We believe that there are 2 potential threats to the validity of Gunter and colleagues' (1) and Park and associates' (2) studies. Daily coffee intake usually begins at approximately 20 years of age in the context of adult student life or incorporation into the job market. Nevertheless, the median age at baseline in both studies was older than 50 years. Persons from the source populations may thus have been exposed to coffee for a median of 30 years before study entry. This prevalent user design may have allowed for a decrease in the number of susceptible persons (who may have had events before qualifying for study entry) from the coffee intake groups, resulting in an exposed study population of healthy survivors. Incident user designs—that is, those that are exposure-naive—are considered the gold standard in observational pharmacoepidemiology, because the selection bias derived from inclusion of prevalent users is minimized and the associations are more consistent with those from randomized trials (3). Of interest, in an analysis of the Johns Hopkins Precursors Study by Klag and colleagues (4) in which the baseline age was younger than 30 years (that is, closer to an incident user design), there was a strong association between coffee use and cardiovascular events in multivariable analyses.
Coffee drinking and mortality in 10 European countries