A greater interest and awareness in the potential of green tea to promote health and prevent disease is underscored by the increased amount of research money dedicated to understanding the biological effects of the bioactive components of green tea. A quick search of the popular scientific search engine Medline with the keyword "green tea" reveals more than 1800 papers in the last 20 years with over a third of those in the last 2 years. Collectively, these studies show green tea to be associated with a wide range of biological effects that improve health.
The bioactive polyphenolic compounds in green tea account for a third of the dry weight of the leaves. The predominant polyphenols are the catechins and include epicatechin, epicatechin-3-gallate, epigallocatechin, and epigallocatechin-3-gallate (EGCG). Most research has focused on EGCG as the predominant active ingredient.
During these experiments, they consumed green tea extract containing 150 mg caffeine and 270 mg EGCG (divided into 3 doses), 150 mg caffeine or a placebo. The whole room respiratory chamber allows for continual measurements of energy expenditure and fat oxidation over a 24-hour period, as opposed to shorter time periods. There were no effects of caffeine compared to placebo but treatment with green tea extract was shown to increase 24 hour metabolic rate by 4% compared to a placebo.
This amounted to an increase in energy expenditure ranging from 65 to 200 kcal/day. This extra fat burning translates into about 0.7 to 1.5 pounds of fat loss over a month period. Furthermore, green tea caused a preferential use of fat over carbohydrate. No adverse effects were noted.
Researchers have found commercial green tea products to vary widely in EGCG content. To get 270 mg of EGCG per day would require drinking at least several cups of green tea and in some cases, more than 10 cups per day.
After 12 weeks, men consuming the green tea had a two-fold greater weight loss (-5.3 vs -2.8 lbs) and fat loss (-3.1 vs -1.5 lbs). Additionally, this study also assessed both subcutaneous and visceral fat in the abdominal region using computed tomography imaging. Compared to placebo, green tea had a dramatic four-fold greater effect on reductions in subcutaneous and visceral fat in the abdomen, a region associated with higher risk for many chronic diseases. The study shows that green tea is an effective method for shedding fat, particularly in the mid-section.
Another study confirming the fat-mobilizing effects of green tea comes was published the International Journal of Obesity.  This study examined different doses of EGCG, the bioactive compound in green tea, in a mouse model of diet-induced obesity. In a dose-dependent manner, EGCG reduced the amount of weight gain, which exclusively came from body fat. The study showed this effect was not due to a decrease in food intake but rather a reduction in the absorption of energy. EGCG also caused an increase in fat oxidation. These studies provide strong evidence for a role of green tea in lowering body fat.
Because of these unknowns, I think supplements that contain standardized extracts of EGCG are a good option. In fact, one recent study actually showed that supplements were better than tea. The study compared the effects of green tea, black tea, and a green tea extract in pill form that all contained the same amount of the flavanol EGCG.  Despite containing the same amount of EGCG, the absorption of total plasma flavanols into the blood after ingestion of the green tea extract supplement was far superior to the liquid teas. The green tea extract also led to higher antioxidant activity in the blood.
Another study found that ingesting supplements containing green tea catechins in a fasted state without food led to greater bioavailability compared to ingesting them with food. 
For weight loss, effective doses of EGCG are probably at least 270 mg a day divided into two or three doses. Some studies used higher or lower doses. For certain health conditions, you may need more. For everyday general health, you may need less.
This safe herb is emerging as a must-have daily addition to your diet. It may provide you with fat-loss benefits and antioxidant protection against a variety of illnesses.