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Osteoporosis International

, Volume 23, Issue 9, pp 2283–2291 | Cite as

Defining physiologically “normal” vitamin D in African Americans

  • N. C. Wright
  • L. Chen
  • J. Niu
  • T. Neogi
  • K. Javiad
  • M. A. Nevitt
  • C. E. Lewis
  • J. R. CurtisEmail author
Original Article

Abstract

Summary

The relationship between serum 25(OH)D and intact parathyroid hormone (iPTH) was evaluated in the Multicenter Osteoarthritis Study (MOST). No further change in iPTH was observed for African Americans with 25(OH)D levels above 20 ng/ml, suggesting that compared to Caucasians, lower vitamin D targets for sufficiency may be appropriate for African Americans.

Introduction

Vitamin D levels ≥30 ng/ml are commonly considered “normal” based upon maximal suppression of iPTH; however, this has recently been challenged and the optimal 25(OH)D level among non-Caucasians is unclear. We evaluated the cross-sectional relationship between serum 25(OH)D and iPTH in a sample of Caucasian and African American adults.

Methods

We used baseline serum samples of participants from the Multicenter Osteoarthritis Study (MOST) for this analysis and used three methods to model the relationship between 25(OH)D and iPTH: ordinary least squares regression (OLS), segmented regression and Helmert contrasts.

Results

Among Caucasians (n = 1,258), 25(OH)D and iPTH ranged from 4 to 51 ng/ml and 2 to 120 pg/ml and from 3 to 32 ng/ml and 3 to 119 pg/ml in African Americans (n = 423). We observed different thresholds between African Americans and Caucasians using each analytic technique. Using 25(OH)D as a categorical variable in OLS, iPTH was statistically higher at lower 25(OH)D categories than the 24–32 ng/ml referent group among Caucasians. However, in African Americans, the mean iPTH was only significantly higher at 25(OH)D levels below 15 ng/ml. Using segmented regression, iPTH appeared to stabilize at a lower 25(OH)D level in African Americans (19–23 ng/ml) compared to in Caucasians (>32 ng/ml). Helmert contrasts also revealed a lower threshold in African Americans than Caucasians.

Conclusion

Among MOST participants, the 25(OH)D thresholds at which no further change in iPTH was observed was approximately 20 ng/ml in African Americans versus approximately 30 ng/ml in Caucasians, suggesting optimal vitamin D levels in Caucasians may not be applicable to African Americans.

Keywords

African American Osteoporosis Parathyroid hormone Racial differences Thresholds Vitamin D 

Notes

Acknowledgements

The MOST study is funded by the following grants from the National Institutes of Health: Boston University (David Felson, MD—1 U01 AG18820), University of Iowa (James Torner, PhD—1 U01 AG18832), University of Alabama at Birmingham (Cora E. Lewis, MD MSPH—1 U01 AG18947) and University of California San Francisco (Michael Nevitt, PhD—1 U01 AG19069). Assays for vitamin D and iPTH were supported, in part, by a research grant from the Investigator-Initiated Studies Program of Merck & Co., Inc to JRC. Dr. Curtis receives support from the NIH (AR053351) and AHRQ (R01HS018517). Dr. Neogi is supported by 5K23AR055127.

Conflicts of interest

None.

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Copyright information

© International Osteoporosis Foundation and National Osteoporosis Foundation 2011

Authors and Affiliations

  • N. C. Wright
    • 1
  • L. Chen
    • 2
  • J. Niu
    • 3
  • T. Neogi
    • 3
  • K. Javiad
    • 4
  • M. A. Nevitt
    • 5
  • C. E. Lewis
    • 6
  • J. R. Curtis
    • 2
    Email author
  1. 1.Department of EpidemiologyUniversity of Alabama at BirminghamBirminghamUSA
  2. 2.Division of Clinical Immunology and RheumatologyUniversity of Alabama at BirminghamBirminghamUSA
  3. 3.Clinical Epidemiology Research & Training UnitBoston University School of MedicineBostonUSA
  4. 4.Oxford NIHR Musculoskeletal Biomedical Research CentreUniversity of Oxford, Nuffield Orthopaedic CentreOxfordUK
  5. 5.Division of Clinical Trials & Multicenter StudiesUniversity of California San FranciscoSan FranciscoUSA
  6. 6.Division of Preventive MedicineUniversity of Alabama at BirminghamBirminghamUSA

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