A systematic review and meta-analysis were conducted on seven observational studies that reported a relationship between Parkinson’s disease and blood levels of vitamin D. The review reported a strong relationship between vitamin deficiency and Parkinson’s disease. Those with vitamin D insufficiency (below 75 nmol/l) had 50 percent increase in risk, and those with vitamin D deficiency (below 50 nmol/l) had a more than double the risk (2.2 times) of the disease.

Lv Z, Qi H, Wang L, Fan X, Han F, Wang H, Bi S. Vitamin D status and Parkinson’s disease: a systematic review and meta-analysis. Neurol Sci. 2014 Nov;35(11):1723-30. PMID: 24847960.