In a study published in the November 2003 Journal of Nutritional Biochemistry, researchers provided additional evidence that green tea can act as a weight loss aid as well as the mechanism behind green tea's fat-burning effects.
The researchers investigated body fat-suppressive effects of green tea in rats fed a high-fat diet. The researchers also sought to determine whether green tea initiates thermogenesis (fat-burning) by activating beta-adrenoceptors in brown adipose (fat) tissue. Beta-adrenoceptors, located on fat cells, assist in the burning of fat and their activation causes the loss of fat in adipose tissue.
Feeding the animals a high-fat diet combined with a water extract of green tea (20 grams/kg) prevented the increase in body fat gain caused by a high-fat diet without affecting energy intake. Energy expenditure was increased by green tea extract, which was associated with an increase in protein content of brown adipose tissue. The simultaneous administration of the beta-adrenoceptor antagonist propranolol (500 mg/kg diet) inhibited the body fat-suppressive effect of green tea extract, indicating that green tea does indeed work by increasing the activity of beta-adrenoceptors. Propranolol also prevented the increase in protein content of brown adipose tissue caused by green tea extract. Digestibility also was slightly reduced by green tea extract and this effect was not affected by propranolol.
The researchers concluded, "It appears that green tea exerts potent body fat-suppressive effects in rats fed on a high-fat diet and the effect resulted in part from reduction in digestibility and to much greater extent from increase in brown adipose tissue thermogenesis through beta-adrenoceptor activation."
Choo JJ. Green tea reduces body fat accretion caused by high-fat diet in rats through beta-adrenoceptor activation of thermogenesis in brown adipose tissue. J Nutr Biochem. 2003 Nov;14(11):671-6.