title: Vitamin D supplementation increases calcium absorption without a threshold effect.
abstract: BACKGROUND
  The maximal calcium absorption in response to vitamin D has been proposed as a biomarker for vitamin D sufficiency.
  
  
  OBJECTIVE
  The objective was to determine whether there is a threshold beyond which increasing doses of vitamin D, or concentrations of serum 25-hydroxyvitamin D [25(OH)D], no longer increase calcium absorption.
  
  
  METHODS
  This was a placebo-controlled, dose-response, randomized, double-blind study of the effect of vitamin D on calcium absorption in healthy postmenopausal women. Seventy-six healthy postmenopausal women were randomly assigned to placebo or 800 IU (20 μg), 2000 IU (50 μg), or 4000 IU (100 μg) vitamin D₃ for 8 wk. The technique of dual isotopes of stable calcium was used with a calcium carrier to measure calcium absorption at baseline and after 8 wk.
  
  
  RESULTS
  Seventy-one women with a mean ± SD age of 58.8 ± 4.9 y completed the study. The mean calcium intake was 1142 ± 509 mg/d and serum 25(OH)D was 63 ± 14 nmol/L at baseline. A statistically significant linear trend of an increase in calcium absorption adjusted for age and body mass index with increasing vitamin D₃ dose or serum 25(OH)D concentration was observed. A 6.7% absolute increase in calcium absorption was found in the highest vitamin D₃ group (100 μg). No evidence of nonlinearity was observed in the dose-response curve.
  
  
  CONCLUSIONS
  No evidence of a threshold of calcium absorption was found with a serum 25(OH)D range from 40 to 130 nmol/L. Calcium absorption in this range is not a useful biomarker to determine nutritional recommendations for vitamin D.
authors: From Winthrop University Hospital, Mineola, NY (JFA, RD, AS, MM, MF, and LR) and Baylor College of Medicine, Houston, TX (SAA).
authors: John F
authors: JF
authors: Aloia
authors: Ruban
authors: R
authors: Dhaliwal
authors: Albert
authors: A
authors: Shieh
authors: Mageda
authors: M
authors: Mikhail
authors: Melissa
authors: M
authors: Fazzari
authors: Louis
authors: L
authors: Ragolia
authors: Steven A
authors: SA
authors: Abrams
mesh_terms: D015415:Biomarkers / Q000097:blood; D050071:Bone Density Conservation Agents / Q000008:administration & dosage* / Q000009:adverse effects / Q000378:metabolism / Q000627:therapeutic use; D002112:Calcifediol / Q000097:blood; D002118:Calcium / Q000097:blood; D002127:Calcium Isotopes; D002136:Calcium, Dietary / Q000008:administration & dosage / Q000378:metabolism*; D002762:Cholecalciferol / Q000008:administration & dosage* / Q000009:adverse effects / Q000378:metabolism / Q000627:therapeutic use; D019587:Dietary Supplements / Q000009:adverse effects; D004311:Double-Blind Method; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D007408:Intestinal Absorption; D008875:Middle Aged; D009518:New York; D009751:Nutritional Requirements; D015663:Osteoporosis, Postmenopausal / Q000097:blood / Q000378:metabolism / Q000517:prevention & control*; D017698:Postmenopause; D012621:Seasons; D014808:Vitamin D Deficiency / Q000097:blood / Q000178:diet therapy / Q000378:metabolism / Q000517:prevention & control
chemical_list: D015415:Biomarkers; D050071:Bone Density Conservation Agents; D002127:Calcium Isotopes; D002136:Calcium, Dietary; D002762:Cholecalciferol; D002112:Calcifediol; D002118:Calcium