Excretion of Phosphorus by the Bowel
Authors: KJERULF‐JENSEN KAJ 
Published: December 2008

In 1939 Hevesy, Hahn and Rebbe suggested a method for determining the endogenous phosphorus in the faeces by means of radioactive phosphorus. So far no method had been found by which enabled us to distinguish between non‐absorbed food phosphorus and endogenous phosphorus excreted to the faeces from the organism. The method is based on the supposition that radioactive phosphorus (introduced into the organism beforehand) is excreted in such a way that the specific activity (activity/mg P) of urinary phosphorus may be considered an indication of the specific activity of plasma phosphorus (and hence of endogenous phosphorus). This supposition, which forms the basis of the method, has been investigated in the present work: Rats, the phosphorus of which had been labelled by injection of a suitable quantity of 32P, were fed a P‐free diet. In this manner it was impossible that the active phosphorus excreted by the organism to the faeces Was mixed with inactive non‐absorbed food phosphorus. Under these conditions uniform values for the specific activity of urinary and faecal phosphorus were found in four rats. In four adult humans, to whom labelled phosphorus had previously been administered, good agreement was also found between the specific activities of urinary phosphorus and of phosphorus from bile‐pancreatic juice (endogenous phosphorus) already one week after the active phosphorus had been administered. These experiments seem to justify the use of radioactive phosphorus for distinguishing between endogenous and exogenous phosphorus in the faeces.

The method was then applied to rats and humans: In 5 normal rats the endogenous phosphorus was found to be on an average 43.5 per cent, of the faecal phosphorus of 6.8 mgs P per 24 hours per 100 grammes body weight of rats on a diet containing 1 per cent. P and 0.54 per cent. Ca.

In four normal humans the fractions of endogenous phosphorus in the faeces was found to be on an average 24.8 – 25.0 – 27.5 – 31.0 per cent, of the total faecal phosphorus or 93 – 102 – 105 – 238 mgs P.

Experiments on rats showed that an increase in the phosphorus administered and in catabolized phosphorus was accompanied by an increased excretion of labelled phosphorus to the faeces, both when phosphorus was given as ordinary mixed food phosphorus and as easily absorbable, inorganic sodium phosphate.

The fraction of endogenous phosphorus in the faeces only represents the difference between the endogenous phosphorus actually excreted and the part which has been reabsorbed in the gut. An attempt was made at obtaining maximum figures for the actually excreted endogenous phosphorus through increased dilution by administration of excessively large, increasing quantities of phosphorus to rats in which the phosphorus had been labelled with radio‐phosphorus beforehand. The consequent increase in excretion of labelled phosphorus to the faeces was so surprisingly large that it seems doubtful whether the labelled phosphorus excreted with the faeces gives a true picture of the excretion of endogenous phosphorus. As an explanation of this disagreement the hypothesis is put forward that in the intestinal lumen non‐labelled phosphorus may by diffusion be exchanged with the labelled phosphorus of the organism (in the intestinal wall). Such an exchange of ions may parallel absorption and would have no influence upon the quantity of phosphorus actually excreted, but upon the concentration of labelled phosphorus in the faeces.

Experiments on rats aiming at demonstrating this exchange of phosphate ions through the mucous membrane of the small intestine have shown, if not the existence, then at any rate the probability of such an exchange.

The possibility of a controlled excretion of endogenous phosphorus to the faeces is discussed; it is pointed out that the increase observed in the excretion of phosphorus following increased administration of phosphorus may be regarded as a natural consequence of the dilution of labelled phosphorus excreted to the gut with exogenous, non‐labelled phosphorus of the food. The present experiments have not been able to prove a controlled excretion into the bowel, but on the other hand the experiments do not preclude this possibility.

The preparations of radioactive phosphorus used were presented to me by Professor G. Hevesy, whom I wish to thank for this present and for the kind assistance given to me at the University Institute of Theoretical Physics. I also wish to express my thanks to my chief, Professor E. Lundsgaard, for his valuable interest in and kind help with the present work.