Adaptation to dietary calcium and phosphorus restriction changes with age in the rat

The purpose of this study was to characterize the changes that take place in adaptation to chronic calcium (Ca) or phosphorus (P) restriction with age. Adaptation in male F344 rats aged 1.5, 3, 12, and 18 mo was studied by feeding rats either a low-Ca diet, a low-P diet, or a high-Ca-P diet for 14 days. Plasma Ca remained relatively constant with age, but plasma P markedly decreased between 3 and 12 mo regardless of diet. Intestinal adaptation was determined by measuring the active transport of Ca by the intestine and by measuring the production of vitamin D-dependent calcium-binder protein. There was significant intestinal adaptation to Ca or P restriction at 1.5 mo, but there was none thereafter because Ca transport declined rapidly with age regardless of diet. The kidney adapted to the low-P diet by significantly reducing P excretion at all ages. In rats on a low-P diet, there was an increase in urinary P, which was due to a decrease in the tubular reabsorption of P and a decrease in urinary Ca with age. These changes in adaptation may reflect a decrease in serum 1,25-dihydroxyvitamin D levels and an increase in parathyroid hormone levels with age.