EXCRETION OF PHOSPHORUS 551 



absorption of the phosphorus contained in the food, as discussed on p. 543. We get, 

 however, trustworthy information on the latter point by compaiing the specific 

 activity of the urine P and the faeces P. As already mentioned, this comparison 

 is based on the assumption that the specific activity of the P contained in the 

 urine is equal to that of the P present in the digestive juices. We tested this assump- 

 tion in the following way: Labelled sodium phosphate was administered to a cat, 

 the animal was sacrificed after 17 days fasting and the phosphorus contained in 

 the last urine produced, and also in the sample removed from the small intestine, 

 investigated. We found 1 mgm P contained in each sample to have, within the 

 error of the experiment (^4%), the same activity. The latter amounted to 0.003% 

 of the total activity administered. 



Information on the amount of endogenous P present in the faeces were also 

 obtained by determining the P content of the faecal output of fasting animals^^^ 

 The conditions prevailing in such experiments are, however, far fiom being phy- 

 siological ones. The amount of the digestive juices, and thus also of the phosphonis 

 secreted into the digestive tract, will much depend on the amount and quality 

 of the food administered. When, for example, 50 gm oil and 300 mgm labelled 

 P as sodiumphosphate were administered^^) fo a fasting dog and the total P content 

 of the intestinal tract investigated after the lapse of 5 hours, the latter was com- 

 posed to an extent of about 75% of P endogenous origin. 



Summary 



After the lapse of 20 minutes, a slight amount (about 0.01%) of the radioactive 

 phosphorus atoms taken by mouth as soditim phosphate can be recorded in the 

 urine of a human subject. In the course of the first day 4—12% of the amount 

 taken was recordered. When administering the labelled phosphate by intravenous 

 injection the figures varied between 4 and 23%. 



Phosphorus contained in the normal diet is less efficiently absorbed than 

 sodium phosphate. As much as about 30% of the former leaves through the bowels. 

 To what extent the latter is unabsorbed (exogenous) P and to what extent it is 

 endogenous, thus derived from the body proper, can be determined by comparing 

 the specific activity of the faeces P with that of the urine P. Such a comparison 

 leads to the result that, in the cases investigated, 70—80% of the phosphoi*us 

 present in the faeces was non-absorbed food P. 



Similar determinations were also carried out on the excreta of rats. 



It is emphasized that important information on the digestibility of different 

 foodstuffs could be obtained by administering foodstuffs containing labelled phos- 

 phorus and other labelled elements, for example of vegetables grown on a soil 

 containing labelled phosphate. 



<i)R. Nicolaysen, Biochem. J. 31, 107 (1937). 



(2>G. Hevesy and E. Lundsgaard, Nature 140, 275 (1937). 



