Ketones are molecules produced by the liver during the breakdown of fatty acids. Supplemental ketones – referred to as endogenous ketones – are popular among many athletes due to their purported beneficial effects on performance. A new study shows that endogenous ketone supplementation impairs athletic performance in endurance athletes.

Researchers asked 23 well-trained adult cyclists to consume a ketone supplement (providing 0.35 grams of ketones per kilogram body weight) or a placebo 30 minutes before completing a 20-minute cycling trial on an exercise bike. The participants repeated the cycling trial a week later with the opposite beverage.

The researchers found that the participants' power output after taking the ketone supplement was 2.4 percent lower than after taking the placebo, regardless of the order in which they performed the two treatments. The cyclists' average heart rate during the trial was lower after taking the ketone ester, but other factors, such as peak heart rate, glucose levels, and perceived exertion, were similar between the two situations. Out of the 22 participants, 11 felt their perceived exertion was greater after taking the ketone ester, seven felt the same in both situations, and four felt less exertion after taking the ketone ester than when taking the placebo.

Although this was a small study, these findings suggest that exogenous ketone supplementation impairs performance in endurance athletes in a setting of short-duration, high-intensity training. However, some evidence suggests that ketone supplementation enhances performance in extreme settings in which hypoxia may be a factor, such as mountain climbing. Learn more in this clip featuring ketone expert Dr. Dominic D'Agostino.

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Coming soon: an interview with Dr. Martin Gibala, one of the investigators involved in this study.