|Activity, Energy Expenditure and Energy Requirements of Infants and Children (International Dietary Energy Consultative Group - IDECG, 1989, 412 pages)|
|The energy requirements of growth and catch-up growth|
|8. Extent to which colonic fermentation of carbohydrates contributes to energy requirements in childhood|
The extent of the production of SCFA and the contribution to metabolizable energy intake appears to depend on several factors.
Firstly, the amount of dietary fibre and the degree to which it is digested. There has been considerable interest in the digestibility of different forms of dietary fibre within the human large intestine (CUMMINGS, 1984). However, the extent to which microbial fermentation takes place and the extent to which the end-products of fibre fermentation are absorbed and contribute to metabolizable energy intake remain an area of controversy. Increases in microbial cell excretion were observed in association with an increased intake of vegetable fibre from cabbage whereas the increased consumption of the much less digestible wheat fibre resulted in only a small change in faecal microbial excretion (STEPHEN and CUMMINGS, 1980). Thus, the higher the fibre intake, particularly that derived from beans, vegetables and fruits, the greater the potential contribution from colonic fermentation.
Secondly, the amount of unabsorbed carbohydrate delivered to the large intestine. Clearly, this may be increased as a result of maldigestion and malabsorption associated with disease (e.g., cystic fibrosis or lactose intolerance). However, it is now clear that some foodstuffs contain relatively large amounts of starch in a form that is resistant to digestion - both in vitro and by human digestive enzymes. A number of studies have demonstrated that not only does the resistant starch largely escape digestion in the human small intestine, but that other types of starch may also pass into the large intestine to undergo fermentation (CUMMINGS and ENGLYST, 1987).
Thirdly, the quantity and nature of endogenous material delivered to the large intestine is not clear. Mucus degradation by colonic microflora has been well documented, and the presence of bacterial sub-populations that produce extracellular glycosidases with the specific role of degrading complex oligosaccharides of mucin in the gut lumen have been identified (HOSKINS and BOULDING, 1981). It has not been possible to directly quantify mucus production and epithelial cell losses, hence the extent of the contribution made by fermentation of mucopolysaccharides and glycoproteins to metabolizable energy remains unknown. However, in circumstances where mucus production is substantially elevated (e.g., in cystic fibrosis), the potential capacity for energy to be salvaged might be substantial (see later).
Finally, what is the overall magnitude of the gross energy intake and to what extent can the intake satisfy the energy needs of the individual? The relative contribution made by colonic fermentation may be relatively small when the gross energy intake is high. In contrast, for individuals on a marginal intake of energy, or in whom requirements are elevated, the relative contribution will be greater and may be critical.