How Effective Are Vitamin D Supplements? Vitamin D is important for bone and muscle strength, but the belief that it significantly reduces the risk of heart disease, cancer and numerous other conditions is questionable. Although Vitamin D supplements are still widely used, well designed studies have established that the supplements are not needed by most people because we need only small-to-moderate amounts of vitamin D. Sunshine and dietary intake provide adequate vitamin D for most people. Dr. JoAnn Manson led one of the largest and most respected randomized trials that contradicted the belief that vitamin D supplements are a cure-all. In this discussion she explains the difference between natural vitamin sources, fortified foods, and supplements. She also addresses the importance of the structure of scientific studies in determining the effects of medications and supplements.