The major components of the diet are starches, sugars, fats and proteins. These have to be hydrolysed to their constituent smaller molecules for absorption and metabolism. Starches and sugars are absorbed as monosaccharides; fats are absorbed as free fatty acids and glycerol (plus a small amount of intact triacylglycerol); proteins are absorbed as their constituent amino acids and small peptides.
The fat-soluble vitamins (A, D, E and K) are absorbed dissolved in dietary lipids; there are active transport systems (section 3.2.2) in the small intestinal mucosa for the absorption of the water-soluble vitamins. The absorption of vitamin B (section 11.10.1) requires a specific binding protein that is secreted in the gastric juice in order to bind to the mucosal transport system.
Minerals generally enter the intestinal mucosal cells by carrier-mediated diffusion and are accumulated intracellularly by binding to specific binding proteins (section 220.127.116.11). They are then transferred into the bloodstream by active transport mechanisms at the serosal side of the epithelial cells, usually onto plasma binding proteins. The absorption of calcium is discussed in section 18.104.22.168, and that of iron in section 4.5.1.
Was this article helpful?