Analyzing vitamin D in foods and supplements: methodologic challenges.

This report briefly reviews existing methods for analyzing the vitamin D content of fortified and unfortified foods. The existing chemical methods are similar; all are time consuming, require experienced technicians, and have only been validated for a few materials (eg, dairy products or animal feed materials). This report also describes the lack of standard reference materials with certified values for vitamin D that laboratories need to guarantee the accuracy of existing analytic methods. Recently, the US Department of Agriculture, as part of a project to update the vitamin D values in the National Nutrient Database of Standard Reference, established an analytic methods committee to compare several existing vitamin D methods and to characterize 5 control materials (skim milk, processed cheese, cereal, orange juice, and salmon). Initial relative SDs for the 5 materials ranged from 35% to 50%. Elimination of systematic biases related to the methods and the standards yielded much more satisfactory relative SDs of 7% to 12%. This research has shown that existing methods for analyzing the vitamin D content in foods can produce accurate results. A new, simpler, and faster method, however, would greatly benefit the field. To guarantee accuracy, we need certified reference materials for foods.