Free Vitamin D: Concepts, Assays, Outcomes, and Prospects

Rene F. Chun, Carrie M. Nielson

Research output: Chapter in Book/Report/Conference proceedingChapter

7 Scopus citations


Total serum 25(OH)D is used clinically as a biomarker for vitamin D status. However, the biochemistry of its transportation, the features of its metabolism, and the mechanisms of its entry into different cell types combine to complicate its designation as the best biomarker of vitamin D sufficiency. Vitamin D-binding protein is the major transport protein of vitamin D metabolites, and it has been proposed that the small portion of 25(OH)D that is unbound (free 25(OH)D) or bound only to albumin (bioavailable 25(OH)D) could be relevant measures of vitamin D status. This chapter reviews the concepts behind free and bioavailable 25(OH)D, the studies that suggest their possible importance, the new assays that are used to measure or estimate them, and the clinical and epidemiologic studies that have evaluated them. The likely utility of these biomarkers is evaluated based on current knowledge, and prospects for the future are offered.

Original languageEnglish (US)
Title of host publicationBiochemistry, Physiology and Diagnostics
PublisherElsevier Inc.
Number of pages13
ISBN (Electronic)9780128099667
ISBN (Print)9780128099650
StatePublished - 2018


  • Albumin
  • Bioavailable 25-hydroxyvitamin D
  • Free 25-hydroxyvitamin D
  • Total 25-hydroxyvitamin D
  • Vitamin D-binding protein

ASJC Scopus subject areas

  • Agricultural and Biological Sciences(all)
  • Biochemistry, Genetics and Molecular Biology(all)
  • Medicine(all)


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