C. MALABSORPTION



The gastrointestinal tract has as its main pur­pose the digestion and absorption of nutrients, either macronutrients required predominantly for energy or micronutrients, for example, trace ele­ments and vitamins. Only 1 u.g of vitamin B12 needs to be absorbed daily in a highly specific and complex process in the terminal ileum. Con­versely, the gastrointestinal tract may be called upon to digest, solubilize, transport, and resyn-thesize 80 grams of fat daily. Beyond the net ab­sorption of dietary components, the gut has a large daily internal circulation of water, electrolytes, and bile salts that are secreted in gastric juice, pancreatic juice, bile, and succus entericus and then reabsorbed in the small intestine and colon for reutilization. The gastrointestinal tract must first prepare for subsequent absorption of the crude mixtures of nutrients and non-nutrients ingested as “food.” These preparatory processes of diges­tion consist of (a) the controlled release into the intestine of food that has been fragmented to small particle size by the grinding action of the stomach; (b) the release of appropriate amounts of enzymes, cofactors, buffers (bicarbonate], detergents (bile salts), and water into the lumen under hormonal pur­pose the digestion and absorption of nutrients, either macronutrients required predominantly for energy or micronutrients, for example, trace ele­ments and vitamins. Only 1 u.g of vitamin B12 needs to be absorbed daily in a highly specific and complex process in the terminal ileum. Con­versely, the gastrointestinal tract may be called upon to digest, solubilize, transport, and resyn-thesize 80 grams of fat daily. Beyond the net ab­sorption of dietary components, the gut has a large daily internal circulation of water, electrolytes, and bile salts that are secreted in gastric juice, pancreatic juice, bile, and succus entericus and then reabsorbed in the small intestine and colon for reutilization. The gastrointestinal tract must first prepare for subsequent absorption of the crude mixtures of nutrients and nonnutrients ingested as “food.” These preparatory processes of diges­tion consist of (a) the controlled release into the intestine of food that has been fragmented to small particle size by the grinding action of the stomach; (b) the release of appropriate amounts of enzymes, cofactors, buffers (bicarbonate], detergents (bile salts), and water into the lumen under hormonal control to allow digestion of food to an isotonic mixture of simple components; and (c) further digestion of disaccharides and oligopeptides by brush border-bound enzymes. All of these steps must precede actual absorption.

Absorption of nutrients occurs from an enor­mous surface area supplied by the intestinal villi and microvilli during an average transit time of approximately ll to 2 hours between the stomach and the ileocecal valve. (Colonic absorption con­sists largely of salvage of fluid and electrolytes only.) The absorption of a given substance may occur diffusely throughout the intestine or be lo­calized to specific sites (e.g., vitamin BJ2 and bile Salts in the terminal ileum). In view of the com­plexities of the above coordinated chemical and physiological processes, it is not surprising that a large number of disorders can result in maldiges­tion or malabsorption, either with considerable specificity or more globally. The purpose of this section is to present a general classification of the disorders of maldigestion and malabsorption, based on current knowledge of pathophysiology, together with a rational approach to their differ­ential diagnosis.