Glucose and lactate interrelations during moderate-intensity exercise in humans

Abstract

To evaluate circulating lactate and glucose kinetics during moderate-intensity exercise, we studied ten healthy endurance-trained men (aged 25 ± 6 years) during 30 to 50 minutes of supine cycle ergometer exercise at 43% ± 5% of maximal oxygen consumption (V̇O 2 max) using isotopic tracer techniques. Seven subjects received [U- 13C]-lactate and [6- 14C]-glucose, and three received [1- 14C]-lactate and [U- 13C]-glucose. Arterial glucose and lactate concentrations were 94.0 ± 4.1 and 5.66 ± 0.87 mg/dL at rest, and 95.7 ± 3.4 and 8.38 ± 3.87 mg/dL, respectively, after 25 minutes of exercise. The rate of glucose disappearance (RdG) increased from 2.41 ± 0.40 at rest to 3.38 ± 0.77 mg × kg −1 × min −1 during exercise, compared with the much larger rise in the rate of lactate appearance (RaL), which increased from 1.25 ± 0.20 to 3.47 ± 0.79 mg × kg −1 × min −1. During exercise RaL was 103% of RdG, compared with only 52% at rest. The rate at which the blood was cleared of lactate increased from 22.7 ± 2.2 at rest to 44.2 ± 11.2 ml × kg −1 × min −1 after 25 minutes of exercise. From secondary labeling of lactate with glucose carbons, the rate of glucose conversion to lactate was estimated to be 0.65 ± 0.16 mg × kg −1 × min −1 during exercise. Twenty percent of the glucose utilization went to lactate formation during exercise, and 20% of the blood lactate appearance came from blood glucose, with the balance presumably coming from muscle glycogen. These results demonstrate that approximately one fifth of the carbohydrate oxidized during moderate-intensity exercise enters the circulating lactate pool before it is completely oxidized.