A review of starch digestion in the lactating dairy cow and proposals for a mechanistic model: 2. Postruminal starch digestion and small intestinal glucose absorption

The objective of this paper is to review the literature concerning postrumi nal starch digestion and glucose absorption in the lactating dairy cow and to propose the framework for a mechanistic model representing these process es. Postruminal starch digestion is of particular importance where high lev els of rumen escape starch flow from the rumen. However, the digestion of s tarch and absorption of the resulting glucose within the small intestine ma y be limited by pancreatic secretion of alpha-amylase and the distribution of SGLT1 glucose transporters respectively. During the investigation, use i s made of data gathered from both in vivo and in vitro studies concerning m ainly lactating dairy cows. The relative importance of ruminal and postrumi nal starch digestion is discussed along with the significance of dietary st arch source and processing method as factors affecting postruminal starch d igestion. Postruminal starch digestion and intestinal glucose uptake become s increasingly important at high starch intakes. Other factors influencing the nature of starch digestion are also presented in order to allow the int erpretation of experimental data and hence the development of a conceptual model of starch digestion. The review subsequently examines postruminal sta rch digestion as it is represented in extant models of ruminant digestion a nd discusses the essential elements of a digestion model that would have th e required capability to accurately account for the fare of rumen escape st arch in a range of practical feeding situations. Whilst the digestion of st arch within the rumen is well represented in several working models present ed in the literature, postruminal starch digestion and glucose uptake has b een largely ignored. Finally, a proposed framework is presented as a scheme upon which a future model of starch digestion in the dairy cow may be buil t.