Measurement of gluconeogenesis in exercising men by mass isotopomer distribution analysis

Departments of 1 Integrative Biology and 2 Nutritional Sciences, University of California, Berkeley, California 94720 We evaluated the hypothesis that coordinated adjustments in absolute rates of gluconeogenesis (GNG ab ) and hepatic glycogenolysis (Gly) would maintain euglycemia and match glucose production (GP) to peripheral utilization during rest and exercise. Specifically, we evaluated the extent to which gradations in exercise power output would affect the contribution of GNG ab to GP. For these purposes, we employed mass isotopomer distribution analysis (MIDA) and isotope-dilution techniques on eight postabsorptive (PA) endurance-trained men during 90 min of leg cycle ergometry at 45 and 65% peak O 2 consumption ( O 2 peak ; moderate and hard intensities, respectively) and the preceding rest period. GP was constant in resting subjects, whereas the fraction from GNG (f GNG ) increased over time during rest (22.3 ± 0.9% at 11.25 h PA vs. 25.6 ± 0.9% at 12.0 h PA, P