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.