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

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

A researcher who reviewed study for WebMD says that while 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 significance of findings, and it is a good model to use to look at effects of caffeine and combination of caffeine and [plant] compounds that are present in green tea, but until better clinical trials are done in humans, it's hard to say what physiological significance of this actually may be."

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

She says green tea may have many health benefits due to its plant compounds, but cautions that it is not 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."

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