Effect of age on calcium and phosphate absorption. Role of 1,25-dihydroxyvitamin D.

In humans, there is a decrease in intestinal absorption of Ca with age and a decrease in the capacity to adapt to a low Ca diet. The rat has been used as an animal model in which to study the mechanisms responsible for this decreased absorption and adaptation with age. Intestinal absorption of Ca and Pi declines with age in the rat. The decline is primarily in the energy- and vitamin D-dependent component of transport found in the duodenum and, in the case of Pi transport, in the proximal jejunum. Administration of 1,25-dihydroxyvitamin D (1,25(OH)2D) increases Ca and P transport in all age groups, but the maximal levels attained are less in older animals compared to the young. Feeding a low-Ca diet increases serum 1,25(OH)2D levels and intestinal Ca absorption only in young animals. However, a low-Ca diet increases intestinal Pi absorption in all age groups, even in the absence of elevated serum 1,25(OH)2D levels. Knowledge of the mechanisms responsible for the age-related decrease in Ca and Pi absorption may suggest new ways of improving mineral absorption in the elderly.