Feeding a hungry microbiome: large bowel fermentation and human health

Dietary fibre largely comprises indigestible plant carbohydrates and was thought to act primarily through its faecal bulking action — as “roughage”. Emerging evidence now shows that many of the actions of fibre are mediated through short chain fatty acids (SCFAs), principally acetate, propionate and butyrate, produced by the fermentation of its constituents by beneficial large-bowel bacteria (Physiol Rev 2001; 81: 1031-1064). Evidence is growing that SCFAs are important for general health as well as for normal gut function. Advances in genomics are enabling detailed identification and characterisation of the hundreds of different microbes that inhabit the large bowel, including fibre fermenters and SCFA generators (Nature 2012; 486: 207-214). Unfortunately, these dazzling technical advances have tended to divert attention away from the substrates, on which the microorganisms depend.

Questions about supply of colonic substrate are increasingly significant for public health. Australians have taken the advice to eat more fibre very seriously and consume fibre at high levels, largely as cereal fibres. This has not led to the anticipated fall in colorectal cancer (CRC) rates, which remain high (Int J Mol Sci 2013; 14: 13525-13541). In comparison, populations at low risk of CRC who consume wholegrains as staples do not have the high cereal fibre intake of Australians. However, the culinary practice of eating foods cold after cooking favours the generation of starch that is resistant to digestion by the small intestine. Resistant starch is fermented extensively in the large bowel, producing SCFAs, which promote a normal colonocyte phenotype — possibly lowering CRC risk (http://tv.csiro.au/?v=xbz189as9jwd). Levels of resistant starch in most Australian processed consumer foods are low, suggesting a general deficiency of fermentable substrate — and thus a collectively “hungry microbiome”. We suggest that filling this gap by increasing the availability of new fermentable fibre-rich foods to feed the colonic microbiota and raise large-bowel SCFA levels has considerable potential to improve the health of the population.