Dietary fibre and the gut microbiota

<jats:p><jats:bold>Summary </jats:bold>The complex human colonic microbiota plays a key role in gut health, and both the composition and metabolism of the gut microbiota are strongly diet related. Controlled dietary intake studies in obese individuals demonstrated that consumption of low‐carbohydrate diets results in low faecal butyrate concentrations together with a low abundance of the major butyrate‐producing bacteria,<jats:italic>Roseburia</jats:italic>spp. and<jats:italic>Eubacterium rectale</jats:italic>group.</jats:p><jats:p>Resistant dietary carbohydrates, including pre‐biotics, escape digestion in the upper gastrointestinal tract and are fermented to short‐chain fatty acids (SCFAs) by bacteria in the colon. The main SCFAs formed are acetate, propionate and butyrate. Model colonic fermentor systems employed to determine which bacterial groups utilise specific substrates indicated that the bacterial composition and production of SCFAs is substrate dependent. The bacterial colonisers of specific substrates were identified using 16S rRNA sequence analysis. Sequences recovered from bran were most closely related to<jats:italic>Roseburia</jats:italic>spp.,<jats:italic>E. rectale</jats:italic>and<jats:italic>Clostridium hathewayi</jats:italic>, while on resistant starch, sequences were most closely related to<jats:italic>Ruminococcus bromii</jats:italic>and<jats:italic>Bifidobacterium adolescentis.</jats:italic></jats:p><jats:p>Differing intakes of dietary carbohydrate also impact on other factors in the colon such as transit and pH. In continuous fermentor systems, a slightly acidic pH (5.5) representative of the proximal colon gave fourfold higher butyrate concentrations than at the higher pH of 6.5, which is more representative of the distal colon.<jats:italic>Bacteroidetes</jats:italic>dominated the fermentor at pH 6.5, while the major butyrate‐producing bacterial groups competed more effectively for carbohydrate substrates when the pH was mildly acidic (5.5). In<jats:italic>in vitro</jats:italic>studies in which pH values were even lower, pH 5.2, lactate and acetate accumulated. In contrast, lactate was not detected at pH 6.4 although stable isotope studies revealed that lactate was actively produced. Lactate accumulation at low pH appears to be as a result of the loss of lactate‐utilising bacteria including<jats:italic>Eubacterium hallii</jats:italic>.</jats:p><jats:p>It is clear that the composition of the colonic microbiota and the balance of its metabolic products is strongly influenced by diet and, in particular, by the intake of resistant carbohydrates and dietary fibre.</jats:p>