Harvard Women's Health Watch

In the news: Report sets new dietary intake levels for vitamin D and calcium

In recent years, many studies have suggested that we take much more vitamin D than we do now — especially those of us living in northern climes who may get too little sunlight to produce adequate amounts in the skin. Many scientists have advocated vitamin D doses of 1,000 to 2,000 international units (IU) a day — much higher than the present recommended dose — to prevent a host of chronic conditions. But the report of an expert panel convened by the Institute of Medicine (IOM) concluded in November 2010 that high doses of vitamin D aren't necessary and might even be harmful. Many people — including many clinicians and researchers — were taken by surprise.

The IOM panel, which included Harvard Women's Health Watch advisory board member Dr. JoAnn Manson, reviewed nearly 1,000 studies, representing more than 15 years of scientific findings. The panel decided that vitamin D's importance for bone building and the prevention of bone disease was well substantiated, but the evidence for its role in preventing other conditions was inadequate to justify its use at much higher doses. Thus, the Recommended Dietary Allowance (RDA) for vitamin D was set on the basis of bone health alone: 600 IU a day for everyone through age 70 and 800 IU a day for those 71 and older. (In setting these standards, the panel assumed no access to vitamin D through sun exposure.)

To reach its decision, the panel evaluated data correlating bone disease with blood levels of 25-hydroxyvitamin D (25OHD), which is made from food and supplements and, in the skin, in response to sunlight. They determined that a blood level of 20 nanograms per milliliter (ng/ml) was adequate to prevent bone disease in at least 97.5% of the population. Citing a lack of standardized lab methods and reporting, the panel also cast doubt on the widespread practice of vitamin D testing. Many labs currently specify 30 to 70 ng/ml as the normal range; that would make most of the North American population vitamin D deficient.

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