The chairman of the Dutch anti-doping agency says he feels "uncomfortable" with the Jumbo Visma team's use of ketones.

Hermann Lamm, who has headed the Dutch anti-doping agency since 2006, told De Limburger that the controversial supplement drink exists in a "gray area" and the official advice from his organization is not to use it.

Because ketones are produced in the liver when there are no more carbohydrates to burn and fat stores begin to burn, synthetic ketones are thought to maintain glycogen stores, reduce lactic acid, and provide an additional energy source to aid recovery.

Synthetic ketones are classified as dietary supplements, not drugs, and are not on the World Anti-Doping Agency's list of prohibited substances. However, their potential for performance enhancement, coupled with a lack of research on their long-term effects on health, has been a source of controversy. Nevertheless, the Jumbo-Visma team publicly stated at last year's Tour de France that they use ketones.

"Ketones are a legitimate nutrition, but at the same time little is known about their health effects.

"Therefore, it is a gray area. We are not on the doping list, but if we are asked by athletes, we advise them not to use ketones. Team Sunweb does not use them for that reason. I am uncomfortable with the fact that Jumbo Bisma uses them."

According to Kieran Clark, a British scientist who helped develop the ketone supplement, at least six teams used ketones in the 2018 Tour de France, but none of them wanted to be named. Last summer, Jumbo Visma confirmed its use of ketones along with Lotto Soudal. Along with Team Ineos, ketones have also been implicated in the success of Deceuninck Quickstep, although the latter has flatly denied it.

In response to Lam's comments, Jumbo-Visma nutritionist Askar Kukendrup defended the team's stance on ketones.

"We believe that ketones are natural and have no negative long-term health effects. We understand that Ram is saying this from his perspective, but if you have a good understanding of how ketones work, there is no need for such concern," Kuykendrup told de Limburger.

"We are doing our own testing because there is not a lot of research available. So we do our own research. We use it primarily in competitions, so different variables also affect it. So it will take years to fully understand how it works."

As for its supposed performance-enhancing effects: "It's not the panacea that many people think it is.