Vitamin D supplementation, moderate sun exposure, and control of immune diseases.

There is considerable debate about the benefits of vitamin D supplementation for multiple sclerosis, allergic asthma, and type 1 diabetes. This has been driven mainly by observational studies linking vitamin D deficiency and insufficiency with increased prevalence of autoimmune and other diseases driven by immune processes. Randomized controlled trials of vitamin D supplementation to treat these (and other) diseases have been disappointing. This review examines the evidence that circulating vitamin D levels provide a surrogate measure of sun exposure and that it is the other molecules and pathways induced by sun exposure, rather than vitamin D-driven processes, that explain many of the benefits often attributed to vitamin D.