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 calcium absorption suggest that elderly women have intestinal resistance to vitamin D action. Thus, in 15 young adult (30 +/- 1 year) and 15 elderly (74 +/- 1 year) women (mean +/- SE), we assessed the responsiveness of intestinal calcium absorption to increases in circulating 1,25(OH)2D induced by 4 days of an experimental diet (150 mg calcium and 1600 mg phosphorus daily). True fractional calcium absorption (FCA) (44Ca mixed with food and 42Ca given intravenously, then their ratio in urine measured by mass spectrometry) was determined. Baseline serum intact parathyroid hormone (PTH) concentration was higher in the older women (P = 0.01) whereas serum 1,25(OH)2D concentration and true FCA were similar. In both groups, serum 1,25(OH)2D concentrations increased (P < 0.002) on the experimental diet. After 4 days on the diet, serum 1,25(OH)2D 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 data suggest that either elderly women have normal intestinal responsiveness to vitamin D or that the resistance to it is too mild to be detected by these methods.