Vitamin D related to neurological disease

Vitamin D deficiency and Parkinson's disease. In the study referenced below a systematic review and meta-analysis were conducted on observational studies that reported the association between blood vitamin D levels and Parkinson's disease. The results indicated that patients with vitamin D deficiency experienced a twofold increased risk of developing Parkinson's disease. 

http://www.ncbi.nlm.nih.gov/pubmed/24847960