Glucose kinetics during high-intensity exercise in endurance-trained and untrained humans

In humans, endurance training reduces the rates of glucose production and utilization during moderate-intensity exercise. It is uncertain, however, whether this is also true during high-intensity exercise. Accordingly, we studied eight endurance-trained cyclists and eight untrained subjects during 30 min of cycling at ~80% of maximal oxygen uptake (V̇O(2 max)). Rates of glucose appearance (Ra) and disappearance (Rd) were determined using a primed, continuous infusion of [6,6-²H]glucose. Average glucose Ra during exercise did not differ in the trained and untrained subjects (34.3 ± 3.6 vs. 36.0 ± 1.7 μmol · min^-1 · kg^-1; mean ± SE; P, not significant). Plasma insulin, glucagon, norepinephrine, and epinephrine concentrations were also similar in the two groups. In contrast, glucose Rd during exercise was 19% lower in the trained compared with the untrained subjects (27.0 ± 2.6 vs. 33.2 ± 1.5 μmol · min^-1 · kg^-1; P < 0.001). Consequently, during exercise, plasma glucose concentration rose significantly (P < 0.05) in the trained subjects but did not change in the untrained subjects. We conclude that utilization of plasma glucose is lower in trained subjects during high- intensity exercise, even when the exercise is performed at the same relative (and therefore a higher absolute) intensity as in the untrained state. Hyperglycemia in trained subjects during intense exercise appears to be due to this lower rate of glucose utilization rather than a higher rate of glucose production.