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

Introduction: Exogenous ketones potentially provide an alternative, energetically advantageous fuel to power exercising skeletal muscle. However, there is limited evidence regarding their relative contribution to energy expenditure during exercise. Furthermore, the effect of blood ketone concentration and exercise intensity on exogenous ketone oxidation rates is unknown. 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 beta-hydroxybutyrate (beta HB) ketone monoester (KME; 252 mg center dot kg BW-1, low ketosis), or (iii) a high-dose beta HB KME (752 mg center dot kg BW-1, high ketosis). The KME contained a C-13 isotope label, allowing for the determination of whole-body exogenous beta HB oxidation rates through sampled respiratory gases. Results: Despite an approximate doubling of blood beta HB concentrations between low- and high-ketosis conditions (similar to 2 mM vs similar to 4.4 mM), exogenous beta HB oxidation rates were similar at rest and throughout exercise. The contribution of exogenous beta HB oxidation to energy expenditure peaked during the 25% W-Max 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 beta HB oxidation contributes minimally to energy expenditure and is not increased by elevating circulating concentrations greater than similar to 2 mM. Despite low exogenous beta HB oxidation rates, exercise efficiency was significantly improved when blood beta HB concentration was raised to similar to 2 mM.

Automated summary

The study showed that exogenous ketones contribute minimally to energy expenditure during exercise, regardless of exercise intensity, and their oxidation rates do not increase even with blood concentrations exceeding 2 mM. Furthermore, exercise efficiency significantly improves when blood beta-hydroxybutyrate concentration is raised to around 2 mM.