About half of U.S. adults take some sort of nutritional supplement. (My wife and I are in the other half.) Many supplements have a crucial specific function (vitamin C, for example, is the best and only treatment for scurvy) but their benefits to the general population are unproven. Vitamin D is frequently prescribed to prevent and treat osteoporosis. It is critical in calcium metabolism and in bone health. Some observational studies have found lower rates of death from cancer and cardiov...