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I read with interest the finding that total dairy and milk consumption were significantly correlated with incidence of Parkinson’s disease (PD) in a large study in Greece [1]. However, the component of dairy and milk that might provide the link was not identified and the possible mechanism was not proposed. This letter does both.
A study from the Health Professionals Follow-Up Study identified milk protein, which is primarily casein, as the component of milk correlated with incidence of PD [2]. At about the same time, I also found that milk protein had the highest correlation (direct) with PD mortality rates in an unpublished 58-country ecological study of dietary factors in which I separated milk into its components fat, protein, and nonfat milk as well as included many other dietary supply factors in the analysis.
The mechanism whereby casein increases the risk of PD is evidently through reducing serum urate or uric acid concentrations [3]. Urate or uric acid is a strong antioxidant, scavenging superoxide, peroxynitrite and hydroxyl radical [4], and has been found inversely correlated with prevalence [4] and risk of PD [5]. Ref. [4] found significant inverse correlations of uric acid with PD disease duration and daily levodopa usage for men but not for women. This finding could help explain the marginally non-significant finding of PD risk for women compared to men in Ref. [1].
References
Kyrozis A, Ghika A, Stathopoulos P, et al. Dietary and lifestyle variables in relation to incidence of Parkinson’s disease in Greece. Eur J Epidemiol. 2013 Feb 2. doi:10.1007/s10654-012-9760-0.
Gao X, Chen H, Choi HK, et al. Diet, urate, and Parkinson’s disease risk in men. Am J Epidemiol. 2008;167:831–8.
Bartges JW, Osborne CA, Felice LJ, et al. Influence of two amounts of dietary casein on uric acid, sodium urate, and ammonium urate urinary activity product ratios of healthy beagles. Am J Vet Res. 1995;56:893–7.
Andreadou E, Nikolaou C, Gournaras F, et al. Serum uric acid levels in patients with Parkinson’s disease: their relationship to treatment and disease duration. Clin Neurol Neurosurg. 2009;111:724–8.
Shen C, Guo Y, Luo W, et al. Serum urate and the risk of Parkinson’s disease: results from a meta-analysis. Can J Neurol Sci. 2013;40:73–9.
Conflict of interest
I receive funding from Bio-Tech Pharmacal (Fayetteville, AR), and the Sunlight Research Forum (Veldhoven) and have received funding from the UV Foundation (McLean, VA), the Vitamin D Council (San Luis Obispo, CA), and the Vitamin D Society (Canada).
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Grant, W.B. The role of milk protein in increasing risk of Parkinson’s disease. Eur J Epidemiol 28, 357 (2013). https://doi.org/10.1007/s10654-013-9786-y
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DOI: https://doi.org/10.1007/s10654-013-9786-y