A team from the Japanese healthcare company Kao has found that mice given green tea extract regularly over 10 weeks increased their endurance in exercise by up to 24 per cent.
They explain in an online edition of the American Journal of Physiology, Regulatory Integrative and Comparative Physiology (10.1152/ajpregu.00693.2004) that green tea extract appears to stimulate the use of fatty acids by the muscle, reducting carbohydrate use and allowing for longer exercise times.
Green tea's effect on fatty acid uptake, speeding up fat breakdown, is also thought to be the reason why it helps weight loss, another area studied by the Kao researchers, and already targeted by supplement makers.
In the new experiments, on Balb/c mice swimming in an adjustable-current water pool, endurance exercise performance was boosted up to 24 per cent when the mice were given green tea extract at a dose of 0.5 per cent of their weight for 10 weeks.
It increased 8 per cent with a 0.2 per cent by-weight addition to food, showing that the effect was dose-dependant.
Like the weight loss category, the sports nutrition sector is growing rapidly, significantly outpacing growth in the more traditional vitamins and minerals categories. In the UK, second only to Germany in terms of size, sports drinks and supplements grew 37 per cent in 2002 to reach a retail value of £166 million in 2003, according to Mintel.
But while sports nutrition has been characterized by innovation and become known as one of the most dynamic segments of the nutraceuticals industry, annual growth in the US market - the world's biggest is slowing - and manufacturers are under pressure to develop novel products with new ingredients.
The Kao researchers claim that green tea extract may be one such opportunity, although results have to be confirmed in human trials.
Nor do they yet not understand the precise molecular mechanism by which green tea stimulates fatty acid metabolism, although the researchers suggest the antioxidant properties of tea catechins may play a role.
They note that in a second part of the study, looking at one of the most potent antioxidant components in green tea (EGCG) fed to mice on its own, they found evidence to show that EGCG has at least some role to play in the overall effects of the green tea extract.
"However, because the effects of EGCG appear weak compared with those of green tea extract, we cannot rule out a possible contribution from other components of green tea," said lead researcher Takatoshi Murase.
He added: "Although long-term intake of green tea extract enhanced endurance capacity, no marked effects were observed after a single dose, suggesting that some biochemical changes induced by habitual green tea extract intake, such as up-regulation of muscular beta-oxidation, contributed to the improvement in endurance capacity."
The study found that plasma NEFA (non-esterified fatty acid) measured immediately after exercise slightly, but significantly, increased in mice fed the tea catechins. Though they conceded that the effect of plasma fatty acid level on endurance capacity is controversial, the researchers said that increased supply of circulating fatty acids would "induce the uptake of fatty acids, and thereby stimulate lipid metabolism in muscle."
Indeed, lab results showed that muscular beta-oxidation was higher in green tea extract-fed mice than controls, suggesting that the supplement enhanced the capacity of muscle to catabolise lipids and use fatty acids as an energy source.
Taken together the experimental results "suggest that habitual exercise and the intake of green tea extract enhance fatty acid availability, catabolism and utilization in muscle, and this is accompanied by a reduction in carbohydrate use, which together result in prolonged swimming times to exhaustion."
The Kao researchers also controlled for possible influences of caffeine by reducing the amount in supplements, and possible weight-fat changes that might affect buoyancy.
Although difficult to extrapolate the findings to a human athlete, Murase said: "We estimate that an athlete weighing 75 kilograms would have to drink about four cups (0.8 litre) of green tea daily to match the effect in our experiments."