Effects of vitamin D supplementation in older African American women

Abstract

Context: Serum 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?

Objectives: The 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 setting: This was a randomized, double-blind placebo trial at Creighton University Medical Center and Indiana University Medical Center.

Participants: Participants were 110 healthy older African American women.

Interventions: The 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 measurements: Change in serum 25OHD and serum PTH levels at 12 months was measured.

Results: Mean 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.

Conclusion: Vitamin 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.

Trial registration: ClinicalTrials.gov NCT00472823.

Figures

Figure 1.

Consort flow chart for study participants. BMD, bone mineral density.

Figure 2.

Vitamin D dose response curve. Final and 6-month serum 25OHD levels (nanograms per milliliter and both nanomoles per liter) are presented according to the dose of vitamin D IU/d or placebo. Serum 25OHD at 6 and 12 months was significantly lower in the placebo group compared with all the vitamin D dose groups individually (P < .05). The blue line is the regression line for 6-month serum 25OHD values, and the black line is the corresponding regression line for the 12-month values The red diamonds represent the values at baseline for each group.

Figure 3.

African American data superimposed on the vitamin D response curve for Caucasians represented by the green lines. The green lines are the 95% prediction intervals for the Caucasian data derived from a published report (2). The blue and black lines are the regression lines for 6- and 12-month serum 25OHD values.

Figure 4.

Effects of different doses of vitamin D or placebo on serum PTH levels (picograms per milliliter) at 12 months. There is no significant effect of dose on serum PTH. The regression lines are shown for baseline data (red), 6-month values (blue), and 12-month values (black).

Figure 5.

Effect of different doses of vitamin D on serum PTH (picograms per milliliter) at 12 months according to the serum 25OHD (nanograms per milliliter). There is a significant decrease in serum PTH with increasing serum 25OHD level (P = .008). The regression lines for African American values are shown as a black line; log PTH = 1.6387–0.00381serum 25OHD and the regression line for Caucasian values as a green line; log PTH = 1.5833–0.00392serum 25OHD (data for Caucasians is from reference 2).