Not long ago, vitamin D was riding high. Beyond its role in calcium homeostasis and bone health, animal studies linked vitamin D deficiency to numerous chronic illnesses including hypertension, diabetes, autoimmunity, and malignancy.1 Corroborating human observational studies reported associations between vitamin D deficiency and increased risks of hypertension, diabetes, cardiovascular disease, autoimmunity, and cancer.2 The lay press seized on this chorus of observational studies, testing of serum 25-hydroxyvitamin D levels proliferated, and supplementation with cholecalciferol (vitamin D3) and ergocalciferol (vitamin D2) increased substantially.3