In this large observational study, we found that higher dairy consumption was associated with increased risk of Parkinson’s disease. The association was stronger in men and was mostly explained by milk consumption. Because this investigation was based on a prospective cohort with a long follow-up period and validated dietary assessment, recall and selection biases are unlikely the explanation of our findings. Reverse causality or confounding by known Parkinson’s disease risk factors such as age and smoking are also not very likely, because the association remained after excluding the first two years of follow-up or adjusting for relevant covariates. However, potential confounding by unmeasured factors such as a lack of novelty-seeking personality (15
) could not be excluded. Another potential limitation of the current study is that we were unable to physically examine individual Parkinson’s disease patients and had to rely on the diagnoses made by treating neurologists for case confirmation. Although a few misdiagnoses are likely, recent clinico-pathological studies revealed that 90 percent of neurologist diagnosed Parkinson’s disease patients could be confirmed at autopsy (16
). Further, any diagnostic error would have probably attenuated the association between dairy and Parkinson’s disease, because case identification was most likely independent of the dietary assessment.
The findings of the present study are consistent with those from two previous prospective investigations: in the first study (5
), men in the highest dairy consumption category had 80 percent higher risk than men in the lowest; in women, the results showed a slightly inverse “U” shape association with higher risk among women with moderate dairy consumptions. In the second study (7
), a study of Japanese American men in Honolulu, Hawaii, Park et al. reported that men with more than 16 ounces of milk consumption had 130 percent higher risk of Parkinson’s disease than those who did not drink milk.
In all studies, the results could not be attributed to measured dairy nutrients such as calcium. A pooled analysis of the current study with the previous ones confirmed a moderate positive association between dairy consumption and risk of Parkinson’s disease, particularly in men. In both the current and previous investigations, the relationship was less clear in women than in men. Among women in the Nurses Health Study, the RRs for dairy intake quartiles were 1.0 (referent), 1.3, 1.3, and 1.1 with a p value for linear trend of 0.9. In the current study, on the other hand, Parkinson’s disease risk among women increased approximately 60 percent at the second dairy consumption quintile and tended to remain at that level for higher consumption categories. However, both analyses were based on a relatively small number of cases; a prospective study with more female cases is needed to better evaluate this relationship in women.
So far, the epidemiological evidence suggests that the dairy and Parkinson’s disease association is unlikely due to calcium, vitamin D, or fat. All three studies generally found that calcium and vitamin D were positively associated with Parkinson’s disease risk only when they were from dairy foods, and fat from either dairy foods or other sources were not related to increased risk of Parkinson’s disease. Further, neither calcium nor vitamin D from supplements was significantly related to increased risk of Parkinson’s disease.
The observation of similar findings in all three well-established prospective studies on dairy and Parkinson’s disease risk suggests that the association is unlikely to be fortuitous. One possibility is that dairy products are contaminated with neurotoxic chemicals. Substantial epidemiological and experimental evidence suggest that pesticides may increase Parkinson’s disease risk (17
) and postmortem studies found higher levels of organochlorines, polychlorinated biphenyls, and dieldrin in the brains of Parkinson’s disease patients than in control brains (18
); some of these compounds present at low levels in dairy products. Further, chemicals such as tetrahydroisoquinolines (20
) and precursors of β-carbolines that induce parkinsonism in rodents and primates (21
) are present in a variety of dairy foods (23
). However, the overall contribution of dairy consumption to exposures to pesticides and other neurotoxins is probably only modest. Another potential explanation for the positive association between dairy and Parkinson’s disease may involve the potential effects of dairy products on circulating levels of uric acid. Higher dairy consumption has been related to lower circulating levels of uric acid and also lower risk of gout (25
). Uric acid has been hypothesized to be neuroprotective by preventing oxidative damage caused by reactive nitrogen and oxygen species, and higher plasma levels of uric acid have been prospectively linked to a lower risk of incident Parkinson’s disease in two cohorts (28
). Because of the lack of experimental data, any potential explanation is speculative.
In summary, accumulating evidence from this and previous prospective studies supports a positive association between dairy consumption and risk of Parkinson’s disease, particularly in men. Future epidemiological and experimental investigations are needed to further evaluate this association and to ascertain the underlying mechanisms.