The effect of age and level of dietary calcium intake on calcium metabolism in sheep.

I . A combination of balance and isotope techniques has been used to study the effects of age and dietary calcium content on Ca metabolism in forty-two wether sheep. 2. The amount of Ca absorbed by young growing animals varied significantly with intake. The percentage of the dietary Ca absorbed, however, remained unchanged. In older animals the amount of Ca absorbed was not altered by changes in intake, but decreased slightly with age. 3. Retention of Ca was directly related to the amount of Ca absorbed and was independent of age or breed. Furthermore, nearly all the Ca absorbed above the minimum mean amount required for maintenance was retained. 4. Faecal endogenous loss of Ca also was related to the amount of Ca absorbed. Values for faecal endogenous Ca were much lower than those used in the calculation of dietary requirements by the Agricultural Research Council (1965). 5 . Urinary Ca excretion was variable, and was not related to age or changes in dietary Ca intake. 6. Increased absorption of Ca in young growing animals was accompanied by a decreased rate of bone resorption, but the rate of bone accretion remained unchanged. Changes in dietary Ca in older animals had no effect on either of these two processes. Results indicate that bone resorption is the main pathway governing Ca homoeostasis. Both the rates of Ca accretion into bone and resorption from bone decreased with age. 7. Neither the rapidly exchangeable Ca pool (P) nor the slowly exchangeable bone pool (E) was altered by changes in dietary intake in young or mature animals. Both, however, decreased in size with age. 8. T h e size of the slowly exchangeable pool (E) was directly related to the rate of Ca accretion into bone. 9. T h e results were used to calculate dietary Ca requirements of sheep gaining weight at different rates, and these values have been compared with values recommended by the Agricultural Research Council (1965).