INFLUENCE OF AGE ON EFFECTS OF ENDOGENOUS 1,25-DIHYDROXYVITAMIN-D ON CALCIUM-ABSORPTION IN NORMAL WOMEN

Recent reports of increases in serum 1,25-dihydroxyvitamin D [1,25(OH2 )D] concentration with aging despite no changes or decreases in calciu m absorption suggest that elderly women have intestinal resistance to vitamin D action. Thus, in 15 young adult (30 +/- 1 year) and 15 elder ly (74 +/- 1 year) women (mean +/- SE), we assessed the responsiveness of intestinal calcium absorption to increases in circulating 1,25(OH) (2)D induced by 4 days of an experimental diet (150 mg calcium and 160 0 mg phosphorus daily). True fractional calcium absorption (FCA) (Ca-4 4 mixed with food and Ca-42 given intravenously, then their ratio in u rine measured by mass spectrometry) was determined. Baseline serum int act parathyroid hormone (PTH) concentration was higher in the older wo men (P = 0.01) whereas serum 1,25(OH)(2)D concentration and true FCA w ere similar. In both groups, serum 1,25(OH)(2)D concentrations increas ed (P < 0.002) on the experimental diet. After 4 days on the diet, ser um 1,25(OH)(2)D increased over baseline by 30.5 and 35.6% and, despite these increases, true FCA was 40 +/- 3 versus 40 +/- 4%/24 hours (NS between groups) in the young and elderly women, respectively. These da ta suggest that either elderly women have normal intestinal responsive ness to vitamin D or that the resistance to it is too mild to be detec ted by these methods.