Green Tea
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Green tea is thermogenic, which means it's a substance that induces your body to jack up its temperature, and, as a result, your metabolism. Therefore, green tea will actually help your body shift from relying only on carbohydrates to also burning fat for energy. (And since green tea naturally contains caffeine, it acts as a mild appetite suppressant.) For endurance athletes, this is a huge advantage, as it means they can go farther on less fuel, because the body is burning stored fat in lieu of stored carbs in the muscles. According to a Japanese study, green tea can actually help you go farther, while burning more fat, which will of course eventually better expose those abs you're so diligently working out. In order to notice an effect though, the research suggests you'd need to become a green tea devotee, consuming four cups per day, every day (no cheating either: mega-doses of green tea extract apparently don't count).