Some Aspects of Dietary Fibre Metabolism in the Human Gut

Abstract

Whilst only 4% of the UK diet is fibre, there are few parts of the process of digestion and absorption which fibre does not influence to some extent. Experimental evidence exists to show that appetite, satiety, gastric emptying, carbohydrate, sterol and mineral absorption may all be affected by fibre.

It is, however, in the human large intestine that fibre exerts its most dominant effects. All types of food fibre when adequately studied can be shown to increase faecal output in man, although wheat fibre remains the most effective, with fibre from fruit and vegetables less so. 50–90 % of ingested fibre disappears during passage through the gut. It is metabolised mainly to short-chain fatty acids, carbon dioxide and methane, all of which are absorbed. Through its metabolism fibre increases colonic bacterial mass, inhibits bile acid dehydroxylation, lowers ammonia levels and dilutes colonic contents. The rate of passage through the colon is also increased.

Fibre is thought to act in the colon by retaining water in its matrix, but since little fibre remains in the human stool, the manner in which it increases faecal weight is not clear. The texture of human faeces is largely cellular—which suggests that many of the large bowel effects of fibre are mediated through its promotion of microbial growth.

The development of ideas about dietary fibre has been delayed because of confusion as to its nature together with inadequate methodology for its measurement. Now, however, new analytical techniques have been reported¹, which, together with modifications of existing ones^2,3, enable a much clearer picture of it to be drawn.