Purpose: We examined the hypothesis that protein coingestion with glucose during endurance exercise increases exogenous glucose oxidation rate and gut comfort and lowers perceived exertion.
Methods: In a randomized crossover design, eight male cyclists rode 150 min at 50% of peak power on three occasions while ingesting solutions containing the following: 8% 13C-enriched glucose and 2% milk protein concentrate (protein–glucose), glucose only (glucose), or noncaloric placebo (water). All solutions contained sodium citrate ([Na+] 60 mmol·L−1) and flavor and were ingested at 150 mL·15 min−1. The exogenous carbohydrate oxidation rate was determined using stable isotope method and indirect calorimetry.
Results: Protein coingestion had no effect on the exogenous glucose oxidation rate but increased endogenous carbohydrate oxidation rate (16%; 90% confidence limits ±7%), relative to glucose. Total carbohydrate and fat oxidation rates were increased (25%; ±6%) and decreased (17%; ±4%), respectively, by protein coingestion relative to water, but the effect relative to glucose was trivial. The plasma glucose concentration relative to glucose (mean ± SD; 6.1 ± 0.8 mmol·L−1) was 5.8% (±3%) lower with protein coingestion; there were no clear differences in glucose concentration for the remaining comparisons or for lactate concentration. Perceived exertion was not altered by protein coingestion; however, there was a small decrease in nausea with the protein–glucose solution relative to water (−0.14 ± 0.08 U); other protein-affected comparisons were without note.
Conclusions: Adding protein to a glucose–sodium solution ingested during exercise had neutral effect on exogenous carbohydrate oxidation and perception and little effect on metabolic measures associated with endurance performance. We conclude that previously reported effects of protein coingestion on endurance capacity were unlikely due to increased exogenous carbohydrate provision.