People accuse doctors and researchers of never being able to make up their mind about certain things, such as how much exercise is healthy and how much is dangerous, should I eat eggs or not, and is coffee good or bad? Part of the reason we "cannot agree" is that each study tests a different group of people with different diseases and looks for different events over different time frames and using different statistical tests.
Well, this post is about coffee and it is a study in the "coffee is okay" category. The study comes from a cohort, which is basically where a whole bunch of people answer a lot of quesitons about their health and their habits, then they get rechecked several times over the following years. So, the people studied were not specially recruited to answer this specific question, and, in fact, the Nurses Health Study has had dozens of publications from it over the years. It started in 1976 and followed over 100,000 nurses for decades.
This is a good method of research because you can ask lots and lots of questions, but the drawback is that it is not conclusive. But, trying to find thousands and thousands of people and making them drink the same amount of coffee every day and then comparing that group to thousands of others not drinking coffee would be nearly impossible, so sometimes we have to settle for this kind of imperfect data.
So, long story short, a study of 12,000 nurses showed that over a 20 year period, regular coffee drinkers were no more likely to die than non drinkers. This was an especially important analysis because it took only the 12,000 nurses who had a history of a stroke or heart attack, suggesting that for people with these diseases, it is likely safe to drink caffeinated coffee. I mentioned these types of studies have limits in what conclusions we can draw and the authors pointed out that since the study was only in nurses, it may not reflect what is true in a general population. While the researchers used statistics to account for the effects of age, blood pressure, and diabetes, there could easily be many other factors that were not measured and therefore could not be accounted for.
So while we might not know the absolute truth about the safety of coffee, I will continue to enjoy mine.