Of all the medical orthodoxies of recent years, few were as ironclad as the prohibition against sunbathing. In a triumph of public education, the notion of a "healthy tan" was turned on its head, as conditions ranging from wrinkles to cataracts, immune-system problems and skin cancers, including deadly malignant melanoma, were linked to ultraviolet exposure. But in the last decade or so researchers have begun asking whether something was lost in the process: the often-overlooked substance that occurs naturally in some foods, especially fish, but is most efficiently produced in the body by exposure to sunlight—vitamin D.
