Acute Ketone Monoester Supplementation Does Not Change Exercise Efficiency During Incremental Cycling in Trained Individuals

METHODS:28 adults (16 males and 12 females) aged 30 ± 10 y [peak oxygen uptake (V̇O2peak): 59 ± 11 ml·kg-1·min-1] completed three experimental trials in a randomized, crossover, and double-blinded manner. Participants ingested either 0.3 (KE-LO) or 0.6 (KE-HI) g·kg-1 body mass of KE or a flavour-matched placebo (PLAC) ~30 min prior to exercise. Exercise involved a 3-minute warm-up, three 5-minute stages at fixed incremental workloads corresponding to 75%, 100%, and 125% of individual ventilatory threshold, followed by a ramp protocol to volitional exhaustion to determine peak power output (PPO). RESULTS:Venous blood [ß-hydroxybutyrate], the major circulating ketone body, was higher after KE ingestion compared to PLAC (KE-HI: 3.0 ± 1.1 ≥ KE-LO: 2.3 ± 0.6 ≥ PLAC: 0.2 ± 0.1 mM; all p ≤ 0.001. There were no differences between conditions in the primary outcome exercise economy, nor gross efficiency or delta efficiency, when analyzed over the entire submaximal exercise period or by stage. Heart rate and ventilation were higher in KE-HI and KE-LO compared to PLAC when assessed over the entire submaximal exercise period and by stage (all p ≤ 0.05). PPO after the ramp was lower in KE-HI compared to both KE-LO and PLAC (329 ± 60 vs 339 ± 62 and 341 ± 61 W respectively; both p < 0.05) despite no difference in V̇O2peak. CONCLUSIONS:KE ingestion did not change indices of exercise efficiency but increased markers of cardiorespiratory stress during submaximal incremental cycling and reduced PPO.