EFFECT OF ENDURANCE TRAINING ON HEPATIC GLYCOGENOLYSIS AND GLUCONEOGENESIS DURING PROLONGED EXERCISE IN MEN

In humans, endurance training markedly reduces the rate of hepatic glucose production during exercise. To determine whether this is due to a reduction in glycogenolysis, in gluconeogenesis, or in both processes, six men were studied at rest and during 2 h of cycle ergometer exercise at 60% pretraining peak O2 consumption (VO2peak), both before and after completion of a strenuous endurance training program (cycling at 75-100% VO2peak for 45-90 min/day, 6 days/wk for 12 wk). The overall rate of glucose appearance (R(a)) was determined using a primed continuous infusion of [6,6-H-2]glucose, whereas the rate of gluconeogenesis (R(gng)) was estimated from the incorporation of C-13 into glucose (via pyruvate carboxylase) from simultaneously infused [C-13]bicarbonate. Training did not affect glucose kinetics at rest but reduced the average R(a) during exercise by 42% [from 36.8 +/- 3.8 to 21.5 +/- 3.6 (SE) mu mol.min(-1).kg(-1); P < 0.001]. This decrease appeared to be mostly due to a reduction in hepatic glycogenolysis. However, the estimated R(gng) during exercise also decreased significantly (P < 0.001) with training, falling from 7.5 +/- 1.6 mu mol.min(-1).kg(-1) (23 +/- 3% of total R(a)) before training to 3.1 +/- 0.6 mu mol.min(-1) kg(-1) (14 +/- 3% of total R(a)) after training. These training-induced adaptations in hepatic glucose metabolism were associated with an attenuated hormonal response to exercise (i.e., higher insulin and lower glucagon, norepinephrine, and epinephrine concentrations) as well as a reduced availability of gluconeogenic precursors (i.e., lower lactate and glycerol concentrations). We conclude that endurance training reduces both hepatic glycogenolysis and gluconeogenesis during prolonged exercise in men.