Respective oxidation of 13C-labeled lactate and glucose ingested simultaneously during exercise.

The purpose of this experiment was to measure, by using 13C labeling, the oxidation rate of exogenous lactate (25 g, as Na+, K+, Ca2+, and Mg2+ salts) and glucose (75 g) ingested simultaneously (in 1,000 ml of water) during prolonged exercise (120 min, 65 +/- 3% maximum oxygen uptake in 6 male subjects). The percentage of exogenous glucose and lactate oxidized were similar (48 +/-3 vs. 45 +/- 5%, respectively). However, because of the small amount of oral lactate that could be tolerated without gastrointestinal discomfort, the amount of exogenous lactate oxidized was much smaller than that of exogenous glucose (11.1 +/- 0.5 vs. 36.3 +/- 1.3 g, respectively) and contributed to only 2.6 +/- 0.4% of the energy yield (vs. 8.4 +/- 1.9% for exogenous glucose). The cumulative amount of exogenous glucose and lactate oxidized was similar to that observed when 100 g of [13C]glucose were ingested (47.3 +/- 1.8 vs. 50.9 +/- 1.2 g, respectively). When [13C]glucose was ingested, changes in the plasma glucose 13C/12C ratio indicated that between 39 and 61% of plasma glucose derived from exogenous glucose. On the other hand, the plasma glucose 13C/12C ratio remained unchanged when [13C]lactate was ingested, suggesting no prior conversion into glucose before oxidation.