(Updated 17 January 2011) Vitamin D is in the news a lot , mostly in stories that suggest “more is better.” But vitamin D has been of interest to the public since the 1920s, when its role in preventing the childhood bone disorder rickets was first understood.

RDA, DRI, and IUs: What’s the Difference?

Recommended Dietary Allowances (RDA) is an old term. The Food and Nutrition Board of the US Institute of Medicine (IOM) now uses nutrient reference values entitled Dietary Reference Intakes (DRI). These include RDAs in some cases and "Adequate Intakes" (AIs) in others where evidence for a recommendation is weaker. The vitamin D DRI is listed as AIs. They are expressed in micrograms per day, which can be converted to International Units, IUs (5 mcg vitamin D is equivalent to 200 IU).

Many expert panels make recommendations that differ from the Board’s, and usually express the recommendations in IUs. In 2010, the Board studied its vitamin D recommendations. (For an update about the Board's decision, go here.)

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