Association between intestinal vitamin D receptor, calcium absorption, and serum 1,25 dihydroxyvitamin D in normal young and elderly women

The exact mechanism for the decrease in intestinal calcium absorption with age is not yet understood. A decrease with age in serum 1,25- dihydroxyvitamin D (1,25(OH)₂D) or a decrease in the intestinal vitamin D receptor (VDR) protein concentration are possible causes. The objective of this study was to examine the effect of age on these factors. Fifty-nine young women age 25-35 years were compared with 41 elderly women age 65-83 years who underwent measurements of VDR, calcium absorption using a 20 mg and 100 mg calcium carrier, and calciotropic hormones. Calcium absorption by both tests was lower in the elderly women compared with the young women (p <0.05). Serum 1,25(OH)₂D and duodenal VDR protein concentration were not significantly different between the two age groups. Serum 1,25(OH)₂D correlated with the 20 mg calcium absorption test in both young (r = 0.35, p <0.007) and elderly women (r = 0.58, p <0.0001) and with the 100 mg calcium absorption in the elderly (r = 0.32; p <0.05). VDR did not correlate with calcium absorption in young women or elderly women, nor did VDR correlate with serum 1,25(OH)₂D and serum 25-hydroxyvitamin D. In summary, the decrease in calcium absorption cannot be explained by a decrease in intestinal VDR. The correlation between serum 1,25(OH)₂D and both calcium absorption tests only accounts for 12-30% of the variance in the age-related change in the calcium absorption tests. Other factors, not yet understood, are responsible for the decline in calcium absorption with age.