A recently published paper in the Journal of the National Cancer Institute reports that heavy coffee consumption can reduce the risk of prostate cancer. This story made the rounds in the popular press as would be expected. The paper is based on a longitudinal study of 47,911 health professionals. What they found was that men who consumed more than six cups of coffee per day lowered their risk of developing prostate cancer by 18%. While this sounds impressive, one must weigh this with the fact that the probability of getting prostate cancer was around 10% over the 20 years of the study. So this means that six cups of coffee per day or more lowered the risk from 10% to 8%. A reduction yes, but probably not enough to start drinking massive amounts of coffee for this purpose alone. Stated in another way, the risk was reduced from 529 cancers per 100 000 person-years to 425 cancers. Now, the study also found that the reduction in risk for severe prostate cancer was around fifty percent. However, the risk of getting lethal prostate cancer is also lower so the risk drops from 79 cancers per 100 000 person-years to 34.
Now the problem with these epidemiological studies is that there are so many confounders, and although the authors were extremely careful in trying to account for them, they are still dealing with very uncertain data. Previous studies on the effects of coffee showed no effects on prostate cancer risk. There is also the problem of multiple comparisons. I’m sure the authors looked at risks for all sorts of diseases and this one turned out to be statistically significant. As I posted before (see here and here), many if not most high-profile clinical results turn out to be wrong and for good systematic reasons. Now, it is biologically plausible that coffee could have some effect in reducing cancer. It contains lots of bioactive molecules and antioxidants and we should test these directly. My take is that until there is a solid biophysical explanation for a clinical effect, the jury is always out.