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The effect of dietary intake of calcium and phosphorus on the absorption and excretion of phosphorus in chimaera-derived sheep

SUMMARY The effect of ratios of calcium to phosphorus in the diet (varying from 0·6 to 3·6) on the absorption and excretion of P was investigated with four sets of 9-month-old chimaera-derived triplets. A pelleted cereal-based diet supplemented to contain 1·5, 3·1 or 6·2 g P/kg D.M. and 3·4 or 5·4 g Ca/kg D.M. was used in a factorial design. The additional Ca in the diet reduced the efficiency of absorption of the P supplements in the diet by 0·18 ± 0·04. The efficiencies of absorption of total P were very similar within but markedly different between (P < 0·001) sets of triplets, the mean values being 0·62, 0·74, 0·82 and 0·84. The fractional absorption of inorganic P alone showed the same ranking and the mean values were 0·47, 0·67, 0·85 and 0·92. These differences between triplets support the view that the individual differences in P metabolism have a genetic basis. The mean absorbability of P in the basic diet (0·77) and in the P supplements (0·73) were similar. The pattern of excretion of absorbed P between the faecal and urinary route of excretion varied between the sets of triplets; as P intake increased, the additional absorbed P was excreted in faeces by one set, in urine by another and in faeces and urine by two sets. The relative importance of the faecal route was dependent upon the fractional absorption of P secreted into the gut.