Diseases affected by vitamin D: sun exposure

Abstract

Solar ultraviolet B (UVB) irradiance, through production of vitamin D, is important in human health. In response to solar UV doses, skin pigmentation adapted over millennia to where people lived, changing from dark in the African plains to light in Northern Europe. Many ecological (geographical) studies have inversely correlated about 20 types of cancer with solar UVB doses. Observational studies along with empirical data from two randomized controlled trials (RCTs) largely support these ecological findings. Research has generally revealed the mechanisms whereby vitamin D can reduce cancer risk. By inducing cathelicidin and defensins, solar UVB reduces the risk of several bacterial and viral infections. Bacterial infections with strong evidence of UVB/vitamin D effects include dental caries, periodontal disease, pneumonia, sepsis, and tuberculosis. Viral infectious diseases with similar evidence include Epstein-Barr virus-linked diseases such as Hodgkin’s lymphoma, multiple sclerosis, and infectious mononucleosis, as well as type A influenza. Solar UVB and vitamin D also reduce risk of other autoimmune diseases, including inflammatory bowel disease/Crohn’s disease, rheumatoid arthritis, and type 1 diabetes mellitus. Good evidence for beneficial roles of UVB/vitamin D in reducing risk of cardiovascular disease (CVD) comes from observational studies of prediagnostic serum 25-hydroxyvitamin D and ecological studies of the seasonality of CVD incidence or mortality rates. That cold temperature also appears to be an important risk factor for CVD tends to somewhat cloud the seasonality studies. However, the same effect applies for type A influenza, yet two RCTs found beneficial effects of vitamin D supplementation. Although RCTs are often called for as definitive proof that vitamin D reduces the risk of various types of disease, the criteria for causality in a biological system that A.B. Hill established in 1965 can serve as an alternative approach.