While some small studies have shown the benefits of exogenous ketones on exercise performance, meta-analyses have found that supplementation with exogenous ketones does not improve aerobic exercise performance.[1][2][3] This includes both running and cycling performance (as measured by time trials and time to exhaustion tests) and aerobic exercise metrics such as VO2max, economy/efficiency, and RPE. Meta-analyses have also found that exogenous ketones do not improve anaerobic exercise performance such as during sprinting.[2] However, as described below, there are some important caveats.
Firstly, several studies have provided an exogenous ketone combined with carbohydrates and lack a comparator group who did not take carbohydrates.[3][1][2][4] This approach could mask any potential benefit of ketones, because ingesting carbohydrates during exercise has a powerful beneficial effect on performance.[5][6][7][8][9][10]
Secondly, many studies have examined the effect of exogenous ketones on exercise performance when participants are fresh and not fatigued.[3][1][2][4] Because ketone bodies become a primary fuel source for tissues during hypoglycemia (when blood glucose concentrations are low) and because hypoglycemia can arise during long-duration aerobic exercise, it would be prudent to study the effects of exogenous ketone supplementation under these conditions. Only one study to date has examined the efficacy of exogenous ketone for improving cycling time trial performance following a fatiguing long-duration bout (4 hours of cycling) and found no benefit.[11] Additional randomized controlled trials are needed to understand whether exogenous ketones can improve performance when people are fatigued and/or hypoglycemic. This would help answer whether exogenous ketones can improve performance during long-duration bouts such as ultra-distance events.