title: Natural calcium isotopic composition of urine as a marker of bone mineral balance.
abstract: BACKGROUND
  We investigated whether changes in the natural isotopic composition of calcium in human urine track changes in net bone mineral balance, as predicted by a model of calcium isotopic behavior in vertebrates. If so, isotopic analysis of natural urine or blood calcium could be used to monitor short-term changes in bone mineral balance that cannot be detected with other techniques.
  
  
  METHODS
  Calcium isotopic compositions are expressed as delta(44)Ca, or the difference in parts per thousand between the (44)Ca/(40)Ca of a sample and the (44)Ca/(40)Ca of a standard reference material. delta(44)Ca was measured in urine samples from 10 persons who participated in a study of the effectiveness of countermeasures to bone loss in spaceflight, in which 17 weeks of bed rest was used to induce bone loss. Study participants were assigned to 1 of 3 treatment groups: controls received no treatment, one treatment group received alendronate, and another group performed resistive exercise. Measurements were made on urine samples collected before, at 2 or 3 points during, and after bed rest.
  
  
  RESULTS
  Urine delta(44)Ca values during bed rest were lower in controls than in individuals treated with alendronate (P <0.05, ANOVA) or exercise (P <0.05), and lower than the control group baseline (P <0.05, t-test). Results were consistent with the model and with biochemical and bone mineral density data.
  
  
  CONCLUSIONS
  Results confirm the predicted relationship between bone mineral balance and calcium isotopes, suggesting that calcium isotopic analysis of urine might be refined into a clinical and research tool.
authors.affiliation: Department of Geology and Geophysics, University of Wisconsin-Madison, Madison, WI 53706, USA. jlskulan@geology.wisc.edu
authors.forename: Joseph
authors.initials: J
authors.lastname: Skulan
authors.forename: Thomas
authors.initials: T
authors.lastname: Bullen
authors.forename: Ariel D
authors.initials: AD
authors.lastname: Anbar
authors.forename: J Edward
authors.initials: JE
authors.lastname: Puzas
authors.forename: Linda
authors.initials: L
authors.lastname: Shackelford
authors.forename: Adrian
authors.initials: A
authors.lastname: LeBlanc
authors.forename: Scott M
authors.initials: SM
authors.lastname: Smith
mesh_terms: D019386:Alendronate / Q000627:therapeutic use; D015415:Biomarkers / Q000652:urine; D018488:Bone Demineralization, Pathologic / Q000378:metabolism; D015519:Bone Density; D050071:Bone Density Conservation Agents / Q000627:therapeutic use; D001842:Bone and Bones / Q000737:chemistry* / Q000378:metabolism; D002118:Calcium / Q000652:urine*; D002127:Calcium Isotopes / Q000652:urine*; D015444:Exercise; D006801:Humans; D018474:Weightlessness Simulation
chemical_list: D015415:Biomarkers; D050071:Bone Density Conservation Agents; D002127:Calcium Isotopes; D002118:Calcium; D019386:Alendronate