Green tea extract is an extremely popular supplement that curbs appetite, increases metabolic rate and boosts energy levels in athletes and people trying to lose weight. This supplement appears to have few side effects other than elevated heart rate and small increases in blood pressure. However, a 12-week study in 60 young adult men and women showed that it had no effect on fat absorption, resting energy expenditure and body composition. The active ingredients in green tea include caffeine and epigallocatechin. Other studies have found that green tea has short-term effects on suppressing appetite and increasing metabolic rate (i.e., caloric expenditure). We can infer from this study that the effects of the supplement do not last.
Source: The Journal of Nutrition, published online March 4, 2015