102 - Digestion and Absorption of Carbohydrate, Protein, and Fat

Abstract

Dietary carbohydrate, protein, and fat are digested in the small intestine and the end products are absorbed into the enterocytes. The digestion products of carbohydrate and protein appear in portal blood whereas those of fat appear in lacteals. The lumen-facing brush-border membrane and the serosa-facing basolateral membrane express various transporters for monosaccharides, peptides, amino acids (AAs), fatty acids, and cholesterol to bring about the directional transfer of the digestion end-products from the lumen across the enterocyte. Carbohydrates are absorbed solely as monosaccharides, whereas proteins are absorbed in the form of small peptides and AAs. The absorption process for fat-soluble vitamins is similar to that of dietary fat. Dietary fat is broken down to smaller components before absorption, but the components are re-assembled and then packaged into chylomicrons in the enterocytes prior to secretion into lacteals. There are a number of diseases associated with the digestion and absorption of dietary carbohydrate, protein, and fat. Diet also contains fiber, which is primarily composed of carbohydrates in various forms that are not digestible by the enzymes in the small intestine; they reach the colon where bacterial enzymes break them down and ferment them to generate short-chain fatty acids. These bacterial fermentation products and other bacterial metabolites have marked effects, mostly beneficial, not only on the colon, but also on the mucosal immune system and other systemic organs such as the liver, pancreas, and brain.