Smoking and Parkinson's disease: using parental smoking as a proxy to explore causality.

In epidemiologic studies and in studies of discordant twins, cigarette smoking has been consistently associated with a lower risk of Parkinson's disease, but whether this association is causal remains controversial. Alternatively, an infectious or toxic exposure in childhood or early adulthood could affect both the reward mechanisms that determine smoking behavior and the future risk of Parkinson's disease. If so, parental smoking, commonly established before the birth of the first child, would be unlikely to be related to Parkinson's disease risk. The authors assessed the association between Parkinson's disease and parental smoking during childhood in the Nurses' Health Study and the Health Professionals Follow-up Study conducted in the United States. During 26 years and 18 years of follow-up, respectively, 455 newly diagnosed Parkinson's disease cases were documented among those who provided information on parental smoking. The age-adjusted, pooled relative rate of Parkinson's disease was 0.73 (95% confidence interval: 0.53, 1.00; P-trend = 0.04) comparing participants who reported that both parents smoked with those who reported that neither did. Adjustment for caffeine and alcohol intake did not materially change the results. If the inverse association between smoking and Parkinson's disease were due to confounding by an environmental factor or were the result of reverse causation, it is unlikely that parental smoking would predict Parkinson's disease.