Abstract Carbohydrates, mostly as starch, are digested by salivary and pancreatic amylases to di-, tri- and oligosaccharides, then to monosaccharides by saccharidases on the wall of the small intestine, following which they are absorbed. Proteins are absorbed as amino acids and small peptides that are broken down further, in the cell, to amino acids. Monosaccharides and amino acids pass to the liver via the portal vein. Fats are digested and absorbed as free fatty acids and glycerides and are then mostly reconstituted to triglycerides in the mucosal cells of the small intestine. They combine with phospholipids and a protein to form chylomicrons, which pass via the lymphatics and the thoracic duct into the general circulation. Fatty acids are released in the tissues and are then either re-esterified and stored as triglycerides in adipose tissue or oxidized for energy. Water is absorbed passively, along the osmotic gradient, secondary to the active absorption of sodium ions.