Increased prevalence of vitamin D deficiency in patients with alopecia areata: a systematic review and meta‐analysis

Abstract Background Alopecia areata ( AA ) is a hair follicle‐specific autoimmune disorder. Vitamin D deficiency has been associated with various autoimmune disorders for its immunomodulatory effects. However, in previous studies, there had been inconsistent association found between AA and vitamin D deficiency. Objective To demonstrate the differences of the mean serum 25‐hydroxyvitamin D level and prevalence of vitamin D deficiency between AA patients and non‐ AA population. Methods A systematic review and meta‐analysis of observational studies on AA and serum vitamin D levels and/or prevalence of vitamin D deficiency was performed searching MEDLINE , Cochrane, Web of Science and Google Scholar databases. Results In all, 14 studies including a total of 1255 AA subjects and 784 non‐ AA control were analysed. The mean serum 25‐hydroxyvitamin D level was significantly lower in AA subjects (−8.52 ng/ dL ; 95% confidential interval; −5.50 to −11.53). The AA subjects had higher odds of vitamin D deficiency (odds ratio of 3.89; 2.02 to 7.49, mean prevalence of 73.8%; 59.1 to 84.6%). However, it was difficult to find clear correlation between serum 25‐hydroxyvitamin D level and extent of hair loss in AA subjects. Conclusion The AA subjects had lower serum 25‐hydroxyvitamin D level, and vitamin D deficiency was highly prevalent compared to non‐ AA controls. Hence, vitamin D deficiency should be assessed in AA patients. Furthermore, nutritional supplementation of vitamin D or topical vitamin D analogues can be considered for AA patients with vitamin D deficiency. The limitation of this study is the highly heterogeneity of the included studies.