Nearly one-half of Americans are deficient in vitamin D. There are geographic and ethnic variations for a condition that can be prevented or controlled by exposure to sunlight or supplements. That’s why a new study challenges a one-size-fits all approach to guidelines for vitamin D supplements. Researchers at Trinity College Dublin say their findings show need for tailored supplements. Their first insight is identifying ambient levels of UVB, a segment of sunlight that promotes vitamin D levels. Northern latitudes have less than where its sunny more of the year. Perhaps more important is age, sex, body mass, cholesterol levels and vitamin D supplements influence how we absorb UVB. The study is in the Journal of Clinical Nutrition.