How Much Vitamin D Do We Need?

Nobody knows.

Over the past year the Institute of Medicine (IOM) has issued one set of guidelines saying we don't really need very much, so supplements are unnecessary and possibly harmful for most people, and the Endocrine Society ("a professional association of 14,000 researchers and clinicians") released its own guidelines calling for much higher doses. Nature:

Why, instead of clearing confusion as was the IOM's goal, has the report sown division and unrest? "The IOM was too definitive in its recommendations," says Michael Holick, an endocrinologist at Boston University School of Medicine in Massachusetts, and an outspoken critic of the IOM panel's conclusions. "Basically, the vitamin-D recommendations are based on low-quality evidence," says Gordon Guyatt, a clinician researcher at McMaster University in Hamilton, Ontario, who has been a consultant on various guidelines. "I think admitting that would have made some of the angst disappear."

Poor data is one reason that the panel did not recommend higher doses, say interested observers. "We are not free to just accept enthusiastic reports, unless they are based on comprehensive, well-characterized data sets," says Paul Coates, director of the Office of Dietary Supplements at the NIH, one of the agencies that had requested the IOM's evaluation. "I think everyone wants to do the right thing, but I would say that the government is inherently more conservative." The episode demonstrates the difficulty of producing public-health advice from disparate and sometimes feeble evidence.

Nature found many researchers and clinicians who think the IOM's guidelines are very conservative, and more vitamin D would prevent many bone fractures and some cases of cancer. But nobody really knows.