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Disaccharides are hydrolyzed by their specific enzymes present in the intestinal mucosa (1-3). Although some current textbooks still state that these enzymes are secreted into the intestinal lu-men (4-7), the concentrations of monosaccharide products free in intestinal contents during disac-charide absorption in vitro (8-10) have been found insufficient to support the concept of intraluminal hydrolysis. In addition, the low disaccharidase activity of intestinal contents during the absorption process in vivo (11-13) strongly suggests that the disaccharide either enters the cell before being hydrolyzed or else is hydrolyzed at the cell surface by mucosa-bound enzyme. The released monosaccharide products presumedly are then transported across the intestinal cell (8, 14, 15). Little information is available that relates disac-charide hydrolysis to absorption of the component monosaccharides. Wilson and Vincent (14) commented on the accumulation of monosaccharides in the mucosal medium during the process of disac-charide absorption in hamster gut sac preparations, and Dahlqvist and Thomson have reported that large amounts of free fructose accumulate intra-luminally during sucrose absorption in the intact rat (16). Despite these findings in animals, Dahlqvist and Borgstr6m found little free intra-luminal monosaccharides during the process of disaccharide absorption in man (12). It was recently demonstrated, however, that appreciable amounts of the hydrolysis products accumulate intraluminally during the process of sucrose absorption in man and that these monosaccharides appear to move back from their mucosal site of hy-drolysis to the lumen (13). The present work is concerned with the relation of hydrolysis of sucrose to the absorption of its monosaccharide components, glucose, an actively absorbed monosaccharide (14, 15, 17), and fructose, which is passively absorbed (14, 15). Methods Thirty-two normal young subjects were studied on 105 occasions by use of a double-lumen tube placed at various levels of the intestine (13, 18). Polyethlene glycol 4000 (PEG) was used as the nonabsorbable water-soluble marker and was determined by a modification of Hyd&i's method (19). Sucrose solution or an equivalent mixture of glucose and fructose made isotonic with NaCl (290 ± 10 mOsm per L) was infused at 15 ml per minute through the proximal orifice of the tube. In some studies , galactose was also infused. Intestinal samples were collected by siphonage from the distal orifice, which was located 15 or 30 cm from the site of infusion. The initial 30 minutes of an infusion period allowed steady state conditions to be approached so that, thereafter, concentration of PEG and the …

Intestinal absorption of sucrose in man: interrelation of hydrolysis and monosaccharide product absorption.