This review examines the beneficial effects of ultraviolet radiation on systemic autoimmune diseases, including multiple sclerosis and type I diabetes, where the epidemiological evidence for the vitamin D-independent effects of sunlight is most apparent. Ultraviolet radiation, in addition to its role in the synthesis of vitamin D, stimulates anti-inflammatory pathways, alters the composition of dendritic cells, T cells, and T regulatory cells, and induces nitric oxide synthase and heme oxygenase metabolic pathways, which may directly or indirectly mitigate disease progression and susceptibility. Recent work has also explored how the immune-modulating functions of ultraviolet radiation affect type II diabetes, cancer, and the current global pandemic caused by SARS-CoV-2. These diseases are particularly important amidst global changes in lifestyle that result in unhealthy eating, increased sedentary habits, and alcohol and tobacco consumption. Compelling epidemiological data shows increased ultraviolet radiation associated with reduced rates of certain cancers, such as colorectal cancer, breast cancer, non-Hodgkins lymphoma, and ultraviolet radiation exposure correlated with susceptibility and mortality rates of COVID-19. Therefore, understanding the effects of ultraviolet radiation on both vitamin D-dependent and -independent pathways is necessary to understand how they influence the course of many human diseases.
Keywords: COVID-19; Cancer; Diabetes; Multiple sclerosis; Sunlight exposure; Ultraviolet radiation; Vitamin D.
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