The influence of diet on the gut microbiota.

Diet is a major factor driving the composition and metabolism of the colonic microbiota. The amount, type and balance of the main dietary macronutrients (carbohydrates, proteins and fats) have a great impact on the large intestinal microbiota. The human colon contains a dense population of bacterial cells that outnumber host cells 10-fold. Bacteroidetes, Firmicutes and Actinobacteria are the three major phyla that inhabit the human large intestine and these bacteria possess a fascinating array of enzymes that can degrade complex dietary substrates. Certain colonic bacteria are able to metabolise a remarkable variety of substrates whilst other species carry out more specialised activities, including primary degradation of plant cell walls. Microbial metabolism of dietary carbohydrates results mainly in the formation of short chain fatty acids and gases. The major bacterial fermentation products are acetate, propionate and butyrate; and the production of these tends to lower the colonic pH. These weak acids influence the microbial composition and directly affect host health, with butyrate the preferred energy source for the colonocytes. Certain bacterial species in the colon survive by cross-feeding, using either the breakdown products of complex carbohydrate degradation or fermentation products such as lactic acid for growth. Microbial protein metabolism results in additional fermentation products, some of which are potentially harmful to host health. The current 'omic era promises rapid progress towards understanding how diet can be used to modulate the composition and metabolism of the gut microbiota, allowing researchers to provide informed advice, that should improve long-term health status.