Many Americans have been warned that they’re deficient in vitamin D and need to take supplements to protect against all the purported ills of deficiency such as heart disease, diabetes, fractures, and a variety of cancers. Yet the latest review of research, conducted by a Tufts Medical Center team, suggests that many of the supposed benefits of supplementation remain unproven and that taking too high a dose of D can increase the risk of problems it’s supposed to prevent.