"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."