Long recognized as important for bone health, vitamin D has attracted recent interest for its possible nonskeletal benefits. Many primary care clinicians now include blood tests to measure vitamin D concentrations as part of routine laboratory work1 and recommend vitamin D supplements, often at high doses, to their patients for the possible prevention of cancer, cardiovascular disease (CVD), diabetes, autoimmune disorders, cognitive decline, and other conditions. Thus, screening rates and sales of vitamin D supplements have increased substantially in recent years.1,2