For decades, vitamin D was appreciated largely for its role in boosting the absorption of calcium, important for bone health. However, over the past decade and especially the past 5 years, research has linked a broad range of additional benefits to having ample vitamin D. It's shown that the nutrient fights cancers and diabetes, is the pivotal feedstock for a hormone that protects muscle, and inhibits autoimmune disorders from multiple sclerosis and lupus to inflammatory bowel disease.