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Green tea has become a very popular weight loss product in recent years. Epidemiological studies have shown that habitual tea consumption improves body fat % and body fat distribution (1). Green tea contains multiple compounds that can potentially induce fat loss, such as several green tea catechins (mainly EGCG) and caffeine.  In this article, I will explain the different mechanisms by which green tea could promote weight loss.

1) Via an increase in energy expenditure:

Energy expenditure is mainly increased after activation of the sympathetic nervous system (SNS) through the secretion of norepinephrine.  Already in 1975, it has been demonstrated that green tea catechins inhibit the enzyme catechol O-methyltransferase (COMT), which has the ability to degrade norepinephrine (2). As a result, green tea catechins prolong the presence of norepinephrine, thereby increasing energy expenditure. The caffeine in green tea works synergistically with the catechins, since it can inhibit phosphodiesterase. This enzyme degrades cyclic AMP, which is released in response to norepinephrine secretion. There is some evidence that green tea extracts containing both caffeine and green tea catechins, can increase 24h-energy expenditure by 4% and even up to 8%, depending in the dose that was administrated (3-4).

2) Through an increase in fat mobilization and oxidation:

It is generally accepted that norepinephrine can increase the release of free fatty acids from the body’s fat stores into the circulation. The previously explained effects of green tea catechins and caffeine on COMT and phosphodiesterase could therefore also indirectly increase fatty acid mobilization and oxidation. An increased mobilization and oxidation of fatty acids could potentially induce fat loss. Dulloo et al. (1999) have shown a decrease in respiratory quotient of 3.4% (4). Another study found similar results (-3.5%) on the respiratory quotient (5). The decreases in respiratory quotient indicate an increased dependence on fat as energy source, indicating that green tea could increase fatty acid mobilization and oxidation.

3) Through a reduction in appetite:

It was already mentioned that green tea can prolong the presence of norepinephrine. A longer manifestation of norepinephrine can increase energy expenditure, free fatty acid release, but it can also reduce appetite. In a study of Belza et al. (2009), an 8% reduction in ad libitum food intake was observed 4 hours after the consumption of 500mg green tea extract (6). This indicates a positive effect of green tea on appetite suppression.

4) Via a reduction in the absorption of nutrients:

Another mechanism, by which green tea might enhance weight loss, is through a reduced nutrient absorption in the gastrointestinal tract. Zhong et al. (2006) have shown through breath analysis, that a mixture of green tea, black tea and mulberry tea decreases carbohydrate absorption with 25%. They did not find any effect on fat malabsorption (7). However, in a polyphenol-enriched oolong tea (also rich in these catechins), Hsu et al. (2006) did find a 50% increase in the fat content of feaces, indicating an inhibitory effect on fat absorption (8).

Conclusion:

It seems that green tea can induce weight loss in 4 different ways. However, I selectively selected studies that found a positive effect. For each of the mechanisms, there were also studies that did not find any effect of green tea consumption. From the studies that did find an effect, it seems that an extract containing 200-300mg EGCG and 150mg caffeine should be sufficient to promote weight loss. These amounts are more easily obtained from a green tea extract, as opposed to drinking green tea.

Excerpt image by activespark

Bibliography:

1. Wu, C. -., Lu, F. -., Chang, C. -., Chang, T. -., Wang, R. -., & Chang, C. -. (2003). Relationship among habitual tea consumption, percent body fat, and body fat distribution. Obesity Research, 11(9), 1088-1095.
2. Borchardt, R. T., & Huber, J. A. (1975). Catechol O-methyltransferase. 5. structure-activity relationships for inhibition by flavonoids. Journal of Medicinal Chemistry, 18(1), 120-122.
3. Dulloo, A. G., Duret, C., Rohrer, D., Girardier, L., Mensi, N., Fathi, M., et al. (1999). Efficacy of a green tea extract rich in catechin polyphenols and caffeine in increasing 24-h energy expenditure and fat oxidation in humans. American Journal of Clinical Nutrition, 70(6), 1040-1045.
4. Bérubé-Parent, S., Pelletier, C., Doré, J., & Tremblay, A. (2005). Effects of encapsulated green tea and guarana extracts containing a mixture of epigallocatechin-3-gallate and caffeine on 24 h energy expenditure and fat oxidation in men. British Journal of Nutrition, 94(3), 432-436.
5. Boschmann, M., & Thielecke, F. (2007). The effects of epigallocatechin-3-gallate on thermogenesis and fat oxidation in obese men: A pilot study. Journal of the American College of Nutrition, 26(4), 389S-395S.
6. Belza, A., Toubro, S., & Astrup, A. (2009). The effect of caffeine, green tea and tyrosine on thermogenesis and energy intake. European Journal of Clinical Nutrition, 63(1), 57-64.
7. Zhong, L., Furne, J. K., & Levitt, M. D. (2006). An extract of black, green, and mulberry teas causes malabsorption of carbohydrate but not of triacylglycerol in healthy volunteers. American Journal of Clinical Nutrition, 84(3), 551-555.
8. Hsu, T. -., Kusumoto, A., Abe, K., Hosoda, K., Kiso, Y., Wang, M. -., et al. (2006). Polyphenol-enriched oolong tea increases fecal lipid excretion. European Journal of Clinical Nutrition, 60(11), 1330-1336.

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