title: Free 25(OH)D and Calcium Absorption, PTH, and Markers of Bone Turnover.
abstract: BACKGROUND
  It has been proposed that serum free 25-hydroxyvitamin D [25(OH)D] may better reflect vitamin D action than total 25(OH)D. An ELISA for serum free 25(OH)D has recently become available, permitting direct assay.
  
  
  OBJECTIVE
  To determine whether serum free 25(OH)D provides additional information in relation to calcium absorption and other biomarkers of vitamin D action compared to total serum 25(OH)D.
  
  
  METHODS
  Ambulatory research setting in a teaching hospital.
  
  
  RESULTS
  Serum free 25(OH)D measured in a previously performed study of varied doses of vitamin D3 (placebo and 800, 2000, and 4000 IU) on calcium absorption, PTH, procollagen type 1 N-terminal propeptide, and C-terminal telopeptides of type I collagen. Free 25(OH)D was measured by ELISA. Calcium absorption was measured at baseline and at 10 weeks using stable dual calcium isotopes.
  
  
  RESULTS
  Seventy-one subjects completed this randomized, placebo-controlled trial. Baseline group mean free and total 25(OH)D varied from 4.7 ± 1.8 to 5.4 ± 1.5 pg/mL, and from 23.7 ± 5.9 to 25.9 ± 6.1 ng/mL, respectively. Participants assigned to the 4000-IU dose arm achieved free 25(OH)D levels of 10.4 pg/mL and total 25(OH)D levels of 40.4 ng/mL. Total and free 25(OH)D were highly correlated at baseline and after increasing vitamin D dosing (r = 0.80 and 0.85, respectively). Free 25(OH)D closely reflected changes in total 25(OH)D. PTH was similarly correlated at baseline and follow-up with total and free 25(OH)D. Serum C-terminal telopeptides of type I collagen had a moderate positive correlation with total and free 25(OH)D at follow-up. The serum 1,25-dihydroxyvitamin D change increased significantly with the change in 25(OH)D but not with the change in free 25(OH)D.
  
  
  CONCLUSIONS
  There was no advantage from measuring free over total 25(OH)D in assessing the response of calcium absorption, PTH, and markers of bone turnover to vitamin D. Free 25(OH)D responded to increasing doses of vitamin D in a similar fashion to total 25(OH)D.
authors: Winthrop University Hospital, Mineola, New York 11501.
authors: John
authors: J
authors: Aloia
authors: Winthrop University Hospital, Mineola, New York 11501.
authors: Ruban
authors: R
authors: Dhaliwal
authors: Winthrop University Hospital, Mineola, New York 11501.
authors: Mageda
authors: M
authors: Mikhail
authors: Winthrop University Hospital, Mineola, New York 11501.
authors: Albert
authors: A
authors: Shieh
authors: Winthrop University Hospital, Mineola, New York 11501.
authors: Alexandra
authors: A
authors: Stolberg
authors: Winthrop University Hospital, Mineola, New York 11501.
authors: Louis
authors: L
authors: Ragolia
authors: Winthrop University Hospital, Mineola, New York 11501.
authors: Melissa
authors: M
authors: Fazzari
authors: Winthrop University Hospital, Mineola, New York 11501.
authors: Steven A
authors: SA
authors: Abrams
mesh_terms: D000328:Adult; D000368:Aged; D015415:Biomarkers; D016723:Bone Remodeling; D001842:Bone and Bones / Q000378:metabolism*; D002118:Calcium / Q000378:metabolism*; D024042:Collagen Type I / Q000097:blood; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D006887:Hydroxycholecalciferols / Q000097:blood* / Q000378:metabolism / Q000494:pharmacology; D008297:Male; D008875:Middle Aged; D010281:Parathyroid Hormone / Q000378:metabolism*; D014815:Vitamins / Q000097:blood* / Q000494:pharmacology; D055815:Young Adult
chemical_list: D015415:Biomarkers; D024042:Collagen Type I; D006887:Hydroxycholecalciferols; D010281:Parathyroid Hormone; D014815:Vitamins; D002118:Calcium