Adaptation of intestinal epithelial hydrolysis and absorption of dietary carbohydrate and protein in mammals and birds.

The small intestine of mammals and birds exhibits fascinating variation across taxa, body size, and life history features such as locomotion and diet. In the intestine's brush border membrane (BBM), hydrolases are more abundant than transporters in both mammals and birds, but there are differences among the groups in abundance of certain hydrolases and possibly in transporters. For example, mammals express two α-glucosidases, sucrase-isomaltase (SI) and maltase glucoamylase (MGAM), whereas songbirds we studied have only SI, and the chicken expresses SI plus another α-glucosidase that functions similarly to MGAM but is not a true ortholog. For intestinal absorption of sugars and amino acids, small fliers rely on a paracellular pathway to a greater extent than do nonflying mammals, which rely more on transporters. Possibly having evolved in fliers as compensation for lower intestinal nominal surface area (NSA), the fliers' reliance on paracellular absorption is supported by their greater villous surface enlargement that leads to more (per cm2 NSA) tight junctions and greater clearance of passively absorbed compounds. To match digestive capacity to nutrient load, a positive relationship is often observed between dietary intake of macronutrients and intestinal activity of the enzymes and transporters of their respective constituents. In enterocytes, rapid, fine-tuned adjustment to high dietary carbohydrate and protein involves rapid, specific correlated increase in activity and abundance of hydrolases and transporters in the BBM and increases in their mRNA.