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Purpose: Preexercise ingestion of exogenous ketones alters the metabolic response to exercise, but effects on exercise performance have been equivocal.

Methods: On two occasions in a double-blind, randomized crossover design, eight endurance-trained runners performed 1 h of submaximal exercise at approximately 65% V[spacing dot above]O2max immediately followed by a 10-km self-paced time trial (TT) on a motorized treadmill. An 8% carbohydrate-electrolyte solution was consumed before and during exercise, either alone (CHO PLA), or with 573 mg[middle dot]kg-1 of a ketone monoester supplement (CHO KME). Expired air, HR, and RPE were monitored during submaximal exercise. Serial venous blood samples were assayed for plasma glucose, lactate, and [beta]-hydroxybutyrate concentrations.

Results: CHO KME produced plasma [beta]-hydroxybutyrate concentrations of approximately 1.0 to 1.3 mM during exercise (P < 0.001), but plasma glucose and lactate concentrations were similar during exercise in both trials. V[spacing dot above]O2, running economy, respiratory exchange ratio, HR, and RPE were also similar between trials. Performance in the 10-km TT was not different (P = 0.483) between CHO KME (mean, 2402 s; 95% confidence interval, 2204-2600 s) and CHO PLA (mean, 2422 s; 95% confidence interval, 2217-2628 s). Cognitive performance, measured by reaction time and a multitasking test, did not differ between trials.

Conclusions: Compared with carbohydrate alone, coingestion of KME by endurance-trained athletes elevated plasma [beta]-hydroxybutyrate concentrations, but did not improve 10-km running TT or cognitive performance.

(C) 2019 American College of Sports Medicine