Green tea may soon show up in locker rooms and doping tests after being found to boost exercise endurance in mice up to 24% while spurring the use of fat as energy.

While the finding is based on green tea extract (GTE) and is difficult to extrapolate to human athletes, Japanese researcher Takatoshi Murase estimates that to match the effects athletes weighing 75 kilograms (165 pounds) would need to drink about four cups of green tea a day&emdash;and over several weeks.

"One of our important findings," says Murase, "was that a single high-dose of GTE or its active ingredients didn't affect performance. So it's the long-term ingestion of GTE that is beneficial."

Exercise boost

The study was conducted by Murase and colleagues at the Biological Sciences Laboratories of Kao Corp. in Tochigi, Japan&emdash;a company that makes green tea beverages and has been investigating the tea's anti-obesity effects.

The researchers say their findings show that green tea extract can boost exercise capacity and support the hypothesis that stimulating the use of fatty acids can improve endurance.

While acknowledging that the impact of dietary interventions on performance is controversial, the researchers note that compounds in green tea called catechins have already been found to have various physiological effects.

These include counteracting obesity from a high-fat diet, for which the researchers recently demonstrated evidence. This finding suggested that catechins stimulate fat oxidation.

It's thought that this might improve exercise performance by allowing the body to get energy from fat rather than carbohydrates during endurance activities.

Supplemental benefits

To test the theory that catechins boost endurance capacity by stimulating fat burning, the researchers used mice swimming in an adjustable-current water pool. Some of the mice received no green tea compounds, others received green tea extract and still others received only a catechin in green tea known as EGCG.

Mice on no supplements could swim about 33 minutes before they were exhausted. Mice on green tea extract consistently performed better after the first week and by week nine those taking 0.5% green tea extract by weight could swim 40 minutes compared to 33 for the controls. A similar effect was observed in mice on EGCG, suggesting that it was at least partly responsible for the benefits.

To support their theory about fat burning, the researchers found that fatty acids in blood increased slightly but significantly in mice on the supplements. They say that their findings suggest that green tea extract enhanced the ability of muscle to use fatty acids as an energy source.

To avoid potential complicating factors in other studies, the researchers controlled for possible influences of caffeine&emdash;a known performance enhancer&emdash;and changes that might have affected the animals' buoyancy.

The next steps are to determine the molecular mechanism by which green tea stimulates fat burning and whether the antioxidant properties of catechins mediate their effects on endurance capacity.