https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5187459/.........Sunlight exposure is essential to ensuring proper levels of circulating Vitamin D, since its synthesis is initiated in the skin with the photoconversion of 7-dehydrocholesterol to previtamin D [29]. Vitamin D is essential to maintain bone homeostasis, but it also has effects on the immune system.............
Yup, nude sunbathing.