In recent years, there’s been a deluge of news about the potential protective effects of green tea against cancer and heart disease. Research shows that most of green tea’s health benefits can be attributed to its high content of antioxidant compounds called polyphenols—more specifically known as catechins. A clinical study suggests that a green tea extract rich in catechins may actually help the body burn fat. Like black tea, which comes from the same plant (Camellia sinensis), green tea contains caffeine in addition to catechins. While there is some scientific evidence that caffeine itself can help burn fat, the results of this study suggest that the fat-burning effect of green tea was not due to its caffeine content alone.
The small, randomized, double-blind study involved ten healthy young men with body-fat ratings that ranged from lean to mildly obese. On three separate occasions, the participants took one of three different treatments three times a day with meals: a green tea extract containing 50 mg of caffeine and 90 mg of epigallocatechin gallate (an important tea catechin), a 50 mg dose of caffeine, or an inactive placebo. The men continued to eat a “typical Western diet” that provided approximately 13 percent of daily calories from protein, 40 percent from fat and 47 percent from carbohydrates. During the treatment periods, each man spent twenty-four hours in a respiratory chamber that enabled researchers to monitor his total energy (calorie) expenditures as well as his respiratory quotients (a measurement of the rate of fat oxidation). Men taking the green tea extract had a small but statistically significant 4 percent increase in energy expenditure, compared with no change for those taking either the placebo or the caffeine alone. In addition, respiratory quotients dropped for those taking green tea, meaning that their fat oxidation was increased and their carbohydrate oxidation decreased.
The study authors concluded that green tea stimulated the oxidation of fat and boosted thermogenesis—in other words, the expenditure of energy (calories) as heat. These effects were not caused by the caffeine in the tea extract, they said, pointing out that the men who took caffeine alone experienced no changes in thermogenesis or fat oxidation. However, they did speculate that the caffeine may have worked in concert with other tea compounds to produce the effects they observed. Because of the small number of participants, their study is far from conclusive, but it offers an interesting foundation for future research. Other plants currently under investigation for their thermogenic effects include cayenne pepper (Capsicum annuum) and bitter orange (Citrus ¥aurantium).
Green tea is probably best avoided by those who are sensitive to the effects of caffeine. However, when compared to coffee or even black tea, green tea is relatively low in caffeine. In addition, green tea is higher than black tea in polyphenols, because it undergoes only minimal processing after harvest.
Dulloo, A. G., et al. “Efficacy of a green tea extract rich in catechin polyphenols and caffeine in increasing 24-h energy expenditure and fat oxidation in humans.” The American Journal of Clinical Nutrition 1999, 70: 1040–1045.