PARIS — Dehydration is the latest possible reason offered for Tour de France winner Floyd Landis' elevated testosterone levels.

"Maybe a combination of dehydration, maximum effort," Jose Maria Buxeda, the cyclist's lawyer, said Thursday after testing began on his backup sample.

That theory was flatly rejected by one of the world's top anti-doping officials.

"In 25 years of experience of testing ... such a huge increase in the level of testosterone cannot be accepted to come from any natural factors," said Prof. Christiane Ayotte, director of Montreal's anti-doping laboratory.

"If dehydration was the case, then marathon runners would be testing positive all the time. Tennis players would be testing positive all the time. Dehydration is a medical condition that requires hospitalization. It has been invoked in the past, but not one case — to my knowledge — has been successful in this argument."

Phonak was notified July 27 that Landis turned up a testosterone-epitestosterone ratio of 11:1 — nearly three times the 4:1 limit — after speeding his way back into contention during Stage 17.

Ever since, the cyclist and his defense team have offered varying explanations, including cortisone shots taken for pain in Landis' degenerating hip; drinking beer and whiskey the night before; thyroid medication; and his natural metabolism.

The latest theory — dehydration — appears to contrast with events. Landis pushed ahead at the 45-mile mark July 20 and rode the rest of the stage alone, which he later called an "advantage" because he was constantly near his team's car and the liquids it carried. He constantly splashed his face and drank fluids as he rode up the Alps.

"It was nice to be alone," Landis said after the stage. "It was an advantage."