• J Christopher Gallagher, Munro Peacock, Vinod Yalamanchili, and Lynette M Smith.
• Bone Metabolism Unit, Creighton University School of Medicine, 601 North 30th Street, Suite 6718, Omaha, Nebraska 68131, USA. jcg@creighton.edu
• J. Clin. Endocrinol. Metab. 2013 Mar 1; 98 (3): 1137-46.

ContextSerum 25-hydroxyvitamin D (25OHD) is lower in women with darker skin color. Is it due to lower skin production, lower absorption, or different metabolism of vitamin D?ObjectivesThe objective of the study was to measure the effect of vitamin D3 on serum 25OHD and serum PTH in older African American women with vitamin D insufficiency and the serum 25OHD 20 ng/mL or less (<50 nmol/L). The results can be used to estimate the Recommended Dietary Allowance (RDA).Design And SettingThis was a randomized, double-blind placebo trial at Creighton University Medical Center and Indiana University Medical Center.ParticipantsParticipants were 110 healthy older African American women.InterventionsThe intervention consisted of participants randomly assigned to placebo, vitamin D3 400, 800, 1600, 2400, 3200, 4000, or 4800 IU daily; calcium supplements were given to maintain total calcium intake of 1200-1400 mg/d.Main Outcome MeasurementsChange in serum 25OHD and serum PTH levels at 12 months was measured.ResultsMean baseline serum 25OHD was 13 ng/mL (33 nmol/L). On 4800 IU, serum 25OHD averaged 50 ng/mL (125 nmol/L) compared with 47 ng/mL (117 nmol/L) in Caucasian women. Serum PTH at 12 months decreased significantly (P = .008) when related to serum 25OHD but not dose. Hypercalcemia occurred in 7% and hypercalciuria in 15%. Events were unrelated to vitamin D dose.ConclusionVitamin D3 800 IU increased serum 25OHD greater than 20 ng/mL (>50 nmol/L) in 97.5% of the African American women just as it did in the Caucasian women, and therefore, the RDA is the same for both groups. Because absorption and metabolism of oral vitamin D absorption is similar in both groups, lower levels of serum 25OHD in African Americans must be due to lower production of vitamin D in skin.

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