The effect of dietary intake of calcium and dry matter on the absorption and excretion of calcium and phosphorus by growing lambs

Abstract

SUMMARYThe isotope dilution technique was used to measure endogenous faecal excretion and absorption of dietary Ca and P by growing lambs. Groups of four lambs were given 0·8 or 1·2 kg/day of a pelleted barley and soya-bean diet containing 4·2 g P/kg D.M. and supplemented to contain 1·79, 3·52 or 4·67 g Ca/kg D.M.At the lowest dietary concentration of Ca, absorption of Ca was insufficient to meet the Ca required for deposition in the newly formed bone matrix of the skeleton; retention was 0·39, 1·16 and 1·12 g/day with increasing dietary Ca concentration. The proportion of dietary Ca absorbed decreased (P < 0·001) with increasing Ca intake, ranging from 0·85 to 0·44. Endogenous faecal excretion of Ca was independent of the absorption of dietary Ca, but increased (P < 0·05) from 17·2 at the lower to 21·1 mg/kg live weight/day at the higher D.M. intake. Plasma concentration of Ca was higher for the female (2·51) than the male (2·33 mmol/1) lambs and increased linearly (P < 0·001) with dietary concentration of Ca from 2·31 to 2·52 mmol/l.The retention of P increased with Ca and D.M. intake. On the other hand, the proportion of dietary P absorbed (0·85) was independent of Ca intake and hence of P retention. Increases in P retention were reflected by decreases in urinary P, but not in endogenous faecal excretion. Endogenous faecal excretion of P increased with D.M. or P intake from 17·9 at the lower to 33·5 mg/kg live weight/day at the higher intake.The results are discussed in relation to dietary allowances of Ca and the mechanism of P homoeostasis.