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The Use and Interpretation of Assays for Vitamin D and its Metabolites^1,2,

Boston University School of Medicine, Vitamin D, Skin, and Bone Research Laboratory, Boston, MA 02118

Vitamin D is essential for the maintenance of calcium and bone metabolism in humans. The recommended daily allowance (RDA) for vitamin D in the United States of 200 KI (5.0 µg) is reasonable for adults who receive some exposure to sunlight; however, in the absence of any exposure to sunlight, this recommendation may be 2 to 3 times lower than that actually required to satisfy the body's needs. Vitamin D was first measured by bioassays. However, bioassays became obsolete in light of the revelation that vitamin D must be activated first in the liver to 25-hydroxyvitamin D (25-OH-D) and then in the kidney to 1,25-dihydroxyvitamin D [1,25(OH)₂D] before becoming biologically functional. Current assays measure circulating concentrations of vitamin D, 25-OH-D or 1,25(OH)₂D. The serum vitamin D concentration is of value for determining the role of sunlight in producing vitamin D in skin and as a provocative test to determine the absorption of vitamin D in patients with malabsorption syndromes. The serum concentration of 25-OH-D is most valuable for determining the overall vitamin D status of an individual, since it is an average of dietary and sunlight-induced vitamin D. The measurement of the serum 1,25(OH)₂D concentrations has been most useful in evaluating disorders in calcium and bone metabolism related to acquired and inborn errors in the conversion of 25-OH-D to 1,25(OH)₂D.

KEY WORDS: • vitamin D • vitamin D metabolites • assays • sunlight • 25-hydroxyvitamin D

¹ Presented as part of a conference, "Nutrition Monitoring and Nutrition Status Assessment", at the first fall meeting of the American Institute of Nutrition, Charleston, South Carolina, December 8–10, 1989. The conference was supported in part by cooperative agreement HPU880004-02-1 with the DHHS Office of Disease Prevention and Health Promotion, the USDA Human Nutrition Information Service, the DHHS National Center for Health Statistics, and the International Life Sciences Institute-Nutrition Foundation.

² The Planning Committee for the meeting consisted of Drs. Helen A. Guthrie, Roy J. Martin, Linda D. Meyers, James A. Olson, Catherine E. Woteki, and Richard G. Allison (ex officio). The symposium papers were edited by a committee consisting of Dr. James Allen Olson (coordinator), Dept. of Biochemistry & Biophysics, Iowa State University, Ames, IA; Dr. Cathy C. Campbell, Division of Nutritional Sciences, Cornell University, Ithaca, NY; Dr. Roy J. Martin, Dept. of Foods & Nutrition, University of Georgia, Athens, GA; and Dr. Catherine E. Woteki, Food & Nutrition Board, National Academy of Sciences, Washington, DC.

Manuscript received 11 February 1990. Revision accepted 11 July 1990.

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