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Green tea extract may not be an equal opportunity fat loss supplement

Does this common fat loss supplement actually work? And how might it produce its effects?

Study under review: Long-term green tea extract supplementation does not affect fat absorption, resting energy expenditure, and body composition in adults

Introduction

Green tea extract is often marketed as a weight loss or fat burning supplement. This is partially based on tradition, but also on studies that have noted an increase in energy expenditure and fat oxidation[1] after supplementation of green tea catechins in combination with caffeine.

Catechins are thought to be some of the active components of green tea. There are four main types of catechins: epicatechin, epicatechin gallate, epigallocatechin, and epigallocatechin-3-gallate (EGCG). EGCG is the most abundant, and thus many green tea extract supplements are standardized to 50% EGCG. Short-term studies have noted beneficial effects at doses of about 400-600 mg EGCG daily. For perspective, one cup of green tea contains about 50 milligrams of EGCG. As shown in Figure 1, teas that are more processed than green tea have a greater percentage of catechins converted to other polyphenols, in addition to having more caffeine.

Figure 1: Processing tea impacts the kind of polyphenols it contains

Source: Adapted from Jane Higdon, Ph.D., http://lpi.oregonstate.edu/research-newsletter

The decrease in body fat, increase in resting energy expenditure, and increase in fat oxidation seen in some studies with green tea extract have been attributed to the combination of catechins and caffeine. The proposed mechanism for the effects of catechins is through the inhibition of catechol-O-methyltransferase (COMT). COMT deactivates compounds like dopamine and adrenaline. Inhibition of COMT by green tea extract could result in higher levels of circulating adrenaline, which could increase fat burning and energy expenditure. In addition, caffeine stimulates heat production and fat oxidation through inhibition of phosphodiesterase. In an alternate mechanism, it has been proposed that catechins may also decrease fat absorption by inhibiting pancreatic and gastric lipases. Previous research[2] has suggested that green tea extract might be more effective for weight loss in Asians than in Caucasians, so the purpose of this study was to determine whether supplementation with green tea extract for 12 weeks had an impact on energy expenditure and dietary fat absorption in Caucasians.
Green tea is thought to aid body fat loss through a combination of the effects of its caffeine and catechin content, which may increase resting energy expenditure as well as decrease fat absorption. The goal of this study was to evaluate these effects in a Caucasian population.

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Other Articles in Issue #08 (June 2015)