The effect of melanism and vitamin D synthesis on the incidence of autoimmune disease

Melanin has several physiological roles in maintaining health, but, notably, it affects the synthesis of vitamin D. Melanin is the primary determinant of the degree of skin pigmentation and protects the body from harmful ultraviolet radiation. Synthesis of 1,25-dihydroxyvitamin D₃ (1,25[OH]₂D₃) in the skin, however, is dependent on ultraviolet B light. Highly pigmented skin, to the level found in people of African origin, abrogates almost all ultraviolet-induced 1,25(OH)₂D₃ synthesis. Numerous animal models and clinical studies have underlined the essential role of vitamin D as a modulator of the different processes of the immune system. Evidence indicates that serum concentrations of 1,25(OH)₂D₃ and the prevalence of autoimmune diseases in a certain population are associated with the latitude at which that population resides. This article explores the relationship between skin pigmentation, vitamin D and the prevalence of autoimmune disease.