The IOC's top anti-doping official said he has seen no convincing evidence that athletes can inadvertently test positive for clenbuterol or other banned drugs by eating contaminated meat.
In an interview Friday with The Associated Press, Prof. Arne Ljungqvist said claims of food contamination in doping cases are ''old stories'' going back 30 years and have never been accepted by an international sports panel.
Ljungqvist, chairman of the International Olympic Committee's medical commission and vice president of the World Anti-Doping Agency, said he remains dubious of claims of accidental doping - a defense that has been used by Tour de France champion Alberto Contador and others.
Ljungqvist said the sports world must continue to work on the principle of ''strict liability,'' whereby athletes are held responsible for what's found in their system.
''Of course, any trial or court hearing would excuse a person if it could be 100 percent proven that it is an accidental matter, that it was totally beyond his or her possibility to check (for banned substances),'' he said. ''So far we haven't had such a case.''
Ljungqvist said he could not comment specifically on Contador, who was cleared by the Spanish cycling federation this month after blaming his positive clenbuterol test during last year's Tour on eating contaminated Spanish beef.
The international cycling federation, UCI, is studying the Contador ruling before deciding whether to appeal. The World Anti-Doping Agency also can appeal to the Court of Arbitration for Sport.
''It's not the first time that a national federation excuses their own athlete,'' Ljungqvist said in a telephone interview from Sweden. ''That's why we have this safeguard of an appeal system.
''We've had cases of that kind even as far back as the 1980s, and so far there has been no hearing panel that has accepted those arguments in the end,'' he added. ''We've had national panels that have excused those athletes, but when it came to the final decision, they have all been ruled guilty.''
In 2009, a CAS panel rejected Polish canoeist Adam Seroczynski's claims that his positive test for clenbuterol at the Beijing Olympics was the result of ''food tampering'' by organizers who fed him contaminated meat. The IOC's expert witness in the case dismissed the possibility of accidental contamination.
Clenbuterol is on WADA's list of banned substances as an anabolic agent that builds muscle and burns fat. It is also used illegally by farmers to bulk up livestock.
Ljungqvist served as chairman of the IAAF medical commission from 1980-2004 and was involved in many of the biggest doping cases in track and field.
''These are so much of the old stories,'' he said of the food contamination defense. ''It's not just clenbuterol. People claim they have been doped from what they had eaten.''
Ljungqvist recalled a hearing in the early 1980s of a Norwegian shot putter who blamed his positive test on food, a case which led to enactment of the strict liability rule.
''You have to make sure yourself that you don't get banned substances into your body in whatever way,'' he said.
Ljungqvist gave little credence to a recent study on clenbuterol contamination by the WADA-accredited lab in Cologne, Germany. The lab found that 22 out of 28 travelers returning to Germany from China tested positive for low levels of clenbuterol, most likely from food contamination caused by misuse of the drug in cattle farming.
The study was prompted by the case of German table tennis player Dimitrij Ovtcharov, who blamed his clenbuterol test on contaminated meat. The German federation decided not to ban him, and WADA chose not to appeal.
''To me this did not seem to be a scientific study with double-checking and control material and all this,'' Ljungqvist said. ''I haven't seen a properly designed scientific design to prove this really.''
Asked about evidence of clenbuterol contamination in China, he said, ''I don't know. If that were the case, I think we should have a number of cases.''
WADA director general David Howman said this week the agency has asked China for information on the use of steroids in raising cattle.
Zhao Jian, deputy director-general of China's Anti-Doping Agency, rejected the Cologne lab's findings.
Zhao called the German lab's warning about Chinese food ''problematic,'' questioned the science behind it and said it unjustly pointed a finger at China.
Zhao noted that Germany had a recent dioxin-tainted food scandal that led to the closure of thousands of farms selling eggs, poultry and pork. He also said there are dozens of clenbuterol cases reported by anti-doping labs around the world each year, not just in China.
''Food safety isn't a problem for just one country or one region,'' Zhao said. ''It's a global problem.''
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AP Sports Writers John Leicester in Paris and Graham Dunbar in Geneva contributed to this report.
