title: Effects of vitamin D metabolites on intestinal calcium absorption and bone turnover in elderly women.
abstract: BACKGROUND
  The relative importance of vitamin D metabolites in the regulation of gut calcium absorption has not been well studied in elderly women living in an environment with abundant sunlight.
  
  
  OBJECTIVE
  The objective was to examine the determinants of active gut calcium absorption ( +/- SD: 42 +/- 11%) after an overnight fast with the use of a low (10 mg) calcium load.
  
  
  METHODS
  One hundred twenty elderly women aged 74.7 +/- 2.6 y underwent an active calcium absorption test with a radioactive calcium tracer, dietary analysis, and measurement of markers of bone turnover and calcium metabolism.
  
  
  RESULTS
  The mean serum 25-hydroxyvitamin D [25(OH)D] concentration at the time of the calcium absorption test was 68 +/- 29 nmol/L. Gut calcium absorption was correlated with 25(OH)D but not 1,25-dihydroxyvitamin D (calcitriol), the free calcitriol index, or dietary calcium intake. After adjustment for age, calcitriol concentration, and dietary calcium intake, the significant determinant of fractional calcium absorption was the 25(OH)D concentration (r = 0.34, P = 0.001). When body weight was included in the regression, both 25(OH)D (beta = 1.20 x 10(-3)) and calcitriol (beta = 1.00 x 10(-3)) were significantly correlated with calcium absorption. Despite the strong relation between 25(OH)D and gut calcium absorption, there was no relation with other aspects of bone turnover or calcium metabolism.
  
  
  CONCLUSIONS
  These data suggest that at low calcium loads, 25(OH)D is a more important determinant of gut calcium absorption than is calcitriol in elderly women exposed to abundant sunlight, but that this relation has little effect on overall calcium metabolism.
authors.affiliation: Department of Medicine, University of Western Australia, Sir Charles Gairdner Hospital, Nedlands, Australia. adevine@cyllene.uwa.edu.au
authors.forename: Amanda
authors.initials: A
authors.lastname: Devine
authors.forename: Scott G
authors.initials: SG
authors.lastname: Wilson
authors.forename: Ian M
authors.initials: IM
authors.lastname: Dick
authors.forename: Richard L
authors.initials: RL
authors.lastname: Prince
mesh_terms: D015652:25-Hydroxyvitamin D 2 / Q000008:administration & dosage / Q000097:blood / Q000494:pharmacology*; D000368:Aged; D001842:Bone and Bones / Q000187:drug effects / Q000378:metabolism*; D002117:Calcitriol / Q000008:administration & dosage / Q000097:blood / Q000494:pharmacology*; D002136:Calcium, Dietary / Q000008:administration & dosage / Q000493:pharmacokinetics*; D003430:Cross-Sectional Studies; D005260:Female; D006801:Humans; D007408:Intestinal Absorption / Q000187:drug effects; D013472:Sunlight; D014904:Western Australia
chemical_list: D002136:Calcium, Dietary; D015652:25-Hydroxyvitamin D 2; D002117:Calcitriol