Effect of vitamin D supplementation on total and free 25 hydroxyvitamin D and parathyroid hormone. An analysis of two randomized controlled trials

L M Smith, J C Gallagher, L M Smith, J C Gallagher

Abstract

Background: It is questionable as to whether total serum 25 hydroxyvitamin D (T25D) levels are lower in African Americans. We measured serum T25D, free 25hydroxyvitamin D (F25D) and serum parathyroid hormone (PTH) in African American and Caucasian women and studied the effect of vitamin D dosing to determine if differences by race or age occur.

Methods: Healthy young and older Caucasian and African American women who were vitamin D insufficient were randomized in two clinical trials to escalating daily doses of vitamin D from 400 to 4800 IU and placebo for 12 months.

Results: Baseline F25D and T25D were significantly lower in young but not older African American compared to Caucasian women. At baseline, the rate of change, or slope, in F25D with T25D was significantly greater in younger women than in older women, but difference in the rate of change in F25D with T25D is similar in African American and Caucasian women. After vitamin D supplementation, there was an increase in F25D, and the dose response was not significantly different by age or race. The ratio of F25D/T25D decreased in all groups once T25D exceeded ~60 nmol L-1 . There was a progressive decrease in serum PTH with increasing vitamin D doses and the per cent change was similar for F25D and T25D.

Conclusion: Serum F25D and T25D are lower in younger African American women, and since dietary vitamin D is similar in the groups, it is likely that the cause of low serum 25OHD in African American women is due to reduced UV exposure and reduced skin production of vitamin D.

Trial registration: ClinicalTrials.gov NCT00472823.

Keywords: free serum 25OHD; race; serum 25 hydroxyvitamin D; vitamin D supplementation.

Conflict of interest statement

Conflict of interest: The authors have no conflict of interest to report.

© 2019 The Association for the Publication of the Journal of Internal Medicine.

Figures

Figure 1
Figure 1
Relationship between serum free 25OHD and total serum 25OHD at baseline and 12 months using a spline.
Figure 2
Figure 2
Baseline comparison of serum free 25OHD and total serum 25OHD by age and race. No difference in was observed by race in older women, but significant age differences are observed as well as race difference in younger women.
Figure 3
Figure 3
12 month serum free 25OHD according to vitamin D dose levels by race in a) older women and b) younger women.
Figure 4
Figure 4
Percent change in PTH from baseline to 12 months by the change in a) serum free 25OHD and b) total serum 25OHD in African American and Caucasian women.

Source: PubMed

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