Green tea is a very rich source of flavon-3-ol compounds that belong to the flavonoid group of phytochemicals. The flavan-3-ol compounds present in green tea are epigallocatechin gallate (EGCG), epicatechin (EC), epigallocatechin (EGC), and epicatechin gallate (ECG), often called catechins. Green tea consumption is inversely associated with cancers of the gastrointestinal tract. In addition, green tea is thought to confer weight loss effects on subject who regularly consume it. Green tea contains the methylxanthine compound caffeine, which is known to be thermogenic and cause weight loss at higher intakes. However, it appears that green tea displays weight loss ability above and beyond that which can be explained by the effects of caffeine. Of the catechins present in green tea EGCG has been researched and found to be the most likely compound to exert the weight loss effect.
For example, a paper published in the American Journal of Clinical Nutrition in 20081, investigated the effects of EGCG on healthy adults. The researchers took 12 healthy men and put them on an exercise bike at 60 % of their VO2 max. Supplementation of the men with 890 mg of polyphenols from green tea with 366 mg of EGCG resulted in a 17 % increase in the oxidisation rate of fatty acids. In a second trial, the researches took 11 healthy young men and gave them a two hour oral glucose tolerance test before and after taking the same supplement. Supplementation caused a reduction in the amount of insulin released in response to the glucose, which the authors reported as a 13 % increase in insulin sensitivity. These data suggest that ECGCG is able to alter carbohydrate and lipid metabolism is health adults.
Green tea EGCG is thought to increase fat oxidation because it is able to inhibit the enzyme catechol O-methyltransferase. This enzyme is responsible for the normal metabolic degradation of noradrenaline. Noradrenaline is known to increase lipolysis thought the adrenergic pathway via the β-adrenergic receptors on adipocytes in fat tissue. Many of the studies looking at the mechanism of action of EGCG have used animal models and so the exact metabolic action of the ECGC is not fully understood in humans. Rats fed or injected with EGCG do lose body fat, and some research has suggested this might be due to a reduced food intake. Cell culture models have also shown that EGCG is able to stimulate the thermogenic effects of brown adipose tissue, which is important in the production of heat in infants, but less important in adults.
Although the mechanism of action are not fully understood, EGCG would appear to be effective in aiding body fat reduction over the long term. Tablets and capsules of EGCG and other catechins are available, but tend to be expensive. If you wish to avoid caffeine, these supplements may be an effective weight loss tool, but you should ensure they contain standardised quantities of EGCG. The caffeine component associated with drinking green tea should further increase fat loss, because caffeine is known to increase resting energy expenditure and fat oxidation in response to exercise. Drinking green tea containing the entire spectrum of catechins will also likely confer a greater protection against the development of cancers of the gastrointestinal tract. Other types of tea such as traditional black tea and Chinese oolong tea do not contain the same levels of EGCG as green tea.
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