Athletes using ketone supplements to boost their performance may be kidding themselves, researchers say.

The literature on the benefit of ketone supplements is mixed. Some studies have shown the supplements improve performance, but others say they have no effect or even worsen performance.

Natural ketones fuel the brain and muscles. A ketogenic diet of very low carbs and high fat causes the body to make more ketone compounds and increase their use for energy.

Ketone supplements are thought to speed up that process, without the strict diet, the researchers noted.

"One of the main perceived benefits is that ketones may serve as an alternative fuel source during exercise or potentially alter the utilization of other major fuel such as carbohydrates and fats, and in turn enhance endurance capacity," said researcher Martin Gibala, a professor in the kinesiology department at McMaster University in Ontario, Canada.

"But our findings suggest that isn't the case," he added in a university news release.

For the study, Gibala and his team tested 23 well-trained endurance athletes who cycled five or more hours a week, selecting them because their athletic performance is consistent from day to day.

Each participant completed two trials that differed only in the drink they were given before they completed a 20-minute cycling time trial that closely predicts 40-km race performance. The drinks contained a ketone supplement or placebo.

"The main observation from this study was that the speed that the cyclists could sustain during the test was lower after drinking the ketone supplement compared to the placebo," said Devin McCarthy, lead author of the study and a graduate student in the department of kinesiology at McMaster.

These findings agree with their earlier work that found ketone supplements increased cardiorespiratory stress during exercise.

The report was recently published in the International Journal of Sport Nutrition and Exercise Metabolism.