THE PROBLEM OF FOOD 3 



we obtain our nitrogen-supply, we see why a certain 

 daily requirement of these substances is put down in 

 diet tables. Other kinds of food-stuffs contain no nitro- 

 gen, and since this element is one of the constituents 

 of our tissues, it is clear that it must be supplied if 

 new tissue is to be made. Familiar examples of pro- 

 teins are: the lean of meat, white of egg, plasmon 

 from milk, the vegetable proteins in wheat and beans. 

 But, it may be said, how does this requirement for 

 growth apply to the adult organism, which has 

 ceased to produce new substance? The ans\ver is 

 that the living cell machinery wears away with use, 

 just like other machinery, and nitrogen is one of the 

 elements that is lost, so that it is wanted to replace 

 the loss. So far as we can make out, however, this 

 loss of nitrogen due to wear and tear is not great, 

 and, in theory, the protein required to replace it is 

 correspondingly small. It is indeed remarkable that 

 the actual waste of muscle substance by wear and 

 tear in activity, instead of being more obvious than 

 that of other cell mechanisms, as would be expected, 

 is so small that no increase of nitrogen excretion can 

 be detected, except under such excessive exertion as 

 to become pathological. But here we come across 

 one of the most disputed problems of nutrition, 

 namely, whether it may be advantageous for other 

 reasons to consume more protein than the minimal 

 amount. This question will be touched upon briefly 

 below. Here I may remark that a person in ill 

 health or with bad digestion may probably be able 

 to do better if a large proportion of his diet be made 

 up of protein, not because of greater wear and tear, 

 but on account of the greater ease with which such 

 individuals can make use of protein for the other 

 great purpose of food, namely, the supply of energy, 

 to which we next turn our attention. 



