Glucose uptake and regulation by intestinal tissues: Implications and whole-body energetics

Abstract

The purpose of this review is to examine mechanisms of glucose absorption by the ruminant small intestine and to relate these processes to available information on the theoretical advantages of shifting site of starch digestion in ruminants. The primary mechanism for glucose absorption across the brushborder membrane of enterocytes is the Na+-dependent glucose transporter, SGLT1. This transport protein is a high-affinity, concentrative, glucose transporter that couples glucose transport to an inwardly directed Na+ gradient. This Na+ gradient is maintained by the activity of Na+/K+-ATPase in the basolateral membrane. The activity of SGLT1 is highest in the developing ruminant and declines after weaning. Glucose transporter activity can be increased by intraluminal glucose; however, glucose absorption or metabolism is not necessary to induce transporter activity. Changes in SGLT1 activity are proportional to SGLT1 protein content. In contrast, SGLT1 mRNA levels are poorly correlated with SGLT1 activity, suggesting post-transcriptional regulation. Forage-fed ruminants have little glucose available in the lumen of the small intestine, whereas considerable quantities can be available in animals fed concentrates. Because of this, glucose absorptive capacity in the small intestine is thought to vary greatly and has made the ruminant, particularly the sheep, a useful model to study the regulation of glucose transport. Theoretical calculations and experimental observations indicate that fermentation losses and differences in partial efficiencies of absorbed substrate use result in ruminally fermented starch or glucose being only 70 to 75% as energetically efficient as starch or glucose that is digested and absorbed in the small intestine. Accordingly, dietary approaches resulting in starch escaping ruminal fermentation should be energetically advantageous if no limitations occur in small intestinal starch assimilation. However, data demonstrating complete digestion of starch in the small intestine and improvements in animal energetic efficiency are limited.