Paris - 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 "supplementary fuel for the muscles," said Jean-Jacques Menuet, the doctor for a rival team, Arkea-Samsic.
Jumbo won four of the first 11 stages on this year's Tour de France.
Mike Teunissen won the opening stage before Jumbo romped to victory in the team time trial the next day. Dylan Groenewegen won stage six in a sprint and Wout Van Aert took stage 10 on Monday.
On the Tour's rest day on Tuesday, Ineos team principal Dave Brailsford described Jumbo as "the most improved team over the past three years."
Brailsford, who has masterminded six Tour de France wins, said Jumbo had an "open-minded and expansive approach."
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 and €90," Menuet said.
Simon Verdonck, team doctor at the French team Cofidis, told AFP that ketones work by extending the amount of time before the body starts using its stored sugars as fuel.
"When you go full gas, your body uses glucids rather than lipids," said Verdonck.
"Ketones delays the use of glucids, saving them for the end of a stage."
The head of the International Cycling Union (UCI) David Lappartient said that ketones may require detailed monitoring.
"At the UCI we look at all elements which may modify performance and which may affect health," he said. "We would not 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."
Verdonck says he first heard of ketones five years ago but "the effects remain mysterious over the long term" and, until tests have been done on potential negative effects, Cofidis will not be using them.
"I don't want to receive a letter in 10 years from a rider telling me that his liver is ruined," he says.
Vincent Lavenu, of the AG2R team, has called for the use of ketones to be halted immediately.
"For equality in this sport we need a swift reaction," he said.