Another obstacle in pinpointing the benefits of coffee is that it's difficult to isolate the effects of coffee from other healthy habits or lifestyles associated with coffee drinking. A 1999 study of coffee and tea consumption in Scotland, for instance, found that coffee drinkers were younger, had higher incomes, and were healthier in general than tea drinkers.
Coffee drinkers in the U.S. seem to fit a similar profile. Seventy percent of Americans with an annual household income of $150,000 or more drink coffee, compared with 54 percent of Americans in a household making less than $25,000 a year, according to consumer market research firm Experian Simmons.
Research has shown for decades that poorer people are more likely to die from virtually any cause than people with a higher socioeconomic status. Wealthier people are more likely to be physically active and eat healthier, and less likely to smoke -- behaviors that could prevent some of the conditions assumed to be affected by coffee.
"That's the problem [with most of the studies done on coffee]," says Vinson. "There's no perfect study out there because they can't control all the variables. The problem with a human study is everybody's different."
A similar warning should be in every single news article about the supposed health benefits of anything, because nearly all conventionally healthy activities correlate with each other. Unfortunately, such a warning only gets stuck into an article about stuff that’s not normally considered healthy.
The only time I’m confident that a health claim is true is when the condition in question is (1) not conventionally considered healthy; and (2) is not prevalent among the upper classes. Thus the continuing research which shows that it’s healthier to be a few pounds overweight is almost certainly the truth. Most everything else is in doubt.