Comment: The quality of evidence is downgraded by inconsistency (variability of results) and imprecise results (wide confidence intervals)
There were 18 randomized trials in a Cochrane review [Abstract] 1, with 1945 participants, examining the effect of green tea preparations for weight loss (15 studies) or weight maintenance (3 studies) in overweight or obese adults. The green tea weight loss preparations are extracts of green tea that contain a higher concentration of ingredients (catechins and caffeine) than the typical green tea beverage prepared from a tea bag and boiling water. Control groups received a beverage or a capsule with closely matched appearance but without active ingredients. Study length ranged between 12 and 13 weeks. Baseline characteristics of participants, including mean body weight and BMI, and the composition or dose of the green tea preparation did not create obvious heterogeneity. Instead, the country of origin was identified a source of heterogeneity: the Japanese studies provided more positive results.
Preparations of green tea used for 12 weeks in overweight or obese patients produced a very small, statistically significant loss of weight, and decrease in waist circumference compared to a control substance. There did not appear to be a trend between the reported daily dose of the cathecins and the effect size.
Twelve studies reported adverse events: in eight studies there were no adverse events reported by participants, and in four studies the prevalence of adverse events (ranging from 5 to 71%) was in two of the studies greater in control group than in intervention groups. The reported adverse events were most frequently mild to moderate hypertension and constipation, and they were judged by the authors to be unrelated to the green tea or control beverage.
| Outcome | Number of participants (trials) | Control:Placebo beverage or capsule | Intervention:Green tea preparation | Effect size (95 % CI) | Interpretation |
|---|---|---|---|---|---|
| Weight change in 3 months, all studies | 1562(14) | From -4.2 kg to +0.08 kg | Mean 950 grams more weight loss that in control group | MD -0.9 kg(-1.7 to -0.1) | Small difference in favor of green tea |
| Weight change in studies from outside Japan | 532(6) | From -4.2 kg to +0.08 kg | Mean 40 grams more weight loss | MD -0.04 kg(-0.5 to 0.4) | No significant difference |
| Change in BMI in 3 months, all studies | 1252(12) | From -2 to +0.2 BMI units | Mean 0,5 BMI units more reduction | -0.5kg(-0.9 to -0.2) | Small difference in favor of green tea |
| Change in BMI in studies from outside Japan | 222(4) | From -2 to +0.2 BMI units | Mean 0,2 BMI units more reduction | MD -0.2(-0.5 to 0.1) | No significant difference |
| Waist circumference change in 3 months, all studies | 1434(13) | From -4.2 cm to +0.3 cm | Mean 1 cm more reduction | MD -0.1(-1.8 to -0.2) | Small difference in favor of green tea |
| Waist circumference in studies outside Japan | 404(5) | From -4.2 cm to -0.2 cm | Mean 0.2 cm more reduction | MD -0.2(-1.4 to 0.9) | No significant difference |
Date of latest search:18.01.2012
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