Green tea has a healthy reputation, principally because previous studies have linked its consumption with a reduced risk of cancer. In recent times, there has also been some evidence, both in animals and humans, that extracts of green tea may stimulate fat-burning in the body, which might help with weight control in the long term.
One particular element of green tea that has been singled out for attention goes by the name of epigallocatechin-3-gallate (EGCG). In a recent study, the physiological effects of a green tea extract rich in EGCG were assessed. The study, published in the American Journal of Clinical Nutrition, was in two parts:
1. In the first study, healthy men underwent a fitness test (VO2 max test). During the test, the rate at which they were burning fat was assessed. They were then given an extract of green tea, which contained a total of about 350 mg of EGCG, after which the fitness test was repeated.
2. In the second study, healthy men were given 75 g of glucose in drink form, after which blood sugar and insulin levels were measured for 2 hours (a glucose insulin tolerance test or "GITT"). Again, the men were given the green tea extract, after which the GITT was repeated.
The results of this study were as follows:
In study one, green tea extract was found to significantly enhance the rate at which individuals metabolized fat.
Green tea extract enhanced the percentage of energy generated by the metabolism of fat by 17 percent.
In study two, blood sugar levels were no different from the placebo or the green tea extract groups.
However, insulin secretion was significantly lowered by green tea extract. This seems to have been related to improved insulin sensitivity (which means that for a given amount of insulin, blood sugar control was better).
The authors of this study offer a number of explanations for these effects. For instance, green tea extract contains caffeine, ¬a substance that stimulates fat burning in the body. Green tea extract is also known to inhibit a substance (known as catechol 0-methyltransferase) which itself breaks down the nerve chemical noradrenaline.
In other words, green tea extract can boost the effects of noradrenaline. This is important because one of noradrenalin's effects is to promote breakdown of fat (lipolysis).
The study authors also suggest several mechanisms by which green tea extract may improve insulin sensitivity. They point out that studies in rats have found the green tea extract can increase the binding of insulin to fat cells (thereby increasing its effectiveness). They also present evidence that demonstrates that EGCG can mimic the effects of insulin.
What these results show is that green tea extract has the ability to enhance fat oxidation during exercise and helps insulin do its job. It might be, therefore, that long-term consumption of green tea may help with weight control and reduce the risk of type 2 diabetes.
Reference:
1. Venables MC, et al. Green tea extract ingestion, fat oxidation, and glucose tolerance in healthy humans. American Journal of Clinical Nutrition 2008; 87:778–784
Dr. John Briffa is a London-based doctor and author with an interest in nutrition and natural medicine.
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