The Effect of Blood Ketone Concentration and Exercise Intensity on Exogenous Ketone Oxidation Rates in Athletes

Six athletes completed 60 minutes of cycling ergometer exercise, in 20-minute intervals at 25, 50 and 75 percent of maximal power output, on three occasions in a single-blind crossover design. They consumed bitter flavored water, a low-dose (252 mg per kg) or a high-dose (752 mg per kg) 13C-labelled beta-hydroxybutyrate ketone monoester, which allowed exogenous oxidation to be measured from respiratory gases. Blood beta-hydroxybutyrate roughly doubled between low and high ketosis (about 2 mM vs 4.4 mM), yet exogenous oxidation rates were similar. Exogenous oxidation contributed at most 4.46 percent of energy expenditure, and delta efficiency was higher in the low-ketosis condition (25.9 percent) than control (24.1 percent, P = 0.027).