Fermentation in the human large intestine and the available substrates

Abstract

Fermentation, the process whereby energy yielding substrates are broken down by anerobic bacteria, is the essence of normal large bowel function in man. Its control is dependent mainly on the amount and type of substrate available to the flora since this will determine the magnitude and rate of many metabolic events in the colon. Many substrates, both dietary and endogenous, are available for fermentation but in the present state of knowledge the principal ones are probably starch, NSP (dietary fiber), and protein. All dietary NSP reaches the colon but the amount of starch and protein that does is dependent on the processing which food has undergone before ingestion. In western countries the amounts of fermentable material are on average 12 g NSP, 12 g protein, and an unknown quantity of starch and other carbohydrates, with a wide range for total substrate available to the flora. In countries where starchy foods, such as rice, maize, plantain, etc, are the main contributors to the diet then the amount fermented may be greater. The end products of fermentation, chiefly short- and branched-chain fatty acids, various gases, phenols, ammonia, and amines, are of physiological importance and will vary according to the substrate available. It is important in understanding large bowel function that these fermentable dietary components are measured separately and exactly. An accurate and detailed method for NSP is available and a new classification of starch for nutritional purposes is proposed which takes account of the site of its breakdown in man. Problems in quantitating protein and endogenous substrates available for fermentation remain to be solved. The large intestine is thus an important and hitherto neglected digestive organ in man. An understanding of fermentation is likely to be important in unravelling the problem of large bowel and possibly other diseases.