Tour de France champion Alberto Contador's claim that the steak he ate is to blame for his positive doping test is plausible, experts say, since the drug he's accused of taking is sometimes illegally given to beef cattle.

The drug clenbuterol is often used to speed up growth and increase muscle mass in animals, including chickens, cattle and pigs. And it's typically been used by people for bodybuilding.

Contador has been provisionally suspended after the International Cycling Union said a "small concentration" of clenbuterol was found in his urine sample on July 21 during the Tour de France, which the Spanish rider won for a third time.

Contador blamed the finding on "food contamination," saying he ate beef brought from Spain to France on a rest day.

Doctors said it would have been nearly impossible for Contador to have received any performance boost from eating clenbuterol-spiked meat.

"The amounts (of clenbuterol he consumed) would be incredibly small unless you were eating vast quantities of meat," said Dr. Andrew Franklyn-Miller, a sports medicine expert at the Centre for Human Performance in London and a team doctor for Britain's rowing team.

"It's very unlikely that the night before a stage in the mountains, anyone would be eating three or four steaks," said Franklyn-Miller, who said cyclists typically eat carbohydrates before a race because they are easier to digest.

Michael Audran, a doping expert who works closely with the World Anti-Doping Agency, said the amount of clenbuterol found in Contador's samples was so small it is unlikely the cyclist was abusing the drug. He said the contamination theory is the only possible explanation for the positive result.

Other experts said the doping tests need to be refined to include a threshold amount for clenbuterol, not simply whether or not the drug is present.

Spain has had several past reports of health problems linked to clenbuterol after people ate beef and veal containing it. In the 1990s, more than 100 people had symptoms including increased heart rate, muscle tremors, nausea, headaches, and anxiety blamed on the drug.

The drug is only legal in Europe for animals that aren't intended for human consumption, but experts say clenbuterol is sometimes found illegally in animal feed to produce more lean meat. It's usually stored in the animals' liver or muscle tissue.

Clenbuterol has been used for years by bodybuilders to increase their muscle mass and reduce fat. The drug also increases the body's aerobic capacity by making more oxygen available for muscles to work. Clenbuterol is also thought to help the body burn more fat, giving athletes a longer energy supply. Its short-term effects are similar to amphetamine drugs.

Athletes who use it can also suffer from side effects such as weight gain, loss of bone density and organ damage. Usage will also lead to decreased taurine levels which again will result in decreased athletic performance.

In some countries, clenbuterol is used to treat breathing disorders like asthma by relaxing the muscles in the airways, but it is not approved for use in humans in the United States and most of Europe.

Contador is not the only athlete to blame meat for a positive clenbuterol test. Last week, German table tennis player Dimitrij Ovtcharov used the same defense after testing positive for the drug, saying he ate meat loaded with clenbuterol during a tournament in China.

In Contador's case, "the quantity is the key to judging" whether it was a case of possible doping or consuming the drug by eating meat, said Giuseppe Banfi, a sports biochemistry expert at the University of Milan. "We don't have the proper parameters right now to know if this was illicit use."

He said that if Contador was doping with clenbuterol, authorities likely would have found more than one positive test result to show systemic drug abuse.

"WADA needs to set a detection limit (for clenbuterol), that is fundamental to defining if it is illicit use or an accidental finding," Banfi said.

Clenbuterol remains in the body for more than a day after it is ingested. The drug can be detected via hair and urine analysis.