For a time, vitamin D was touted as a potential miracle vitamin, thought to prevent everything from heart disease to cancer to diabetes. But several recent randomized controlled trials showed no significant benefit of vitamin D for any major condition. To be sure, vitamin D plays a vital role in health, but most people get all they need in several minutes of daily sunlight.

Why this matters: Overtesting by doctors of vitamin D serum levels remains widespread. Experts disagree about how to interpret the test and many doctors still unnecessarily recommend vitamin D supplements. The supplements represent more than a $1 billion market, despite the lack of evidence that they are necessary for the majority of people (not to mention the fact that vitamins are not independently tested for purity or dosing). 

What the experts say: “There’s a religiosity around vitamin D,” says Clifford Rosen, an endocrinologist at the Maine Medicine Center’s Research Institute. “The evidence is out there. People don’t want to pay attention to it.”