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Manuscript received 28 January 1997. Initial reviews completed 24 February 1997. Revision accepted 11 April 1997.
USDA/ARS Children's Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030
Quantification of the metabolism of dietary glucose by the splanchnic tissues is incomplete. Whether habitual carbohydrate intake affects splanchnic glucose metabolism is not known. Female mice were offered isoenergetic and isonitrogenous quantities of diets containing high (HCD) or low (LCD) amounts of carbohydrate, 5% of which was [U-13C]-glucose. Four mice from each dietary group were killed after 24, 48 and 120 h. The 13C-isotopomer distribution in blood glucose, lactate and alanine and in hepatic alanine and glycogen was measured by selected ion monitoring mass spectrometry. [U-13C]-Glucose and its products, [U-13C]-lactate and alanine, were in complete isotopic equilibrium in the blood. The tracer:tracee ratio of hepatic [U-13C]-alanine was significantly higher (P < 0.01) than that of circulating alanine. In both groups, the tracer:tracee ratio of circulating [U-13C]-glucose was significantly (P < 0.001) lower than that of the dietary carbohydrate, and the ratio of [13C3]-glucose:[U-13C]-glucose [0.57 (HCD) and 0.78 (LCD); diet effect P < 0.05], a measure of glucose metabolic cycling, was between two- and fivefold higher than published values obtained with intravenous tracer glucose. The tracer:tracee ratio of [U-13C]-glycogen glucose was significantly (P < 0.05) higher than that of arterial glucose. We conclude the following: 1) dietary glucose is extensively recycled, via pyruvate, within the liver; 2) this metabolic cycle is maintained in mice consuming low carbohydrate diets; and 3) dietary carbohydrate is channelled to hepatic glycogen. We speculate that the metabolic cycling of enteral glucose is related to the hepatic catabolism of dietary protein.
Key words: dietary glucose metabolism, stable isotopes, gluconeogenesis, mice.
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