Map of life expectancy at birth from Global Education Project.

Wednesday, September 13, 2006

You might want to try this . . .

You may have heard about the Japanese study that finds that drinking green tea is significantly protective against dying from heart disease and stroke. (Abstract here.) If you want my opinion, this is about as persuasive as an epidemiological cohort study (as opposed to a randomized controlled trial) can get.

The effect is very large, particularly for women, and particularly for people who drink a lot of it. Women who drank 5 or more cups of green tea a day had 26% lower total mortality over 11 years than abstainers; men had 17% lower mortality. For heart disease specifically, the effects were even more dramatic. The researchers controlled for everything they could think of, but it didn't make much difference, except that the effect was weaker in men who had been, or were currently, smokers.

Now, this kind of study can't prove that drinking green tea is really the cause of this effect. It could conceivably be something associated with green tea drinking that they just didn't think of and couldn't control for. But it's hard to see what that might be. And it might not work as well in other populations because it might depend on other dietary, environmental or lifestyle components prevalent in that part of Japan. But the effect size is so large it seems awfully compelling.

Black tea, by the way, didn't work in this study. (Black tea comes from the same plant, but it's fermented, so some of the chemicals in the tea leaves are destroyed.) It will be bad if people decide that if they drink green tea, it's okay to get fat, not exercise, smoke, and eat doughnuts. But if you don't use it as an excuse for doing bad stuff, it looks like a winner. Oh, and it's fine to drink it cold.

(BTW, unlike other studies, this one didn't show any protective effect against cancer. In fact, cancer death rates were non-significantly higher for the green tea drinkers, although I suspect that's because they weren't being killed off first by heart disease, so they were still around to die of cancer.)

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