long-term health risks of an infrequent bowel habit. Our obsession with processed and convenience foods has driven this change and we no longer recognise what a healthy bowel habit is. With such a taboo subject it is unlikely we can start a public campaign but maybe we, as health professionals, can be more proactive at starting conversations and normalising a healthy high fibre bowel habit.

And what about my patient? Well, she has decided to try increasing her fibre in her diet again – this time a lot more gradually…

The Trouble with Fibre…

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