Relation between calcium absorption and serum calcitriol in normal men: evidence for age-related intestinal resistance to calcitriol

Abstract

<h4>Objective</h4>To obtain information on the causes of age-related bone loss in men and the concomitant decline in calcium absorption.<h4>Design</h4>Cross-sectional study.<h4>Setting</h4>Adelaide, South Australia, Australia.<h4>Subjects</h4>A total of 95 healthy, Caucasian men (age range 27-87 y).<h4>Results</h4>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.<h4>Conclusions</h4>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.