Oxidation of an oral [C-13]glucose load at rest and prolonged exercise in trained and sedentary subjects

The purpose of this study was to compare the oxidation of [C-13]glucose (10 0 g) ingested at rest or during exercise in six trained (TS) and six sedent ary (SS) male subjects. The oxidation of plasma glucose was also computed f rom the volume of (CO2)-C-13 and C-13/C-12 in plasma glucose to compute the oxidation rate of glucose released from the liver and from glycogen stores in periphery (mainly muscle glycogen stores during exercise). At rest, oxi dative disposal of both exogenous (8.3 +/- 0.3 vs. 6.6 +/- 0.8 g/h) and liv er glucose (4.4 +/- 0.5 vs. 2.6 +/- 0.4 g/h) was higher in TS than in SS. T his could contribute to the better glucose tolerance observed at rest in TS . During exercise, for the same absolute workload [140 +/- 5 W: TS = 47 +/- 2.5; SS = 68 +/- 3 %maximal oxygen uptake ((V) over dotO(2max))], [C-13]gl ucose oxidation was higher in TS than in SS (39.0 +/- 2.6 vs. 33.6 +/- 1.2 g/h), whereas both liver glucose (16.8 +/- 2.4 vs. 24.0 +/- 1.8 g/h) and mu scle glycogen oxidation (36.0 +/- 3.0 vs. 51.0 +/- 5.4 g/h) were lower for the same relative workload (68 +/- 3% (V) over dotO(2max): TS = 3.13 +/- 0. 96; SS = 2.34 +/- 0.60 l O-2/min), exogenous glucose (44.4 +/- 1.8 vs. 33.6 +/- 1.2 g/h) and muscle glycogen oxidation (73.8 +/- 7.2 vs. 51.0 +/- 5.4 g/h) were higher in TS. However, despite a higher energy expenditure in TS, liver glucose oxidation was similar in both groups (22.2 +/- 3.0 vs. 24.0 +/- 1.8 g/h). Thus exogenous glucose oxidation was selectively favored in T S during exercise, reducing both liver glucose and muscle glycogen oxidatio n.