ABSTRACT
Most of the defined formula diets are nutritionally complete and satisfy the recommended daily allowances (RDAs) for all macro-(nitrogen, carbohydrate, and fat) and micronutrients trace elements, fat, and water-soluble vitamins. In contrast to micronutrients, knowledge of the physiology of absorption of the macronutrients has greatly influenced the formulation of the polymeric and predigested chemically defined formula diets. The ensuing text will, thus, review relevant aspects of the physiology of protein, carbohydrate, and fat absorption. Hydrolysis of proteins to small peptides and free amino acids occurs within the intestinal lumen gastric, jejunal, ileal and at the enterocyte brush border. There appear to be four major, group-specific, active transport systems in the mammalian enterocyte: monoamino, monocarboxylic neutral amino acids; glycine, proline, hydroxyproline; dibasic amino acids and cysteine; and dicarboxylic acidic amino acids.
