8.5. Faecal energy and non-starch polysaccharide

Whilst there is evidence that NSP may contribute to the metabolizable energy of an individual, there is also evidence that increasing the amount of NSP in the diet may also lead to an increased excretion of fat, nitrogen and energy; the result being a decrease in the apparent digestibility of fat and protein and a reduction in available energy (SOUTHGATE and DURNIN, 1970; WISKER, MALTZ and FELDHEIM, 1988). In these studies, the total increase in energy losses associated with the increased intake of NSP exceeded the gross energy contained in the NSP itself. On the basis of these observations, FAO/WHO/UNU (1985) have proposed that no extra correction needs to be made to the metabolizable energy derived using Atwater factors, when the diet contains small amounts of dietary fibre. With increasing amounts of dietary fibre the calculated metabolizable energy should be reduced by about 5%. It is proposed that metabolizable energy may need to be further reduced when the consumption of dietary fibre is high: of the order likely to be ingested in many developing countries. This recommendation should not be applied uncritically, without an improved understanding of the potential contribution made by colonic fermentation to metabolizable energy and the nature and origin of the energy within the stool.