There have been a moderate number of studies investigating the effects of catechins in green tea and caffeine on metabolic rate and fat burning. In respect to green tea
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 a paper published several years ago showed that intake of caffeine with 270 mg of EGCG, the main polyphenol in green tea, elevated 24-hour metabolic rate by 80 kcal/day compared to a placebo. Furthermore, EGCG caused a shift to greater fat oxidation. Studies indicate that single doses up to 1,600 mg EGCG are well tolerated and result in rapid circulating levels of EGCG. Higher doses of EGCG may not be necessary to elicit significant biological effects. For example, one study showed a significant increase in 24-hour energy expenditure with a dose of 90 mg of EGCG which was no different than the increase with 400 mg. Given the variable effects of green tea on metabolism, researchers set out to analyze the various trials using a statistical approach called meta-analysis. They found that collectively the studies indicated that catechin-caffeine mixtures and caffeine-only supplementation significantly increased energy expenditure by nearly 5% or roughly 100 kcal per day. Only the combination of catechins and caffeine resulted in a significant increase in fat oxidation. This analysis of work done on green tea catechins and caffeine provides compelling evidence that the combination does have a significant impact on resting metabolism and shift fuel to a greater reliance on fat.
  . Hursel R, Viechtbauer W, Dulloo AG, Tremblay A, Tappy L, Rumpler W, Westerterp-Plantenga MS. The effects of catechin rich teas and caffeine on energy expenditure and fat oxidation: a meta-analysis. Obes Rev. 2011 Jul;12(7)