Introduction: Acute ketone monoester (KE) supplementation affects exercise responses but there are equivocal data regarding the effects on exercise efficiency. We examined the effect of ketone monoester ingestion on exercise efficiency during cycling and probed further the influence of supplement dose and exercise intensity. This study was registered prior to data collection at ClinicalTrials.org (NCT05665855). Methods: Twenty-eight trained adults were recruited [16 males, 12 females; peak oxygen uptake (V̇O2peak): 59±11 ml·kg-1·min-1]. Participants completed three experimental trials in a randomized, crossover, and double-blinded manner, each separated by ~1 week. Participants ingested either a 0.3 (KE-LO) or 0.6 g/kg (KE-HI) body mass dose of KE or a flavour-matched placebo (PLAC) ~30 min prior to exercise. The incremental cycling protocol involved a 3-minute warm-up, three 5-minute stages at 75%, 100%, and 125% of individual ventilatory threshold, and a ramp increase to volitional exhaustion. Expired gases and heart rate were measured continually during exercise. Results: Venous blood [ß-hydroxybutyrate], the major circulating ketone body, was higher in both KE conditions compared to PLAC and also different between conditions (3.0±1.1 and 2.3±0.6 vs 0.2±0.1 mM; all p<0.05). There were no differences in submaximal exercise V̇O2, exercise economy, gross efficiency, or delta efficiency between conditions (all p>0.05). Submaximal exercise heart rate and ventilation were higher in both KE conditions compared to PLAC (141±11 and 141±12 vs 137±12 beats/min; 63±14 and 62±13 vs 60±13 L/min, respectively; all p<0.05). Peak power output at V̇O2peak was lower in KE-HI compared to both KE-LO and PLAC (329±60 vs 339±62 and 341±61 W; both p<0.05). Conclusion: KE supplementation did not alter exercise efficiency during submaximal cycling. KE ingestion increased cardiorespiratory stress during submaximal exercise and the higher dose reduced peak aerobic power output. Future studies should investigate the mechanisms by which KE ingestion alters exercise responses. / Thesis / Master of Science (MSc) / Endurance exercise performance is determined by many variables including the efficiency of the individual. This can be measured during cycling by calculating the ratio of oxygen uptake relative to power output. Ketone supplements have been suggested to alter exercise efficiency. We investigated this issue by having trained adults complete an incremental cycling protocol on three occasions. Before exercise the participants ingested either a small or large dose of a ketone supplement or a taste-matched placebo drink. Exercise efficiency was not different between the conditions but ventilation rate and heart rate were higher during the ketone supplemented trials compared to the placebo. The power output that the participants could achieve at maximal exercise was reduced in the high dose ketone condition. Our study does not support the use of ketone supplements as a strategy to enhance endurance exercise performance. Future studies should investigate the mechanisms by which ketones affect exercise responses.