title: Calcium absorption varies within the reference range for serum 25-hydroxyvitamin D.
abstract: BACKGROUND
  Calcium absorption is generally considered to be impaired under conditions of vitamin D deficiency, but the vitamin D status that fully normalizes absorption is not known for humans.
  
  
  OBJECTIVE
  To quantify calcium absorption at two levels of vitamin D repletion, using pharmacokinetic methods and commercially marketed calcium supplements.
  
  
  METHODS
  Two experiments performed in the spring of the year, one year apart. In the first, in which participants were pretreated with 25-hydroxyvitamin D (25OHD), mean serum 25OHD concentration was 86.5 nmol/L; and in the other, with no pretreatment, mean serum concentration was 50.2 nmol/L. Participants received 500 mg oral calcium loads as a part of a standard low calcium breakfast. A low calcium lunch was provided at mid-day. Blood was obtained fasting and at frequent intervals for 10 to 12 hours thereafter.
  
  
  METHODS
  Relative calcium absorption at the two 25OHD concentrations was estimated from the area under the curve (AUC) for the load-induced increment in serum total calcium.
  
  
  RESULTS
  AUC(9) (+/- SEM), was 3.63 mg hr/dL +/- 0.234 in participants pretreated with 25OHD and 2.20 +/- 0.240 in those not pretreated (P < 0.001). In brief, absorption was 65% higher at serum 25OHD levels averaging 86.5 nmol/L than at levels averaging 50 nmol/L (both values within the nominal reference range for this analyte).
  
  
  CONCLUSIONS
  Despite the fact that the mean serum 25OHD level in the experiment without supplementation was within the current reference ranges, calcium absorptive performance at 50 nmol/L was significantly reduced relative to that at a mean 25OHD level of 86 nmol/L. Thus, individuals with serum 25-hydroxyvitamin D levels at the low end of the current reference ranges may not be getting the full benefit from their calcium intake. We conclude that the lower end of the current reference range is set too low.
authors.affiliation: Creighton University, 601 N. 30th Street, Suite 4841, Omaha, NE 68131, USA. rheaney@creighton.edu
authors.forename: Robert P
authors.initials: RP
authors.lastname: Heaney
authors.forename: M Susan
authors.initials: MS
authors.lastname: Dowell
authors.forename: Cecilia A
authors.initials: CA
authors.lastname: Hale
authors.forename: Adrianne
authors.initials: A
authors.lastname: Bendich
mesh_terms: D019540:Area Under Curve; D002118:Calcium / Q000493:pharmacokinetics*; D018592:Cross-Over Studies; D004305:Dose-Response Relationship, Drug; D005260:Female; D006801:Humans; D007408:Intestinal Absorption; D008875:Middle Aged; D009753:Nutritive Value; D012016:Reference Values; D014807:Vitamin D / Q000031:analogs & derivatives* / Q000097:blood* / Q000494:pharmacology
chemical_list: D014807:Vitamin D; C104450:25-hydroxyvitamin D; D002118:Calcium