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

Péronnet, F., Y. Burelle, D. Massicotte, C. Lavoie,and C. Hillaire-Marcel. Respective oxidation of¹³C-labeled lactate and glucoseingested simultaneously during exercise. J. Appl.Physiol. 82(2): 440-446, 1997.The purpose ofthis experiment was to measure, by using¹³C labeling, the oxidation rateof exogenous lactate (25 g, as Na⁺,K⁺,Ca²⁺, andMg²⁺ salts) and glucose (75 g)ingested simultaneously (in 1,000 ml of water) during prolongedexercise (120 min, 65 ± 3% maximum oxygen uptake in 6 male subjects). The percentage of exogenous glucose and lactateoxidized 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 amountof exogenous lactate oxidized was much smaller than that of exogenousglucose (11.1 ± 0.5 vs. 36.3 ± 1.3 g, respectively) andcontributed to only 2.6 ± 0.4% of the energy yield(vs. 8.4 ± 1.9% for exogenous glucose). The cumulative amount ofexogenous glucose and lactate oxidized was similar to that observedwhen 100 g of[¹³C]glucose wereingested (47.3 ± 1.8 vs. 50.9 ± 1.2 g, respectively). When[¹³C]glucose wasingested, changes in the plasma glucose¹³C/¹²Cratio indicated that between 39 and 61% of plasma glucose derived fromexogenous glucose. On the other hand, the plasma glucose ¹³C/¹²Cratio remained unchanged when[¹³C]lactate wasingested, suggesting no prior conversion into glucose before oxidation.