LONDON -- A study by a German doping laboratory has found that humans can inadvertently ingest clenbuterol from eating meat, a finding that would support claims by Alberto Contador and other athletes that contaminated beef caused their positive drug tests.
The German Sports University lab in Cologne -- accredited by the World Anti-Doping Agency -- is warning athletes of the risks of accidental clenbuterol doping when traveling to China.
The lab carried out an investigation that found that 22 out of 28 travelers returning to Germany from China tested positive for low levels of clenbuterol. The samples were tested between Sept. 15 and Jan. 15.
"The figures are most probably due to a food contamination problem, potentially caused by misuse of clenbuterol as growth promoter in stock-breeding," the lab said in a statement.
Clenbuterol is on WADA's list of banned substances as an anabolic agent that builds muscle and burns fat. It is also used by farmers to bulk up livestock. In China, in particular, clenbuterol's illegal use in farming is well documented.
The German findings were presented at a doping workshop Monday in Cologne. Prof. Wilhelm Schanzer, director of the Cologne lab, said the results will be submitted to sports federations for their consideration.
"We've shown there is a problem and that athletes should be careful," Schanzer told The Associated Press in a telephone interview Tuesday. "You should try to be careful with those things that you eat."
The Cologne study appears to offer the strongest proof yet that athletes can test positive inadvertently from food. It challenges WADA's principle of strict liability, which holds that athletes are responsible for all substances found in their body.
The German table tennis federation recently decided not to ban Dimitrij Ovtcharov after he blamed his positive clenbuterol test on contaminated meat.
Dozens of Chinese athletes have tested positive for clenbuterol in the past three years and received bans despite claims of contamination.
Contador -- whose samples were tested at the Cologne lab last year -- blamed his positive clenbuterol test at the Tour de France on eating beef that was brought across the border from Spain. He was cleared of doping Tuesday by the Spanish cycling federation, which reversed an earlier recommendation to suspend him for one year.
Schanzer stressed that the Cologne study did not involve athletes. He declined to comment on Contador or any specific cases.
"We only made a scientific investigation, and it's up to federations to decide how this issue will be handled," he said. "We are delivering scientific data independent from any case, so this problem should be discussed and people should be aware of this problem, especially in sports drug testing."
Schanzer said the people tested in the study were told they could eat whatever they wanted. They came from Hong Kong, Shanghai, Beijing and other cities.
"There was no clear trend," he said, adding that researchers plan to expand the study to a larger group.
Schanzer said the clenbuterol problem appears mainly centered in China and possibly in Mexico -- "those countries which had a problem with illegal substances for animal feeding."