132 METABOLISM AND NUTRITION 



body must draw on its own store of these, the mineral constituents are set 

 free, find their way into the fluids of the body, and are eliminated through 

 the kidneys. 



If an animal receive more salts in his food than he needs the excess is like- 

 wise eliminated in the excretions. 



The importance of the individual elements has already been discussed on 

 general lines at page 25. It remains for us to mention briefly the behavior 

 of some of them in metabolism. It will be necessary to limit the discussion 

 to phosphorus, calcium and magnesium. With regard to iron cf. Chapter X. 



B. PHOSPHORUS 



Phosphorus, like several other inorganic foodstuffs, is eliminated mainly in 

 the faeces. In following the metabolism of proteids containing phosphorus we 

 have therefore to consider both the urine and the faeces, whereas the metabolic 

 products of proteids containing no phosphorus are almost all given off in the 

 urine. 



For a long time it was assumed that only the inorganic phosphorus com- 

 pounds are absorbed from the intestine. This was based partly on results indi- 

 cating that the excretion of phosphorus in the urine plainly rose after the addi- 

 tion of phosphates, and partly on the supposition that the phosphorus component 

 of different proteids was indigestible. It has been shown, however: that pan- 

 creatic juice, acting for one to two hours, dissolves from one-half to one-third 

 of the phosphorus in the nuclein of thymus (Popoff) ; that in the digestion of 

 casein by gastric juice the greater part of the phosphorus passes into the soluble 

 digestive products (Salkowski) ; and that under the action of pancreatic juice 

 almost all of the phosphorus of casein is brought into solution (Sebelien). 



It is not a difficult matter to show that the phosphorus from these substances 

 is actually absorbed from the intestine. In some experiments on dogs Marcuse 

 found that at least eighty-one to eighty-four per cent of the phosphorus of casein 

 was absorbed ; and in experiments on man Loewi observed an absorption of about 

 seventy-nine per cent of phosphorus derived from nuclein. 



But we cannot decide positively from these experiments whether the phos- 

 phorus is actually absorbed in organic combination or not. We know, indeed, 

 that phosphorus is very easily split off from such substances by all sorts of 

 agencies. In any case if it could be shown that phosphorus is stored in the body 

 only after feeding with a proteid containing phosphorus in its molecule, we 

 should then know that the body must depend upon such compounds as a source 

 for this, element. The experiments of Zadig seem in fact to prove that this is 

 the case ; but in a more recent series of experiments Leipziger was able to demon- 

 strate a storage of phosphorus after feeding a P-free edestin and a phosphate. 

 Hence there is as yet no proof that phosphorus may not be supplied as well in 

 inorganic as in organic form. Experiments by Loewi have shown however that 

 the ratio of N retained to P retained after excessive feeding with the nucleins 

 agrees fairly well with the ratio of their percentages in the food, and indicates 

 therefore that nucleins may be absorbed from the intestine, partly at least, in 

 unchanged form. 



In order to determine the absolute requirement of the human body in phos- 

 phorus it is necessary to measure directly the income and the output. 



In human faeces the quantity of P varies, according to the character of the 

 food and the P contained in it, from 0.25 g. to 3 g. and more per day. In 

 the urine the limits for an adult are 0.4 g. and 2.8 g. When the phosphorus is 



