Image 1 of 3 Ketone supplements Image 2 of 3 Jumbo-Visma celebrate winning the team time trial at the Tour de France (Image credit: Bettini Photo) Image 3 of 3 Lotto Soudal riders try out the 2019 kit (Image credit: Bettini Photo)

The confirmation that several of the most successful teams at this year's Tour de France are using ketones has sparked debate amongst teams, and even a call for the revolutionary dietary aid to be banned to ensure that riders are competing on a level playing field.

Related Articles Ketone supplements used at Jumbo-Visma

Ketones are naturally created by the body during extreme diets, while synthetic ketones are classified as a food supplement rather than a drug. They are not on the World Anti-Doping Agency's list of doping products but reportedly give a 1.5 per cent performance boost and can significantly aid recovery during a three-week race like the Tour de France when taken at the right time.

The Jumbo-Visma team acknowledged to Dutch newspaper De Telegraaf that it was using ketones, with Lotto Soudal also telling Sporza they use them. Professor Peter Hespel of the University of Leuven, who works closely with the Deceuninck-QuickStep team, also pointed to ketone supplements as being "probably a piece of the puzzle" of the Belgian team's success.

According to Kieran Clarke, the British scientist who invented the ketone supplement, upwards of six teams were using ketones as a dietary supplement during the 2018 Tour de France but none of the teams wanted to be named. Reports of the use of ketones among the professional peloton dates back to the 2012 Olympic Games in London.

In July 2015, Sporza reported that Belgian coach Paul van den Bosch suggested that Chris Froome use ketone supplements. However, Team Sky team manager Dave Brailsford denied to Cyclingnews that his riders had ever used ketones.

The confirmation of the use of ketones, matched with the success of several teams this season, including at the Tour de France, has reignited debate about their use and impact on racing at this year's Tour.

"Ketones are a dietary supplement. You can use them just like vitamins. The substance is not on the prohibited list, and it's also known that other teams use ketones," Jumbo-Visma team manager Plugge told De Telegraaf.

Rival team doctors agree with the WADA definition but are concerned about the long-term effects drinking ketones could have on the liver. Team Sunweb told De Telegraaf that they do not plan to use ketones until they are sure of the consequences.

"They naturally occur when the liver turns lipids (fats) into glucid (sugar)," Jean Jacques Menuet, the Arkea-Samsic team doctor, told the AFP news agency.

"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, explaining why they have recently become more widespread in the peloton.

Simon Verdonck, team doctor at the French Cofidis team told AFP that ketones can extend the amount of time before the body starts using its stored sugars as fuel.

"When you go full gas, your body uses carbohydrates rather than lipids," said Verdonck. "Ketones delay the use of carbs, saving them for the end of a stage."

Verdonck says he first heard of ketones five years ago but admitted "the effects remain mysterious over the long term". According to AFP, Cofidis will not use ketones until tests have been done on potential negative effects. Menuet feels the same.

"I don't want to receive a letter in 10 years from a rider telling me that his liver is ruined," he said.

AG2R team manager Vincent Lavenu has called for the use of ketones to be stopped.

"For equality in this sport we need a swift reaction," he said.

UCI president David Lappartient was at the Tour de France on Thursday, but his hands seem tied by the World Anti-Doping Code, which regulates the use of medicines and anti-doping.

"At the UCI, we look at all elements that may modify performance and that may affect health," he said. "We wouldn't hesitate to take the initiative and refer the matter to the World Anti-Doping Agency as we have done in the past with tramadol and corticosteroids.

"I am confident that WADA will monitor all performance-enhancing drugs. Ketones aren't on the list of banned drugs, but we must monitor usage."