856 PHYSIOLOGY 



condition of nitrogenous equilibrium with amounts of protein in the 

 diet of man which might vary between 40 and 200 grm. per diem. 

 The more protein that is taken in with the food the greater is the 

 relative amount which is applied to the energy needs of the body. 

 If therefore we would attempt to find out what are the end-products 

 of the tissue metabolism we should confine the energy metabolism of 

 proteins within the smallest possible limits by reducing the quota of 

 protein in the diet to its minimum. Folin has shown that if we 

 compare the composition of the urine obtained under these two 

 conditions, namely, on a diet containing a normal quantity of protein 

 and on a diet containing a minimal amount, we find evidence of a 

 qualitative difference between the two kinds of metabolism. The 

 difference is well brought out in the tables just quoted. On a large 

 diet the greater part of the nitrogen can be regarded as derived 

 directly from the food, whereas on a small diet a relatively larger 

 proportion of it must come from protein which has been previously 

 built up into the tissues. Folin distinguishes these two sources of the 

 nitrogen of the urine as exogenous, i.e. that from the food, and endo- 

 genous, i.e. derived from the tissues. Two facts stand out in comparing 

 these two urinary analyses. In the first place, on a normal protein diet 

 the urea accounts for 87 per cent, of the total nitrogen of the urine. On 

 an excessive protein diet this percentage may rise to 90 or 95. On 

 the low protein diet the percentage of nitrogen appearing as urea is 

 reduced to 60. On the other hand, practically identical amounts of 

 creatinin are obtained under the two conditions, so that whereas on 

 the full diet it amounts only to 3-6 per cent., on the low protein diet 

 it forms as much as 17 per cent, of the total nitrogen output. We 

 are therefore justified in regarding urea as to a large extent exogenous 

 in origin, and as derived directly from the nitrogenous moiety of the 

 protein molecule, which may not at any time have formed part of the 

 living tissues of the body. 



On giving a large protein meal to a dog the urea in the urine 

 rapidly rises, and at the end of four or five hours 50 per cent, of the 

 total nitrogen taken in with the food has appeared in the urine as urea 

 (Fig. 351). If we take into account that the digestion of a meat meal in 

 this animal may go on for eight hours, we are justified in the statement 

 that by far the greater portion of the protein nitrogen taken with the 

 food is excreted almost directly after absorption as urea in the urine. 

 Urea is therefore to be regarded in the first place as an index to the 

 amount of protein absorbed. We have seen that the end-products of 

 protein digestion in the intestine are the amino-acids ; and that these 

 are the immediate precursors of the urea is shown by the fact that 

 the administration of these bodies is followed very rapidly by the 

 appearance of the whole of their nitrogen in the urine as urea. Many 



