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I wondered that too.
Quick search produced this poss reason: (NakedScience site)"When the body is cold ( and it often was when I was swiming) the blood vessels near the skin contract to keep the warm blood away from the skin. This saves losing heat to the water. However it also raises the blood pressure in the body. The body senses this and does something about it- it causes the excretion of some water to drop the blood volume."
"The sports scientist responsible for the salbutamol regulations that left Chris Froome fighting to save his reputation has admitted that the World Anti-Doping Agency (Wada) rules are flawed and need an overhaul because of the risk of false positives.
Ken Fitch said that he had to support Froome’s case, which he did with a written submission, because he felt that the Wada threshold, based on his studies, was catching innocent athletes. Professor Fitch believes that Wada’s statement clearing Froome of an adverse analytical finding (AAF) from La Vuelta last year was “unprecedented”.
Professor Fitch, who works for the University of Western Australia, told The Times: “The outcome of this is groundbreaking. It’s big not just for Chris but for asthmatic athletes and for the Wada rules. Most significantly, they have accepted that the salbutamol you take and the level in your urine do not necessarily correlate . . . They should have accepted it years ago.”
Those Wada regulations, including a maximum dose of 1,600 mcg per 24 hours (16 puffs) and a decision limit for an AAF of 1,200 ng/ml urinary concentration were based on work that Fitch led in the 1990s. Fitch was a member of the IOC medical commission for 28 years and pushed it to carry out studies to distinguish between oral and inhaled salbutamol.
“I’ll admit I made a terrible blunder,” he said. “The sport with the highest prevalence was swimming so that’s who we tested. But what happens after an hour of swimming? A full bladder. Cycling for five hours is completely different, you have little but quite concentrated urine. And a major error with our studies was that we did not measure the urine for specific gravity.
“From those studies came the threshold, which Wada increased to the 1,200 decision limit, but it was based on a false premise. The studies were never performed with the aim of finding the amount of salbutamol in urine after inhaling the allowable quantity. As I had a major role in these decisions, I acknowledge my error . . . I feel quite concerned about cases like Chris Froome.
“If I had wanted to clarify the salbutamol levels of athletes in urine after taking the permitted dose, I would have done multiple studies, administering different doses and collecting urine over a period of time, not just once an hour later. A number have been carried out . . . but they have shown the problem that the metabolism and excretion of salbutamol is capricious.”
Fitch, who served on Wada committees, has opposed Wada in cases, including that of Alessandro Petacchi, the Italian sprinter who served a one-year ban after a high salbutamol reading at the Giro d’Italia in 2007. Wada did not allow urine concentration to be corrected for specific gravity, ie dehydration, but changed the rules in the past year. “I was arguing [for that correction] in 2007. Petacchi was innocent . . . They [Wada] have to accept that the rules need changing,” Fitch said.
Dr Olivier Rabin, the agency’s director of science, has argued that “the rules are right” but said that the details of the Froome case would be sent to Wada’s listing committee for assessment.