Absorption, the Microbiome, and Hormonal Regulation

Nutrient absorption occurs predominantly in the small intestine, where digested nutrients cross the single layer of epithelial cells lining the villi and enter either the bloodstream or the lymphatic system. Monosaccharides such as glucose and galactose are absorbed via active transport coupled to sodium, while fructose is absorbed by facilitated diffusion; amino acids and small peptides are absorbed through a variety of specific transporter proteins, many of which, like the glucose transporters, are also sodium-dependent. Fat digestion products—free fatty acids and monoglycerides—are packaged, together with bile salts, into micelles that ferry them to the intestinal cell surface for absorption; inside the cell, they are reassembled into triglycerides and packaged into lipoprotein particles called chylomicrons, which enter the lymphatic system rather than the bloodstream directly, eventually draining into venous circulation near the neck. Water-soluble vitamins and minerals generally use dedicated transport proteins, while fat-soluble vitamins are absorbed together with dietary fat via the same micelle-dependent pathway, which explains why very low-fat diets can impair the absorption of vitamins A, D, E, and K.