Harmful Effects?
To date, the only negative side effect reported from drinking green tea is insomnia due to the fact that it contains caffeine. However, green tea contains less caffeine than coffee: there are approximately thirty to sixty mg. of caffeine in six - eight ounces of tea, compared to over one-hundred mg. in eight ounces of coffee.
How much Green Tea should you drink?
There are as many answers to this question as there are researchers investigating the natural properties of green tea. For example, Herbs for Health magazine cites a Japanese report stating that men who drank ten cups of green tea per day stayed cancer-free for three years longer than men who drank less than three cups a day (there are approximately 240 - 320 mg of polyphenols in three cups of green tea). Meanwhile, a study by Cleveland's Western Reserve University concluded that drinking four or more cups of green tea per day could help prevent rheumatoid arthritis, or reduce symptoms in individuals already suffering from the disease. And Japanese scientists at the Saitama Cancer Research Institute discovered that there were fewer recurrances of breast cancer, and the disease spread less quickly, in women with a history of drinking five cups or more of green tea daily.
Consider the following - One pound of fat (.45 kg) is equivalent to approximately 3500 calories. Whether using green tea for weight loss or any other method, in order to lose one pound of fat over the period of a week, a person must maintain a calorie deficit of 3500 below their normal "set point", that "set point" being the amount of calories consumed in one's diet where a person neither loses nor gains fat. This calorie deficit can come through the modification of diet, through exercise, or a through a combination of both. In the case of a pound per week weight loss, that works out to a needed calorie deficit of 500 calories per day.
Research by Japanese and other researchers(1) suggest that if the average person were to drink five cups of green tea a day, they would burn an extra 70 to 80 extra calories through an effect known as thermogenesis - the process of heat production in organisms. While much of the thermogenesis effect in green tea is derived from caffeine, studies also suggest(2) that thermogenesis in green tea occurs to some degree beyond its caffeine content from epigallocatechin gallate (EGCG.)
Regardless, 70-80 calories per day does not quite come close to the 500 calorie deficit needed per day to lose a pound a week, and for sure it's nowhere close to the calorie deficit needed to lose "inches per week" from one's waistline as some green tea web sites would have consumers believe. According to the research, if a person were to change nothing outside of adding five cups of green tea to their diet per day, then over the course of 43 days one could expect to lose 1 pound. Over the period of one year, one could expect a loss of about 8 pounds. Yes, there is some correlation between green tea and weight loss, unfortunately it's not on the level many would have you believe.
Clearly, drinking green tea alone is not going to be satisfactory for significant weight loss. Still, green tea can still be quite useful as a weight loss aid. The extra 80 calories per day can be just what a person needs to tip the scale from a calorie surplus to a calorie deficit, especially when following a balanced nutrition and exercise regime.
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(1) Dulloo AG, Efficacy of a green tea extract rich in catechin polyphenols and caffeine in increasing 24-h energy expenditure and fat oxidation in humans, Am J Clin Nutr 70: 1040-1050, 1999
(2) The Effects of Epigallocatechin-3-Gallate on Thermogenesis and Fat Oxidation in Obese Men: A Pilot Study Michael Boschmann, MD and Frank Thielecke, PhD. Universitary Medicine Berlin, Charité Campus Buch, Franz Volhard Center for Clinical Research, and HELIOS Clinic Berlin. J Am Coll Nutr. 26 (4); 389-3995. 2007