Vitamin D status and the risk of multiple sclerosis: a systematic review and meta-analysis.

To estimate the associations between vitamin D status and multiple sclerosis (MS). We searched electronic databases of the human literature in PubMed, EMBASE and the Cochrane Library up to February, 2014 using the following keywords: 'vitamin D' or '25(OH)D' and 'status' or 'deficiency' or 'insufficiency' and 'multiple sclerosis'. A systematic review and meta-analysis were conducted on observational studies that reported the association between blood vitamin D levels and MS. Eleven studies met the inclusion criteria. 1007 patients and 829 controls were included. Results of our meta-analysis show that MS patients had lower mean levels of 25-hydroxyvitamin D [25(OH)D] than healthy controls (weighted mean difference[MD], -14.52, 95% confidence interval [CI], -23.83 to -5.22). There were statistically significant heterogeneity (P<0.00001; I(2)=92%). The significant heterogeneity may be due to the differences in ethnicity, country, season of blood sampling and age of the participants studied. To sum up, low vitamin D levels are associated with an increased risk of MS.