The net movement of fluid in the small bowel and in the colon in response to an oral lactose load was determined by intraluminal intubation in 5 lactase-deficient and 2 normal subjects. The major net secretion of fluid in lactase-deficient individuals occurred in the stomach, duodenum, and jejunum. The test solution had been diluted approximately 5-fold when it reached midileum. The fluid response was similar to that seen with an osmotically equivalent amount of mannitol. The response to an osmotic stimulus was the same as occurred in the control subjects fed mannitol. Ileal contents were neutral or alkaline and the osmolality was 300 milliosmoles per kg after lactose feeding. In the colon there was interference with net fluid absorption when lactose was fed by mouth to lactase-deficient subjects or perfused into the ileocecal region of 4 subjects. The decrease in net absorption was demonstrable when the result was compared with that of mannitol. Net secretion did not occur in the colon. Stools were acid (mean pH 5.8) and the osmolality had risen to 379 milliosmoles per kg presumably as a result of bacterial fermentation of the undigested lactose. Lactose-induced diarrhea in lactase-deficient subjects results, in large part, from the combination of net fluid secretion by the small intestine in response to an osmotic load and interference with net fluid absorption in the small bowel and colon. The products of fermentation of lactose may play a role in the decreased colonic absorption.
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