The Digestion and Absorption of Lactose by the Intact Rat

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Abstract
The digestion and absorption of 2 different doses of lactose was compared with that of equivalent amounts of glucose and galactose. The sugars were administered intragastrically together with a water‐soluble, non‐absorbable marker, polyethylene glycol, in order to study the rate of movement of the sugars through the gastrointestinal tract and to determine their site of absorption. Only the larger dose of lactose showed an effect on the rate of transit of the sugars, and this effect was confined to the distal third of the small intestine. When the monosaccharides were fed, they were almost completely absorbed in the upper two‐thirds of the small intestine, glucose being absorbed more rapidly than galactose. Lactose was absorbed in all parts of the small bowel, and, after the larger dose, as much as 25% of the amount of sugar fed was absorbed from the colon. Large amounts of sugar were found in the distal small intestine and colon as long as 6 hours after lactose administration. The ceacal contents contain large amounts of lactase, a considerable fraction of which is enzymatically different from small‐intestinal mucosal lactase. This caecal lactase is probably of bacterial origin. In the small intestine, the rate of hydrolysis of lactose was rate‐limiting for its absorption. In the large bowel, lactose was hydrolysed more rapidly than the monosaccharides could be absorbed.