The LA Times has a good article discussing recent findings about whether coffee is good or bad for our health.
Definitive proof that coffee is good for the blood vessels is unlikely to emerge anytime soon, Hemmen says. Such studies would need to randomly select people to drink either a lot of coffee or a little coffee, and then researchers would have to closely monitor their coffee intake and health for decades.
What we currently have is circumstantial evidence, in other words, evidence of correlation only. Such evidence is not definitive for a number of reasons, including:
- The studies examined certain subgroups of the population, which makes generalizing to the whole population a bit of a stretch. For example, the 2008 Finnish study concerned male smokers who drink 8 or more cups a day compared to those who don't drink at all. How comfortable are you that this result applies to the larger population?
- Multiple studies with roughly similar findings do help increase our confidence in the result but having many studies does not mean that all relevant segments of the population have been covered, nor should we treat every study with equal weight. The Times did great to give us some details of each study, such as the sample size and the composition of the population under study.
- The data comes from "observational studies", meaning that people choose to drink whatever amount they choose. The challenge with this type of study is that we cannot eliminate the possibility that coffee drinkers tend to be more health conscious, and therefore, it's not the coffee that is doing the magic but other unobserved traits. Now, you may not agree that this correlation is plausible but you also can't claim it is not possible. As the article points out, research does show that there are differences in behavior between coffee drinkers and non-drinkers.
As one of the interviewees explained: "[The observed benefits] may be due to some other factors we haven't even taken into consideration." The reason why a randomized experiment, in which people are randomly assigned to drink either more or less coffee, is more reliable is that random selection ensures that if some unknown factor is affecting the results, it affects the results of both test groups evenly, not biasing the relative results.
Given the nature of these results, the Times does well not to hyperventilate, telling readers that "Such studies reveal that coffee isn't harmful, as once thought, and might even be beneficial."