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Summary. The metabolism of circulating disaccharides was studied in adult humans and rats. After iv infusions of 10 g of either lactose, sucrose, or maltose in four adults, no rise in blood glucose was noted. A mean of 8.7 + 1.89 g of the lactose and 6.3 + 1.39 g of the sucrose was excreted in the 24-hour urine sample. Only 0.11 ± 0.03 g of the infused maltose was recovered in the urine, suggesting that the maltose was metabolized.After injection of 14C-labeled lactose and sucrose in rats, 6.2 + 2.7 and 7.6 ± 2.4%, respectively, was oxidized to 14CO2 in 24 hours; 62.1 ± 13.5 and 68.4 ± 10.8% of the respective disaccharides was excreted into the urine. Conversely, after injection of 14C-labeled maltose 54.6 + 7.0% was oxidized to 14CO0 and 4.8 + 3.9% excreted in the urine. The per cent of maltose oxidized to CO2 was similar to that of glucose.In addition to small intestinal mucosa, homogenates of rat kidney, brain, and liver as well as serum were found to have measurable maltase activities.

A B S T R A C T The utilization of circulating maltose was compared to that of glucose in six normal fasting subjects after intravenous injection of 25 g of either sugar. Blood samples were obtained over a 2 hr period and were assayed for free fatty acids (FFA), insulin, glucose, and total reducing substances. Urine was collected for 2 hr after maltose administration and assayed enzymatically for glucose and maltose. Blood glucose concentrations did not increase after maltose infusion, although a significant rise in total reducing substances was noted, indicating the presence of this disaccharide in the blood. Less than 3% of the administered maltose was excreted in the urine either as maltose or glucose. Initially, there was a fourfold increase in serum insulin concentration after glucose and a threefold increase after maltose infusion. Therefore, serum insulin concentrations gradually declined in a similar manner for both sugars. The plasma FFA at 15 min decreased 371 uEq/ liter after glucose and 338 uEq/liter after maltose infusion.In other studies, 10 g maltose containing 5 GCi maltose-U-"C were injected into five human subjects and expired C02 collected for 6 hr. Maximal "C02 specific activity was noted at 170 min and a mean of 61.1% of the injected radioactivity was recovered as "CO2. Less than 8% of the injected "C was excreted in the urine.These results indicate that maltose administered intravenously has similar metabolic effects when compared to glucose, and may be efficiently utilized as a carbohydrate substrate. The oxidation of intravenously administered maltose-U-"C to "CO2 demonstrates that circulating maltose is readily metabolized. A solution of maltose could provide twice the mass of sugar (and of calories) per milliliter as an equimolar solution of glucose. Parenterally administered maltose may be of clinical value and should be further studied.

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