In a recent study a group of researchers managed to separate the normal fat-burning effects produced by exercise and compare them to what happens when green tea is drunk before the exercise.1
The two-part study featured 12 young men, average age 26. In the first part they took a capsule containing ingredients equivalent to 3 1/2 cups of green tea three times a day, the last one hour before exercise. Other subjects got a placebo containing corn flour. The exercise consisted of 30 minutes of cycling at an intensity equal to 60 percent of maximum heart rate—low-intensity exercise that past studies have shown is optimal for fat oxidation. In the second part of the study 11 of the men took an oral glucose-tolerance test before and after taking green tea capsules.
Those who took the green tea capsules had a 17 percent greater contribution of fat to energy expenditure during exercise than those who took the placebo. That scale of fat oxidation was over and above what usually occurs during exercise, pointing to a definite green tea effect. The second part of the study showed that green tea increased insulin sensitivity by 13 percent, which reduces the insulin response to a glucose load by 15 percent.
Green tea is known to modulate glucose metabolism. It boosts the activity of GLUT4, the primary carrier of glucose into muscle, which itself would increase insulin sensitivity. It also mimics the effects of insulin cell receptor binding, thus boosting glucose uptake into cells. Through encouraging the oxidation rather than the storage of fat, green tea reduces intramuscular fat increase. That’s significant because that kind of fat can also produce insulin resistance—although not in those who work out regularly.
As for fat oxidation, one mechanism involves the inhibition of an enzyme that breaks down catecholamines, such as norepinephrine, which are involved in fat oxidation. Prolonging the activity of catecholamines results in greater fat oxidation. Green tea also elevates the amount of various proteins involved in fat transport and oxidation, especially if used for longer periods. Green tea is known to reduce the content of malonyl coenzyme-A, the substance produced by carbohydrates that blocks fat oxidation during exercise. The effect would be to increase the potency of carnitine transport of fat into the mitochondria for fat oxidation, a.k.a. beta oxidation.
Another study, this time involving isolated fat cells, found that green tea can inhibit the development of new fat cells.2 The mechanism involved an inhibition of GPDH, an enzyme, along with the inhibition of PPAR-gamma, a protein that boosts bodyfat. Green tea acts as an oxidant in fat cells—but that’s good because it activates AMPK, a protein that stimulates fat oxidation in muscle during exercise.
Green tea is not a miracle fat burner, and taking green tea supplements is not a license to eat whatever you want. It’s a dietary aid that enhances fat oxidation. Unlike many other touted “fat burners,” green tea at least has significant research backing up the claim. IM
1 Venables, M.C., et al. (2008). Green tea extract ingestion, fat oxidation, and glucose tolerance in humans. Am J Clin Nutr. 87:778-784.
2 Seuk-Moon, H., et al. (2007). Inhibitory effect of epigallocatechin-3-gallate on lipid accumulation of 3T3-L1 cells. Obesity. 15:2571-2582.