The banned substance clenbuterol has grabbed as many headlines as the riders in the build-up to this year's Tour de France after Alberto Contador's positive test during last year's race. But what is it? ITV.com's James Appell has the details
Barely a sentence has been spoken about the 2011 Tour de France without some reference to the case of Alberto Contador, who tested positive for a small amount of clenbuterol while on his way to victory in last year's event.
Contador was exonerated by the Spanish cycling federation, after he argued that the positive test was due to eating contaminated meat.
Some have argued that no cyclist would take clenbuterol during a race, while others claim the positive test points towards a blood doping offence.
Regardless, it's important to understand what clenbuterol is. Its primary use is as a treatment for breathing disorders such as asthma both in humans and animals - though it is illegal in the UK.
It acts by opening up the bronchi (airwaves within the lungs), relieving congestion and allowing the user to breathe more easily.
The by-product of this - which makes it attractive to athletes - is that it increases oxygen flow from the lungs into the blood, improving aerobic capacity and endurance. Using clenbuterol to artificially boost performance, a cyclist can pedal faster for longer.
In addition, taking clenbuterol has the added effect of enabling athletes to more effectively convert fat into muscle. In recent years it has become a lifestyle drug, taken by bodybuilders to aid the production of muscle, or by many women seeking a miracle drug for rapid weight loss.
The use of clenbuterol is not without risk, however. The increase in blood oxygen levels produced by the drug stimulates the production of red blood cells, which thickens the blood. This can cause huge strain on the heart as it tries to push the thicker blood around the body, and can lead to heart attacks.
Interestingly, however, a different use of clenbuterol's fat-burning potential - to produce leaner, firmer beef or lamb for human consumption - has somewhat muddied the waters in the case against Contador.
Investigations into the case of five Mexican footballers who tested positive for clenbuterol in June revealed that the use of the drug in rearing cattle in Mexico is "chronic". The players' cases are under review, but contaminated meat is now the prime suspect.
Even the World Anti-Doping Agency (WADA) have begun to make allowances for athletes unwittingly consuming clenbuterol in their food. German table tennis player Dimitrij Ovtcharov was cleared of any wrongdoing last October after successfully arguing that his positive test was caused by contaminated Chinese beef.
The Ovtcharov case may even have some bearing on the fate of Contador, who will attend a further hearing at the Court of Arbitration for Sport in August where his case will be conclusively ruled upon.
On top of all this, the UCI have recently announced that they are considering changing the rules on clenbuterol. At the moment, and trace of the substance triggers a positive test. But it may soon be the case that a minimal level of "clen" in the bloodstream would not result in a positive test and a ban.
Whatever the outcome, it's clear that Contador's case isn't quite as simple as many would make out.
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