What has happened?

One of Landis's samples showed high levels of testosterone after winning the Tour de France.© ALESSANDRO TROVATI/AP/EMPICS

In the past week, cyclist Floyd Landis and sprinter Justin Gatlin have both been singled out by doping tests - not for any new-fangled drug, but for the old-fashioned sex hormone testosterone. Landis made a spectacular comeback in the 17th leg of the Tour de France to take first place in the famous cycle race. But urine samples taken following that stunning performance showed unusually high levels of testosterone. And Gatlin, who shares the world record for the 100-metre sprint, tested positive for synthetic testosterone after a race in April this year. Both men maintain their innocence.

Why would anyone take testosterone?

The hormone, which occurs naturally in the bodies of both men and women, builds muscle mass, enhancing speed and aiding recovery from injury. Some experts have questioned the usefulness of taking testosterone, especially for endurance sports where muscle bulk can slow the body down. But the hormone is one of the more common findings of the labs at the World Anti-Doping Agency (WADA), according to David Cowan, director of the Drug Control Centre at King's College London.

How would they do it?

Athletes could swallow or inject a synthetic version of the hormone made from plant steroids. This confers the muscle-enhancing properties of testosterone without the sex-hormone side effects.

How do labs test for it?

Labs often look at urine samples for the ratio of testosterone to epitestosterone, which is produced alongside the natural version of the hormone. The normal ratio (T/E) in men is around 1:1, although there is a lot of natural variation from person to person, and the testosterone ratio thought to be "suspiciously high" has changed over the years. At the beginning of 2005, WADA set a threshold of 4:1 as the cut-off for declaring a test positive. The T/E ratio in one of Landis's samples was reported by his doctor to be 11:1.

But athletes could inject synthetic epitestosterone along with testosterone, making the test fallible. Samples from Gatlin were sent straight for more advanced analysis with isotope ratio mass spectroscopy (IRMS).

What's IRMS?

This technique, developed a few years ago, looks at isotopes of carbon in the various compounds in a urine sample. A person's natural hormones should contain a consistent ratio of carbon-12 to carbon-13. But synthetic testosterone tends to contain a smaller amount of the heavier isotope.

This test is considered to be far better than T/E ratios at distinguishing synthetic from natural testosterone. But it can't always be used, as there is often too little of the individual hormones, including testosterone, in a urine sample for a reliable result. "It is really remarkable when it works," says Cowan. "Unfortunately, it's not really sensitive enough in many of the sports cases to give us that miracle answer." Landis's sample is also undergoing this test; results are expected this weekend.

Might someone who has naturally high testosterone levels test positive?

It is possible — but rare — for an athlete to have consistently high T/E ratios. Diet or alcohol may affect the ratio on shorter time scales. But variations over days or weeks may be indicative of doping.

Are other naturally occurring substances used for doping?

"The only other substance that is widely used is creatine," says Michael Rennie, a clinical physiologist at the University of Nottingham, UK. "But that's not illegal - it's in meat." Taking creatine supplements increases an athlete's ability to produce power through ATP, the body's energy currency, so they can train harder. Rennie has occasionally come across athletes taking growth hormone to increase muscle mass, but there is little evidence that it has any positive effect on sporting results. It can also cause diabetes if taken long term.

What can be done about it?

Cowan would like to see all athletes individually profiled, creating an athlete's 'passport' system for monitoring them as a matter of course. If their normal levels are disturbed, one explanation would be that they are doping. Cowan hopes to see this system in place for the London Olympics. "It's a theme that I'm very keen on for London 2012, one that I keep pushing," he says.

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