Many in the news media claimed that high coffee consumption would prevent Parkinson's disease based on a report of an observational study of 8000 Japanese-American men in Hawaii. Although this was a large study, only 100 men developed PD during 30 years of follow-up.
Japanese men drink more tea than coffee.
Caffeine from non-coffee sources was significantly protective, but much less so than that from coffee. A strength of the study is that coffee consumption was assessed while the participants were all healthy. But there was the assumption that coffee intake did not change during the study. An observational study like this cannot prove cause and effect. Most of the cases in this study occurred at older ages, and early and late onset PD appear to have different degrees of genetic and environmental factors.