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 postruminal starch digestion and glucose absorption in the lactating dairy cow and to propose the framework for a mechanistic model representing these processes. Postruminal starch digestion is of particular importance where high levels of rumen escape starch flow from the rumen. However, the digestion of starch and absorption of the resulting glucose within the small intestine may be limited by pancreatic secretion of alpha-amylase and the distribution of SGLT1 glucose transporters respectively. During the investigation, use is made of data gathered from both in vivo and in vitro studies concerning mainly lactating dairy cows. The relative importance of ruminal and postruminal starch digestion is discussed along with the significance of dietary starch source and processing method as factors affecting postruminal starch digestion. Postruminal starch digestion and intestinal glucose uptake becomes increasingly important at high starch intakes. Other factors influencing the nature of starch digestion are also presented in order to allow the interpretation of experimental data and hence the development of a conceptual model of starch digestion. The review subsequently examines postruminal starch digestion as it is represented in extant models of ruminant digestion and discusses the essential elements of a digestion model that would have the required capability to accurately account for the fare of rumen escape starch in a range of practical feeding situations. Whilst the digestion of starch within the rumen is well represented in several working models presented in the literature, postruminal starch digestion and glucose uptake has been 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 built.