Conclusions
Regardless of exercise intensity, exogenous βHB oxidation contributes minimally to energy expenditure and is not increased by elevating circulating concentrations greater than ~2 mM. Despite low exogenous βHB oxidation rates, exercise efficiency was significantly improved when blood βHB concentration was raised to ~2 mM.
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 (WMax). 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 13C 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 peaked during the 25% WMax exercise intensity but was relatively low (4.46% ± 2.71%). Delta efficiency during cycling exercise was significantly greater in the low-ketosis (25.9% ± 2.1%) versus control condition (24.1% ± 1.9%; P = 0.027). Conclusions: Regardless of exercise intensity, exogenous βHB oxidation contributes minimally to energy expenditure and is not increased by elevating circulating concentrations greater than ~2 mM. Despite low exogenous βHB oxidation rates, exercise efficiency was significantly improved when blood βHB concentration was raised to ~2 mM.