Variations in starch digestibility in non-ruminants

Abstract

Starch is the major energy-yielding ingredient in diets for both pigs and poultry, and its effective digestion will have a major impact on their energy balance; a common presumption is that it is completely digested but, whilst this may well be the case over the entire digestive tract (through excreta and faecal analyses), this is rarely found within the small intestine. Starch disappearance from the large intestine is through microbial fermentation whose products, volatile fatty acids, are not used with the same efficiency of metabolic utilisation in energy-yielding pathways as is glucose (the product of amylolysis in the small intestine). Other consequences of incomplete starch digestion in the small intestine may include excessive large intestinal fermentation leading to diarrhoea and dehydration. Variations in small versus large intestinal digestibility for starch have not received attention but it should be in future assessments. Starch from peas is digested less well in the small intestine than from cereals. Separation of starch into different categories (rapidly digested, slowly digested and resistant to digestion) could be a useful development. There is some debate as to whether there is amylase adequacy in the newly hatched chick/post-weaned piglet. However although the activity of α-amylase may well increase beyond these ages, it is still generally believed that poor digestibility values of starch in young broilers cannot be attributed to inadequate levels of α-amylase and is more likely to be attributable to extrinsic and/or intrinsic factors. With respect to piglets, there appears to be a link between feed intake and enzyme activity; thus reduced intake (common in the immediate post-weaning period) may be responsible for limited amylase secretion rather than a capacity for amylase synthesis. There have been numerous studies evaluating the digestibility of starch from wheat in poultry; isolated wheat starch was readily digested in vitro by chick pancreatic α-amylase, even that from wheat with relatively low apparent metabolizable energy (AME) values. Variations in starch digestibility have, nevertheless, been recorded. Thus, it is not starch per se that is poorly utilised in some samples but other factors within wheat may be reducing starch digestibility. The starch/protein interface in the endosperm of wheat (responsible for the ‘hard’ and ‘soft’ endosperm texture) might be responsible for problems with starch utilisation, but no firm evidence exists for this. A key recent development has been the emergence of near-isogenic lines which are very similar except for key characteristics. Thus, lines near-isogenic except for hardness have been used to establish that hard wheats are less well digested in poultry than soft ones. Similarly, wheats containing the IBIR rye translocation are less well digested. Such developments have been crucial as the independent effects of hardness and IBIR cannot be established conclusively if there are other unknown factors present. Thus, random variety trials are of no value in investigations of what influences starch digestion. It is possible that the negative effects of the IBIR translocation may be offset by the positive effects of soft endosperm although it is crucial to point out that endosperm texture is a continuum between very hard and very soft, not simply hard and soft. There have been many attempts to assess starch digestibility in vitro as a means of screening samples. Certainly there has been success in terms of linking in vivo digestibility to rate of in vitro digestibility. However it has been suggested that slowly digested starch will lead to better performance than rapidly digested starch.