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How much does green tea really affect body weight?

By Lori Bricker, MS, RD

green tea and body weightThere has been so much media hype over green tea and weight loss in the last couple of years that a lot of consumers are becoming very annoyed with what they perceive as slick marketing or outright lies.  It is extremely common, especially on the Internet, to find all kinds of “too good to be true” health claims for tea, particularly as it pertains to weight control, a topic of interest to so many.   As is frequently the case with exaggerated nutrition claims, there may be some truth behind the hype.  So how much can you really depend on green tea to help you slim down?  A new review published in The American Journal of Clinical Nutrition demonstrates how complicated this issue is and suggests that green tea is no magic bullet.

The authors of the paper searched for recent studies that had evaluated the effects of green tea catechins, both with caffeine and without, on body weight, BMI (body mass index), waist circumference, and waist-to-hip ratio.  To be included in the review, the studies had to be randomized and had to meet validity criteria as determined by use of the American Dietetic Association Research Design and Implementation Checklist.  This means that the research was, for example, bias-free, provided a clear statement of the research question, and used appropriate statistical analysis, among other things.

Fifteen research studies met these criteria and were evaluated in this paper.  The studies fell into three groups:   In seven studies, green tea catechins with caffeine were compared to caffeine-matched controls (we’ll call these Group 1); in six studies, green tea catechins with caffeine were compared to caffeine-free controls (Group 2); and in two studies, caffeine-free green tea catechins were compared to caffeine-free controls (Group 3).

They found the following:

  • For Group 1, doses of green tea catechins of 583-714 mg/day over a median of 12 weeks had a statistically significant benefit on BMI, body weight, and waist circumference, but no effect on waist-to-hip ratio.
  • For Group 2, green tea catechin intake of 141-1207 mg/day significantly reduced body weight, but had no effect on the other parameters.
  • For Group 3, (green tea catechin intake of 282-548 mg/day) there were no significant changes in any of the parameters.

So, off-hand, it looks promising with 2 groups showing “statistically significant” changes in the observed parameters.  But here are some things to note:

  • Just because a result is statistically significant doesn’t mean it is a big difference; it just means it is unlikely to be caused by chance.  For the statistically significant weight loss results observed for Group 1, the amount of weight lost averaged just 3 pounds over 12 weeks for the tea group compared to the control group.  That’s something, but it is not like these people lost 10 or 20 pounds in a two weeks by ingesting green tea catechins.  For Group 2, the results were even less impressive at an average of <1 pound.
  • On a similar note, the authors pointed out that the waist circumference measurements, although reduced by statistically significant amounts for Group 1 (an average of <2cm), practically speaking could have occurred as part of normal measurement error.
  • How much green tea catechin were the subjects consuming?  The studies evaluated here used varying amounts of green tea catechin, the equivalent of roughly 1-8 cups of tea per day.  This makes it very difficult to generalize, and the authors noted they were unable to determine if the effects of green tea catechin were dose-dependent.
  • There are also several different green tea catechins, and their specific effects are not fully understood.  Some studies reviewed here specified which catechins were given, but others did not.
  • The authors also noted that the ingestion of green tea catechins with or without food can affect its absorption (it is increased on an empty stomach), and these studies were inconsistent with respect to when catechins were given to the subjects.
  • The populations studied in these trials also varied and included children, men only, postmenopausal women, healthy adults, and adults with various conditions (such as obesity, diabetes, high cholesterol).   The authors pointed out that they were unable to say which of these populations might benefit the most from the green tea catechins.
  • Group 3 only contains 2 studies, so perhaps we shouldn’t generalize too much about decaffeinated green tea catechins.
  • Something to keep in mind in general when looking at tea studies is that how the tea is brewed can effect the quantities of catechin present in the final beverage, so that is often an unknown factor.  I also wonder if green tea catechin in capsule form has the same effects as that in brewed tea?  This question is not typically addressed.

The authors concluded that the observed reductions in weight loss and other parameters, although statistically significant in some cases, practically speaking are not all that important.  As a registered dietitian, if I were counseling someone on a weight loss plan, I would suggest drinking green tea if they like it (why not?), but I certainly would not make it a requirement and would explain that, while it appears to aid in weight loss a bit, it won’t have a dramatic effect.  Changing your overall diet to include more vegetables, fruits, whole grains, and fiber while getting plenty of exercise will do far more than green tea.

Source:  Phung, O.J., et. al. Effect of green tea catechins with or without caffeine on anthropometric measures:  a systematic review and meta-analysis.  Am J Clin Nutr 2010;91:73-81.

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