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Green tea for weight loss: does it really work?

Study under review: The effect of green tea supplementation on obesity: A systematic review and dose-response meta-analysis of randomized controlled trials

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What was the question?

This meta-analysis was designed to determine the effects of green tea supplementation on bodyweight and waist circumference, and to investigate a potential dose-response relationship to try to determine if an optimal dose exists.

Why was the question worth asking?

Green tea, a type of tea made from the fresh leaves of the Camellia sinensis plant species, has a long history of many uses, one of which is helping people manage their bodyweight. Green tea has been suggested to aid with bodyweight management because of its high concentration of catechins and epigallocatechin gallate[1] (EGCG).

EGCG, which constitutes more than 50% of the total catechin content in most green tea products, is proposed to exhibit anti-obesity effects through several mechanisms. As shown in Figure 1, these include: (i) increasing[2] energy expenditure, (ii) reducing[3] dietary fat absorption, (iii) increasing[3] fat utilization, and (iv) inhibiting[4] fat cell formation.

Figure 1: Some proposed mechanisms for EGCG’s anti-obesity effects

Reference: Thielecke et al. Phytochemistry. 2009 Jan.[3]

Although the purported anti-obesity effects of green tea supplementation have been examined in clinical trials and meta-analyses, no consensus has emerged due to conflicting results. As such, the goal of the study under review was to conduct a meta-analysis of the effects of green tea supplementation on bodyweight and waist circumference that would examine the potential sources of heterogeneity, and that would also include a dose-response analysis to try to determine if an optimal dose exists.

The potential anti-obesity effects of green tea have been evaluated in a number of studies. However, due to the heterogeneity of the results, no consensus has been obtained. The study under review is a meta-analysis that investigated the effects of green tea supplementation on bodyweight and waist circumference, which also examined the potential sources of heterogeneity and included a dose-response analysis.

How was the question answered?

The authors conducted a systematic review and meta-analysis of randomized placebo-controlled trials in adults over 18 years of age that looked at the effects of green tea supplementation compared to a control group on bodyweight and waist circumference. Studies were excluded if they included children or pregnant women, or if they used green tea in combination with other supplements. Ultimately, 25 studies comprising 26 comparisons (one study included two separate arms for male and female participants) were included in the meta-analysis.

The trials included between 18 and 104 participants with overweight or obesity and/or a variety of metabolic disorders. Studies ranged in duration from two weeks to five months, with an average duration of around 11 weeks. Out of the 26 trials, 14 included males and females, 11 included only female participants, and one included only male participants. The daily dose of green tea ranged from 99 to 20,000 milligrams per day, with most trials (n=20) using between 300 and 1,500 milligrams. In terms of outcomes, 22 trials (involving 2,357 participants) reported bodyweight, 22 trials (involving 1,124 participants) reported body mass index (BMI), and 13 trials (involving 685 participants) reported waist circumference.

The outcome data of the included trials were summarized in a random-effects model. Researchers conducted subgroup analyses for the different BMI categories, supplementation dosages, and treatment durations. Non-linear dose-response meta-analyses of green tea dosage and treatment duration were also conducted.

Heterogeneity was tested using the I2 statistic, publication bias was evaluated by visual inspection of the funnel plot, and the risk of bias of the included trials was assessed with the Cochrane Collaboration’s risk of bias assessment tool for RCTs[5]. The study was not preregistered, but did follow PRISMA guidelines.

This was a systematic review and meta-analysis of 26 randomized placebo-controlled trials that examined the effects of green tea supplementation on bodyweight, BMI, and waist circumference.

What was the answer?

The overall meta-analyses found that, relative to the controls, green tea supplementation resulted in significantly greater reductions in bodyweight (-1.78 kilograms or -3.92 pounds) with no significant heterogeneity between studies, and BMI (-0.65 kg/m2) with moderate levels of heterogeneity between studies. No significant effects on waist circumference were observed. These findings, along with their 95% confidence intervals, are shown in Figure 2.

Figure 2: This study’s main outcomes

In the subgroup analyses according to baseline BMI, bodyweight and BMI reductions were significant only in participants with overweight or obesity. In the subgroup analyses according to green tea dosage, bodyweight and BMI reductions were significant with doses of both less than 800 and at least 800 milligrams per day. In the subgroup analyses according to treatment duration, bodyweight and BMI reductions were significant only in trials that lasted 12 weeks or less.

The dose-response analysis showed evidence of a more pronounced reduction only in bodyweight with a green tea dose of under 500 milligrams per day and when the supplementation period lasted 12 weeks.

Finally, there was no evidence for publication bias, and most trials were assessed as being of good quality.

The findings of this meta-analysis indicate that green tea supplementation results in meaningful reductions in bodyweight and BMI in people with overweight or obesity over the course of 12 weeks.

How much should you trust the answer?

There is reason to be fairly confident in the results of this meta-analysis, as they are mostly based on good quality trials. It would have been informative, however, for the subgroup analyses to include testing according to study quality, so that the effects of green tea from only good quality trials could have been examined in isolation.

The lack of a significant degree of heterogeneity between the trials in the overall bodyweight analysis also boosts confidence in these results. On the other hand, the moderate levels of heterogeneity (48%) observed in the overall BMI analysis leave some room for uncertainty about the effects of various moderating factors. However, the subgroup analyses offer some insight into the potential sources of the heterogeneity, as no heterogeneity was observed in the subgroups that looked only at participants with obesity or with intervention durations of less than 12 weeks. Moreover, the fact that changes in both bodyweight and BMI moved in the same direction and in the same subgroups also boosts confidence in these results.

While the dose-response analysis showed evidence of a more pronounced reduction in bodyweight with a green tea dose of under 500 milligrams per day and when the supplementation protocol lasted 12 weeks, these results are greatly influenced by one outlier study[6] that examined the effects of a patented green tea product (Monoselect® Camellia) on bodyweight. This single study’s strong influence, which is made clear in Figure 3, suggests that this result should be taken lightly. In addition, the fact that the dose-response analysis did not show any other statistically significant effects of green tea dosage or treatment duration on BMI or waist circumference raises further doubts about the dose-response findings.

Figure 3: The dose-response analysis for bodyweight, with the outlier highlighted

Although the effect of green tea on waist circumference did not reach statistical significance, there was a trend for a reduction with green tea supplementation (-1.5 centimeters, p=0.08; 95% CI: -3.19, 0.19). Considering that these data are mostly consistent with a reduction in waist circumference, and that fewer trials looked at waist circumference as an outcome (n=13), compared to bodyweight (n=22) and BMI (n=22), the meta-analysis may have simply been underpowered to detect an effect of green tea on waist circumference.

The results of this meta-analysis suggest that green tea supplementation has a statistically significant and practically meaningful effect on reducing bodyweight and BMI, with a larger effect in people with overweight or obesity, and during the first 12 weeks of supplementation. While it’s possible that there may also be an effect on waist circumference, more research is needed to examine the effects of green tea on this outcome.

What’s the take-home?

This meta-analysis suggests that green tea supplementation, relative to control, reduces bodyweight and BMI in the short term in people with overweight or obesity.

Doses of about 500 milligrams per day for 12 weeks were found to be most effective for bodyweight reduction. However, this result should be interpreted cautiously because it rests heavily on a single study involving a proprietary supplement formula, and other measures like BMI and waist circumference didn’t show a clear dose-response pattern.

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See other articles with similar topics: Green Tea, Weight Loss, Bmi.

See other articles in Issue #70 (August 2020) of Study Deep Dives.

Other Articles in Issue #70 (August 2020)