a little piece from an article by an MD
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There is one herb that has shown promise in a small-scale test and appears quite safe: green tea extract. In a study published in the December 1999 issue of the American Journal of Clinical Nutrition, researchers studied the effects of green tea extract on energy expenditure and fat burning in 10 men who took either the extract (which contains caffeine), caffeine alone, or a placebo. The men who took the extract increased their energy expenditure and burned more fat than did those who took either the caffeine alone or the placebo. No side effects were reported.
Green tea extract is available in health food stores in capsule and tablet forms. But bear in mind that although this research is promising, we still don't know whether people who take this extract will really lose weight. Nor is it clear whether drinking regular green tea might have similar effects. For shedding pounds, the tried-and-true method of exercising more and eating less is still your best bet.