Previous animal studies showed that green tea extract increased thermogenesis, which is the generation of body heat that occurs as a result of normal digestion, absorption, and metabolization of food. In previous human studies, the authors showed that consumption of green tea increased thermogenesis as well as energy expenditure and fat loss in healthy men, suggesting that green tea in liquid or capsule form may be an effective way to aid weight loss.

In the new study, conducted by Abdul Dulloo, from the Institute of Physiology at the University of Fribourg in Switzerland, researchers exposed a particular type of fatty tissue from rats to caffeine and to green tea extract containing small concentrations of caffeine.

Green Tea Containing Caffeine Can Raise Metabolism

Green tea containing caffeine significantly increased thermogenesis by 28% to 77%, depending on dose, whereas caffeine alone resulted in no significant increase. When the stimulant ephedrine was added to green tea with caffeine, the increase was even more significant compared with caffeine alone and ephedrine alone. Caffeine and ephedrine are used together in some herbal weight loss preparations, but there are many safety concerns regarding ephedrine because it raises heart rate and blood pressure.

Dulloo and colleagues also tested the plant compound EGCG found in green tea. They found that the stimulant ephedrine alone had no effect on thermogenesis, but that caffeine plus ephedrine resulted in an 84% increase. However, adding EGCG to the caffeine plus ephedrine mix increased thermogenesis even further.

"Our studies ... raise the possibility that the therapeutic potential of the green tea extract, or indeed a combination of EGCG and caffeine, may be extended to the management of obesity," Dulloo and co-authors write.

A researcher who reviewed the study for WebMD says that while the work is interesting and extends this group's previous findings by showing that compounds in green tea other than caffeine are involved in thermogenesis, caution should be used in interpreting animal data and applying it to humans.

"They used [a particular type of fatty tissue] from rats and we don't really know how significant that tissue is in humans or if it is different in obese vs. non-obese people," says Sheri Zidenberg-Cherr, PhD. "It doesn't rule out the significance of the findings, and it is a good model to use to look at the effects of caffeine and the combination of caffeine and the [plant] compounds that are present in green tea, but until better clinical trials are done in humans, it's hard to say what the physiological significance of this actually may be."

Zidenberg-Cherr, who is an associate professor of nutrition at the University of California, Davis, also points out that thermogenesis plays only a very small role in energy expenditure in adults. Most of the energy expended is used to maintain basic body functions such as breathing and the flow of blood throughout the body.

She says green tea may have many health benefits due to its plant compounds, but cautions that it is not the answer to weight-loss woes. "Green tea can't be used, and it shouldn't be used, as a 'magic bullet' for weight loss," she tells WebMD. "You've got to combine it with other changes, including increasing physical activity and reducing a high-calorie diet."