Physiology and pathophysiology of intestinal absorption.

Final digestion and absorption of carbohydrates (CHO) occur after intraluminal hydrolysis by pancreatic alpha-amylase at the surface of the mucosal membrane in close relationship between disaccharide hydrolysis and the glucalogue carrier system. In general, Na(+)-dependent transport is the rate-limiting step of CHO absorption. The rate of absorption is determined by mode of ingestion, chemical composition of the meal, gastric emptying, pancreatic digestion, intestinal digestion and absorption, and intestinal motility. A delay of absorption of CHO may be achieved by dietary fibers, alpha-amylase inhibitors, or alpha-glucosidase inhibitors. Final protein digestion is achieved by a dual mechanism: intact peptide absorption with subsequent intracellular hydrolysis to free amino acids (AA) and membrane hydrolysis of peptides followed by absorption of free AA. More complex is the mechanism of lipid absorption: emulsification, lipolysis, micellar formation, membrane translocation, intracellular resynthesis, chylomicron formation, and lymphatic drainage. The most critical steps in lipid digestion are lipolysis and micellar formation.