Primary Research · 2009
The effects of green tea on weight loss and weight maintenance: a meta-analysis
Hursel R, Viechtbauer W, Westerterp-Plantenga MS · International Journal of Obesity, 2009
Key finding
Meta-analysis of 11 trials found green tea catechins modestly reduced body weight (~1.3kg) over 12 weeks, with effects attenuated in high-caffeine consumers.
Abstract
PubMed · PMID 19597519 →INTRODUCTION: Different outcomes of the effect of green tea on weight loss (WL) and weight maintenance (WM) have been reported in studies with subjects differing in ethnicity and habitual caffeine intake. PURPOSE: To elucidate by meta-analysis whether green tea indeed has a function in body weight regulation. METHODS: English-language studies about WL and WM after green tea supplementation were identified through PubMed and based on the references from retrieved articles. Out of the 49 studies initially identified, a total of 11 articles fitted the inclusion criteria and provided useful information for the meta-analysis. Effect sizes (mean weight change in treatment versus control group) were computed and aggregated based on a random-effects model. The influence of several moderators on the effect sizes was examined. RESULTS: Catechins significantly decreased body weight and significantly maintained body weight after a period of WL (microcirc=-1.31 kg; P<0.001). Inhibition of this effect by high habitual caffeine intake (>300 mg per day) failed to reach significance (microcirc=-0.27 kg for high and microcirc=-1.60 kg for low habitual caffeine intake; P=0.09). Also, the seemingly smaller effect of catechins in Caucasian (microcirc=-0.82 kg) subjects compared with Asians (microcirc=-1.51 kg; P=0.37) did not reach significance. Interaction of ethnicity and caffeine intake was a significant moderator (P=0.04). CONCLUSIONS: Catechins or an epigallocatechin gallate (EGCG)-caffeine mixture have a small positive effect on WL and WM. The results suggest that habitual caffeine intake and ethnicity may be moderators, as they may influence the effect of catechins.
Abstract sourced from PubMed, a database of the U.S. National Library of Medicine. Displayed in the authors’ own words for context; our critique is in the sections below.
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How to read a study like this
The same questions worth asking about any research paper, not just this one. Worth a minute even if you trust the grade.
Who was studied, and do you resemble them?
Supplement effects often depend on baseline status. Vitamin D helps people who are deficient; iron helps people who are anemic. A result in people unlike you may not apply to you.
What was measured, and does it matter in daily life?
A study that shows a blood marker moved isn't the same as a study that shows people felt or functioned better. Ask what the outcome means in practice.
How large was the effect — not just whether it was significant.
'Statistically significant' only means the effect is unlikely to be zero. It doesn't tell you the effect is large enough to notice. Look for effect sizes, not just p-values.
Who paid for the trial, and what did they stand to gain?
Industry-funded trials are several times more likely to report positive results than independent ones. It's not usually fraud — it's subtle design and reporting choices. Weight accordingly.
Has anyone else replicated this?
Single positive trials are hypotheses. Replication by independent groups is what turns a hypothesis into reliable evidence. If the only positive trial is the one you're reading, wait.
Does the dose in the trial match what's being sold?
Supplement marketing routinely cites trials that used 5–10× the dose in the product. If the effective dose was 2 g/day and the capsule has 200 mg, expect roughly no effect.
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