Some observations on phosphorus homoeostasis and requirements of sheep

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Abstract
Summary: The results of phosphorus kinetic studies from 149 sheep used in previous studies of calcium and P metabolism have been analysed statistically, to provide information on the mechanism of control of P homoeostasis.Whereas the rate of absorption of dietary P was increased in direct relation to P intake, the efficiency of absorption was decreased. Both the rate and efficiency of absorption, however, were increased in response to increased P demands. These results suggest that P may be absorbed by two processes: a passive one, related to P intake, and an active one, related to P demands.Endogenous loss of P in the faeces was directly related to both P intake and P absorption but inversely related to P demands. Salivary secretion of P was directly related to P absorption and increased at a rate of 1 mg/day/kg body weight for each 1 mg/day/kg body weight increase in absorption. This finding supports the suggestion that the inevitable endogenous faecal loss of P, in animals fed exactly according to their P requirements, is not constant but varies in direct relation to the rate of P absorption, and hence to the P intake needed to supply these requirements.Urinary loss of P was generally low, but high values were found in some animals and were associated with increased rates of P absorption and decreased rates of endogenous faecal loss of P. Possible reasons for these high rates of urinary P are discussed.Increased P demands were met by a combination of an increase in P absorption and a decrease in endogenous faecal loss of P, and P retention was directly related to Ca retention.The relevance of these results to the calculation of P requirements and to the control of P homoeostasis is discussed.