Green tea could mask testosterone doping.

Athletes who cheat by doping themselves with testosterone may be able to mask their actions by drinking green tea. Large quantities might even provide a legal performance boost by raising levels of testosterone in the blood.

Abuse of testosterone is hard to spot because the steroid hormone is found naturally in men and women. Tests rely on detecting an unusual ratio of testosterone to a hormone called epitestosterone in urine.

In lab tests, Declan Naughton and colleagues at Kingston University London discovered that compounds called catechins, found in tea, inhibit an enzyme called UGT2B17. This enzyme attaches glucuronic acid to testosterone, making it more likely to be excreted in urine.

Catechins are present in green and white tea but not in black. "Levels from a strong cup of green tea match those we used in our experiments," Naughton says.

Athletes dosing themselves with testosterone may be able to reduce the amount of hormone entering their urine by drinking green or white tea. Tea alone may also boost circulating levels of testosterone by blocking its excretion.

The researchers have informed the World Anti-Doping Agency in Montreal, Canada, of their findings. If the same effect occurs in human bodies, one answer would be to test athletes' blood, rather than urine.

WADA is aiming to add regular checks of blood steroids to its "biological passports", which monitor athletes for suspicious changes in their physiology. That should foil any attempt to fool the urine test, says Olivier Rabin, WADA's science director.

Rabin also doubts that tea alone will have much effect on performance, compared with the big boosts in testosterone seen in doping cases. "It would be fairly modest," he says.