Ketone drink raised ketones but not race performance

Exogenous ketone supplements are marketed on the premise that raising circulating ketone bodies alters fuel use and spares muscle glycogen, which should in theory help endurance performance. One argument is that people who have restricted carbohydrate for a long time may have upregulated their capacity to burn ketones, and so might respond better to a ketone drink than a typical mixed-diet athlete. That hypothesis had not been tested directly in chronically keto-adapted athletes. This trial asked whether topping up ketones on top of carbohydrate improves endurance performance in exactly that group.

Nine recreationally active males, each with at least one year of ketogenic diet adherence, attended the laboratory four times. The first visit established maximal oxygen uptake and the second familiarised them with the test procedures. In a counterbalanced crossover design, the remaining two visits compared a 60 g carbohydrate beverage against a beverage containing 60 g carbohydrate plus 0.5 g/kg 1,3-butanediol. Each trial involved a 60-minute set-intensity exercise bout followed by a 16.1 km time trial. Blood βHB, lactate, glucose, substrate oxidation, oxygen consumption at fixed workload, heart rate and rating of perceived exertion were measured.

The supplement did what it was supposed to biochemically: βHB rose from 0.72 ± 0.33 mmol/L at rest to 1.69 ± 0.30 mmol/L post-exercise with the ketone drink, while the carbohydrate-only trial went from 0.80 ± 0.53 to 0.55 ± 0.30 (p < 0.001). Time-trial performance did not differ between conditions (1606 ± 138 s versus 1620 ± 134 s, p > 0.05). During the set-intensity bout there was no significant effect of supplementation on lactate, substrate oxidation, oxygen consumption at a fixed workload, perceived exertion or heart rate. The one difference was lower blood glucose at minutes 40 and 60 with the ketone drink.

The finding complicates the idea that keto-adaptation primes an athlete to benefit from exogenous ketones, since raising βHB clearly did not translate into faster times here. It is consistent with a broader pattern in which ketone supplements reliably shift blood chemistry without reliably shifting the stopwatch. The authors are explicit that with nine participants and limited statistical power this is an absence of a detectable effect rather than evidence of no effect. Larger trials, and work covering women and different exercise durations, remain the obvious next step.

This is a reason to be sceptical that a ketone drink will make you faster simply because it raises measurable ketone levels in the blood. Biochemical change and performance change are separate things, and only the first one happened in this trial. The lower blood glucose later in the steady-state bout is an interesting detail but was not accompanied by any change in fuel oxidation or perceived effort. Given the small sample and the very specific population of long-term keto-adapted men, treat this as an early signal rather than a verdict on the whole supplement category.