592 METABOLISM 



and who take comparatively little protein, with those living in temperate 

 zones, who consume much more. After discounting all of these other 

 factors, it has been quite clearly shown that, when the protein allowance 

 is materially reduced, the people as a whole are less robust, mentally in- 

 ferior, and, instead of being less prone to the very diseases which are 

 usually supposed to be due to overloading of the organism with useless 

 excretory products, are more liable to suffer from them. 



That a decided reduction in protein weakens the defense of the organ- 

 ism against infection is probably due to the fact that the fluids of the 

 body normally contain a great variety of so-called antibodies that is, 

 of highly complex substances that are largely protein in nature. When 

 bacteria, or the poisons produced by them, enter the body, they are met 

 by one or more of these defense substances and destroyed or neutralized. 

 Now it is clear that there should always be a surplus of protein-building 

 materials from which the antibodies may be constructed. Such an excess 

 will constitute a "factor of safety" against disease. And there are fac- 

 tors of safety of another nature to be provided for, two of which we are 

 in a position to appreciate. In the first place, there must always be an 

 adequate supply of tryptophane, of lysine, and of cystine, not only to 

 meet the bare necessities of the protein constructive processes that go on 

 under normal conditions, but also to make good the larger amount of 

 protein wear and tear that greater degrees of tissue activity will entail. 

 Although moderate muscular exercise does not appear to cause any im- 

 mediate consumption of protein (carbohydrate and, later, fat being the 

 fuel material used to produce it), yet it does throw a greater strain on 

 the tissues and causes a greater wear and tear of the machinery, and 

 hence a demand for more protein-building material. In the second place, 

 there are certain of the internal secretions of the body, such as epineph- 

 rine (adrenaline), that are essential for life, and as crude materials 

 for the manufacture of which certain amino acids are essential. Tyro- 

 sine is one of these, and since proteins, as we have seen, differ from one 

 another quite considerably in the amount of this amino acid which they 

 contain, it is advisable to provide an excess, so that an adequate supply 

 of tyrosine may always be available. 



The answer to one of the most important practical questions in die- 

 tetics namely, "What proportion of protein should the diet contain?" 

 depends on these scientific principles. The source of the protein is the 

 important thing. With animal protein there is no doubt that we could 

 get along with perfect safety by taking daily not more than 50 or 60 

 grams, which is about half of what we actually consume. If the protein 

 is of vegetable origin and part of it of the first quality, as wheat and 

 Indian corn preparations, more should be taken so as to allow for the 



