[Mechanisms of intestinal absorption of nutrients]

The nutrient uptake from the intestinal lumen into the distributing blood circulation is mediated by the epithelial cell of the small intestine. The transfer process through this distinctly polar cell consists of three partial events: entrance of substances through the brush-border membrane, traversal of a metabolic active intracellular space and exit through the baso-lateral membrane. The fundamental transfer mechanisms--simple diffusion, facilitated diffusion, antiport and symport systems, electroneutral and electrogenic processes--are described. The significance of nutrient metabolization for transport processes is discussed: proton secretion by the epithelial cell coupled to the glucose and lactate metabolization is quoted as an illustration. The "acid microclimate" resulting from this proton secretion on the mucosal surface has a significant influence on weak-electrolyte absorption. This effect was clearly demonstrated for in vitro uptake of nicotinic acid into the intestinal tissue. It can be assumed that--similar to the role of a Na+-gradient--the proton gradient on the surface of absorptive epithelia is highly significant as a driving force of nutrient absorption.