Absorption of glucose and galactose and digestion and absorption of lactose by the preruminant calf

Abstract

1. Uptakes of glucose, galactose, glucose+galactose and lactose from the intestine were studied in preruminant calves fitted with cannulas in the proximal small intestine, and in slaughtered calves. Responses in concentration of blood sugar to the presence of sugar in the small intestine were also investigated.2. Glucose and galactose were absorbed at about the same rate when present in the intestine separately. The rate for each sugar approached a maximum with increasing sugar concentration in the lumen.3. When glucose and galactose were present in the intestine together, the rate of galactose absorption was greatly depressed; that of glucose was virtually unaffected. Consequently, when calves were fed with mixtures of glucose and galactose, absorption of galactose in the proximal intestine, where the relative concentration of glucose was high, was depressed. In the more distal intestine where the glucose concentration had fallen, absorption of galactose was rapid. Both sugars were absorbed efficiently during their passage through the whole small intestine, but at different sites.4. Galactose appeared in the blood after galactose alone was given but conversion of some galactose into glucose always occurred. Results of experiments in which [¹⁴C]galactose, together with glucose, was infused into the duodenum of a calf suggested that under these conditions the rate of galactose absorption was depressed to such an extent that nearly all the galactose entering the blood was converted into glucose.5. Lactose hydrolysis occurred much more rapidly than the absorption of its constituent monosaccharides. Thus, after lactose was fed to the calves, glucose and galactose accumulated in the lumen of the proximal small intestine. A similar accumulation occurred when lactose was infused into loops of small intestine. The rate-limiting step in the total absorption of lactose in the young calf was not, therefore, the hydrolysis stage, as reported for other mammals. The absorption of the released glucose and galactose occurred in a manner similar to that described above for mixtures of glucose and galactose.