title: Relation between calcium absorption and serum calcitriol in normal men: evidence for age-related intestinal resistance to calcitriol.
abstract: OBJECTIVE
  To obtain information on the causes of age-related bone loss in men and the concomitant decline in calcium absorption.
  
  
  METHODS
  Cross-sectional study.
  
  
  METHODS
  Adelaide, South Australia, Australia.
  
  
  METHODS
  A total of 95 healthy, Caucasian men (age range 27-87 y).
  
  
  RESULTS
  Calcium absorption declined with age (r=-0.46, P<0.0001), as did 24-h urine calcium, phosphate and creatinine (r>-0.21, P<0.05 for all); serum calcitriol and 25 hydroxyvitamin D did not change with age. Calcium absorption was related to serum calcitriol (r=0.20, P=0.05). An inverse relation between the residual deviations in calcium absorption, after allowing for its dependence on calcitriol, and age (F=5.4, P<0.005) was observed. The 24-h urinary calcium, phosphate and creatinine were all related to calcium absorption (r>0.41, P<0.0001). Forearm bone density fell with age (r=-0.45, P<0.0001) but was not related to calcium absorption, or markers of bone turnover.
  
  
  CONCLUSIONS
  In healthy Caucasian males (i) calcium absorption falls, but serum calcitriol does not change with age, (ii) the relation between calcium absorption and serum calcitriol changes with age, indicative of an intestinal resistance to calcitriol and (iii) calcium absorption is a significant determinant of 24-h urinary calcium excretion.
authors.affiliation: Division of Clinical Biochemistry, Institute of Medical and Veterinary Science, Adelaide, South Australia, Australia. franca.scopacasa@adelaide.edu.au
authors.forename: F
authors.initials: F
authors.lastname: Scopacasa
authors.forename: J M
authors.initials: JM
authors.lastname: Wishart
authors.forename: M
authors.initials: M
authors.lastname: Horowitz
authors.forename: H A
authors.initials: HA
authors.lastname: Morris
authors.forename: A G
authors.initials: AG
authors.lastname: Need
mesh_terms: D015502:Absorptiometry, Photon; D000328:Adult; D000367:Age Factors; D000368:Aged; D000369:Aged, 80 and over; D001315:Australia; D015519:Bone Density; D002117:Calcitriol / Q000097:blood*; D002118:Calcium / Q000097:blood / Q000493:pharmacokinetics* / Q000652:urine; D003404:Creatinine / Q000652:urine; D003430:Cross-Sectional Studies; D005542:Forearm / Q000000981:diagnostic imaging; D006801:Humans; D007408:Intestinal Absorption; D008297:Male; D008875:Middle Aged; D010710:Phosphates / Q000652:urine; D011877:Radionuclide Imaging
chemical_list: D010710:Phosphates; D003404:Creatinine; D002117:Calcitriol; D002118:Calcium