title: Vitamin D metabolites in captivity? Should we measure free or total 25(OH)D to assess vitamin D status?
abstract: There is general consensus that serum 25(OH)D is the best biochemical marker for nutritional vitamin D status. Whether free 25(OH)D would be a better marker than total 25(OH)D is so far unclear. Free 25(OH)D can either be calculated based on the measurement of the serum concentrations of total 25(OH)D, vitamin D-binding protein (DBP), albumin, and the affinity between 25(OH)D and its binding proteins in physiological situations. Free 25(OH)D can also be measured directly by equilibrium dialysis, ultrafitration or immunoassays. During the vitamin D workshop held in Boston in March 2016, a debate was organized about the measurements and clinical value of free 25(OH)D, and this debate is summarized in the present manuscript. Overall there is consensus that most cells apart from the renal tubular cells are exposed to free rather than to total 25(OH)D. Therefore free 25(OH)D may be highly relevant for the local production and action of 1,25(OH)2D. During the debate it became clear that there is a need for standardization of measurements of serum DBP and of direct measurements of free 25(OH)D. There seems to be very limited genetic or racial differences in DBP concentrations or (probably) in the affinity of DBP for its major ligands. Therefore, free 25(OH)D is strongly correlated to total 25(OH)D in most normal populations. Appropriate studies are needed to define the clinical implications of free rather than total 25(OH)D in normal subjects and in disease states. Special attention is needed for such studies in cases of abnormal DBP concentrations or when one could expect changes in its affinity for its ligands.
authors.affiliation: VA Medical Center and University of California San Francisco, San Francisco, CA 94158, USA. Electronic address: Daniel.bikle@ucsf.edu.
authors.forename: Daniel
authors.initials: D
authors.lastname: Bikle
authors.affiliation: Clinical & Experimental Endocrinology, KULeuven, Herestraat 49 ON1 Box 902, 3000 Leuven, Belgium. Electronic address: Roger.bouillon@kuleuven.be.
authors.forename: Roger
authors.initials: R
authors.lastname: Bouillon
authors.affiliation: Division of Nephrology, Massachusetts General Hospital, Boston, USA. Electronic address: rthadhani@mgh.harvard.edu.
authors.forename: Ravi
authors.initials: R
authors.lastname: Thadhani
authors.affiliation: Medical Research Council (MRC), Human Nutrition Research, Elsie Widdowson Laboratory, 120 Fulbourn Road, CB1 9NL Cambridge, UK; Department of Medicine, Faculty of Medicine and Health Sciences, University of East Anglia, NR4 7TJ Norwich, UK. Electronic address: i.schoenmakers@uea.ac.uk.
authors.forename: Inez
authors.initials: I
authors.lastname: Schoenmakers
mesh_terms: D000818:Animals; D006801:Humans; D007118:Immunoassay / Q000379:methods; D011110:Polymorphism, Genetic; D011485:Protein Binding; D014462:Ultrafiltration / Q000379:methods; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood / Q000378:metabolism; D014808:Vitamin D Deficiency / Q000097:blood / Q000235:genetics / Q000378:metabolism; D014809:Vitamin D-Binding Protein / Q000235:genetics / Q000378:metabolism; D014815:Vitamins / Q000097:blood* / Q000378:metabolism*
chemical_list: D014809:Vitamin D-Binding Protein; D014815:Vitamins; D014807:Vitamin D; C104450:25-hydroxyvitamin D
keywords: Centrifugal ultrafiltration; Free vitamin D metabolites; Genetic polymorphism; Keratinocytes; Liver disease; Pregnancy; Vitamin D binding protein