The reduction in renal function with age giving rise to reduced renal 1,25(OH)2D production may be a contributing factor.
Intestinal calcium absorption also decrease with age, partly because of decreased renal hydroxylation of 25(OH)D, and partly because of decreased responsiveness of the intestinal mucosa to circulating 1,25(OH)2D. This intestinal resistance to 1,25(OH)2D has been observed in postmenopausal women showing an intestinal malabsorption of calcium.