NORMAL ABSORPTION



Malabsorption can be understood only in the context of normal absorption. Only t’le absorption of the three major classes of caloric nutrients will be described here. The intestinal absorption of water and electrolytes is described in Section D of this chapter, since this analogous form of “mal­absorption” largely results in diarrhea. The im­portant specific mechanisms for the absorption of iron (ferrous and heme forms), calcium (under the control of vitamin D). and vitamin Blz are de­scribed elsewhere.

Digestion and Absorption of Fat . Most dietary fat is’in the form of triglycerides of long-chain fatty acids (e.g., saturated—palmitic and stearic; unsaturated—oleic and linoleic). Fat from the stomach enters the duodenum as an oil-liquid emulsion; its presence in the stomach stim­ulates the flow of bile (via cholecystokinin [CCK]) and pancreatic juice (via secretin and CCK) . Pancreatic lipase, bound to the lipid surface by colipase in the presence of the deter­gent action of amphophilic bile salts, releases two free fatty acids from each molecule, leaving a 2-monoglyceride. These products of lipolysis are in­corporated into complex mixed micelles with bile salts, which enhance their solubility and allow them to traverse the unstirred water layer that ov­erlies the surface of the epithelial cells. The fatty acids and 2-monoglycerides then diffuse from the micelles into the cell cytosol, where they are largely resynthesized into triglycerides and pack­aged into chylomicrons and very low density li­poproteins (VLDL) for transport via lymphatics. The bile salts remain in the intestinal lumen for reutilization and are finally reabsorbed from the terminal ileum. The process of fat absorption is normally highly efficient; approximately 95 per cent of ingested neutral fat is absorbed. Fat soluble vitamins are probably “carried along” in this same general process.

Digestion and Absorption of Proteins. The digestion and absorption of proteins are simpler than those for fat, which may explain why mal­absorption of fat figures more prominently inmost malabsorptive syndromes. Hydrolysis of proteins begins in the stomach with pepsin but continues more completely in the upper small in­testine catalyzed by pancreatic trypsin, chymo-trypsin, and carboxypeptidase. The products of hydrolysis are free amino acids, dipeptides, and oligopeptides, the latter of which may undergo further hydrolysis by dipeptidases or oligopepti-dases in the brush border of microvilli on the sur­face of the enterocytes. Many amino acids are ab­sorbed as dipeptides, which are subsequently hydrolyzed to the free amino acids in the mucosal cells. There are several specific transport systems for amino acids: (a] the dibasic amino acid system, which is often abnormal in cystinuria; (b) the neu­tral amino acid system, which is abnormal in Hartnup disease; (c) the imino acidglycine sys­tem; and (d) the dicarboxylic acid system. Amino acids are absorbed by active cotransport with so­dium, predominantly in the jejunum.

Digestion and Absorption of Carbohydrates. Ingested carbohydrates consist largely of starch (a complex hexose polysaccharide] and the disac-charides sucrose and lactose. Salivary and pan­creatic amylases hydrolyze starch intraluminally to oligosaccharides and disaccharides. In contrast to dipeptides, disaccharides cannot be absorbed. The final stages of digestion are catalyzed by spe­cific enzymes in the microvillous surface—limit dextrinase, sucrase, lactase, and maltase. The re­leased glucose and galactose are absorbed by ac­tive transport in association with sodium; fructose absorption is by facilitated diffusion.