... In essence, the picture seems to be that in fast metabolizers, a few cups of coffee a day reduces your risk of heart attack. The working theory is that some of coffee's other beneficial ingredients, like polyphenols, lower your risks. In these people, caffeine works its mood-boosting magic in the brain and then is cleared out of the body before it can have negative effects. (Once you get up to four-plus cups a day, the caffeine dose is high enough that the apparent benefits start to reverse.)

In slow metabolizers, on the other hand, the caffeine sticks around in the body after it has done its job in the brain, and this ends up raising your heart attack risk if you're drinking more than two cups a day. ...