Back in December I reported on a study which found that supplementing men with an extract of green tea led to a significant reduction in their weight, waist size and ‘fat mass’.
The extract in question, which goes by the name of epigallocatechin-3-gallate (EGCG), is believed to boost metabolism by stimulating the burning of fat in the body. Previously experiments had found that plying mice with extracts of green tea enhanced their ability to metabolise fat and reduced their tendency to gain weight.
Recently, a study was published which again assessed the physiological effects of green tea extract in humans. 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 which 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 hrs (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 metabolised fat.
Green tea extract enhanced the percentage of energy generated through the metabolism of fat by 17 per cent (the amount of energy derived from carbohydrate tell by the same amount).
In study two, blood sugar levels were unaffected compared to placebo by the green tea extract.
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 which 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 noradrenaline’s effects is to promote break down 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 which 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 reduced the risk of type 2 diabetes.
1. Venables MC, et al. Green tea extract ingestion, fat oxidation, and glucose tolerance in healthy humans. Am J Clin Nutr 2008;87:778-784