CHAPTER XI 

 THE HISTORY OF THE FOODSTUFFS 



SECTION I 

 PROTEIN METABOLISM 



IN dealing with the metabolism of the body as a whole we saw reason to 

 believe that the proteins taken in with the food might be regarded as having 

 a twofold destiny. One part, and under normal circumstances the greater 

 part, is applied to the production of energy, in this respect discharging a 

 function which might equally well be performed by the fats and carbohy- 

 drates of the food. In its second function protein cannot be replaced by any 

 other foodstuff, since it alone contains the necessary elements as well as the 

 groupings of these elements which are essential for the building up of the 

 living tissues. We saw reason to believe that this tissue metabolism ac- 

 counted however for a small part only of the nitrogen of the food. For this 

 reason it is possible to ensure health and a condition of nitrogenous equili- 

 brium 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 pro- 

 tein in the diet to its minimum. Folin has shown that if we compare the com- 

 position 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 given 

 here. 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 y i. e. that from the food, and endogenous, 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 

 51 801 



