Glucose and lactate interrelations during moderate-intensity exercise in humans.

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 (VO2 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 x kg-1 x 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 x kg-1 x 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 x kg-1 x 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 x kg-1 x 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.(ABSTRACT TRUNCATED AT 250 WORDS)