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It’s not easy to get a handle on caffeine and health. One month scientists seem to say that it’s bad for you. The next month they say that a cup or two of coffee a day is harmless.

“Trying to link our health to what we eat is always tough, but it’s especially complicated with caffeine,” says Alan Leviton, a neuroepidemiologist at the Harvard Medical School.

That’s because we rarely consume caffeine by itself. We swallow it mixed with sugar or hundreds of other chemicals in coffee, tea, cocoa, and colas. And how much caffeine you get depends on the type of coffee or tea you drink, how it’s brewed, how big your mug is — even the type of coffee — maker you use. Researchers rarely have all those details.

To complicate the picture, decaf drinkers are more likely than other coffee drinkers to take care of themselves. They tend to take more vitamins, exercise more faithfully, and eat more cruciferous vegetables like broccoli. They’re even more likely to use seat belts when they drive.

And heavy-coffee-drinkers generally smoke more, drink more alcohol, and eat more fatty foods than non-coffee-drinkers.