The Tour de France is again embroiled in a debate over what cyclists put in their bodies after Dutch team Jumbo acknowledged they were using a dietary aid believed to boost stamina.

The team responded to questions from Dutch newspaper De Telegraaf by saying they were using drinks based on a substance called ketones, which occur naturally in the body.

Jumbo team manager Richard Plugge says the use of ketones is nothing unusual and is widespread among Tour de France riders.

“It’s a food supplement, like vitamins,” he told De Telegraaf. […]

Ketones are produced by the liver during intense dieting, but a laboratory version is now cheaply and easily available.

Ketones are classified as a food supplement rather than a drug and are not on the World Anti-Doping Agency’s list of doping products.

Two of three WADA criteria would have to be met before they are added — namely, if it enhances, or potentially enhances, performance, if it violates the spirit of sport or if it is an actual or potential health risk.

For the moment, ketones do not appear to be harmful.

“They naturally occur when the liver turns lipids (fats) into glucid (sugar),” Menuet, the Arkea-Samsic team doctor, told AFP, adding that this would happen during an extreme diet.

Artificial ketones, first created in laboratory conditions in the English city of Oxford, have the same effect.

“At first it cost thousands of euros for one bottle, but now you can get a bottle on the internet for between 30 euros and 90 euros,” Menuet said.

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