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Methods

Six athletes completed cycling ergometer exercise on three occasions within a single-blind, random-order controlled, crossover design study. Exercise duration was 60 min, consisting of 20-min intervals at 25%, 50%, and 75% maximal power output (W Max). Participants consumed (i) bitter flavored water (control),(ii) a low-dose β-hydroxybutyrate (βHB) ketone monoester (KME; 252 mg· kg BW− 1,“low ketosis”), or (iii) a high-dose βHB KME (752 mg· kg BW− 1,“high ketosis”). The KME contained a 13 C isotope label, allowing for the determination of whole-body exogenous βHB oxidation rates through sampled respiratory gases.

Results

Despite an approximate doubling of blood βHB concentrations between low-and high-ketosis conditions (~ 2 mM vs~ 4.4 mM), exogenous βHB oxidation rates were similar at rest and throughout exercise. The contribution of exogenous βHB oxidation to energy expenditure …