THE NORMAL DIET OF MAN 653 



tion to the amount of protein in the diet. Especially is this marked in man, 

 where the power of storing protein in the body seems to be minimal or absent 

 in the normal adult. As we have seen, the protein taken in the food has a 

 twofold destiny. Part of it, probably the smaller portion, is needed to be 

 built up into the tissues, and to form living protoplasm in replacement of 

 wear and tear. The other, the larger portion, serves, like the fats and carbo- 

 hydrates, for meeting the energy requirements of the body. The ammo- 

 acids produced by the disintegration of the proteins in the alimentary canal 

 are rapidly absorbed and apparently undergo deamination in the body. 

 The nitrogen so split off is at once excreted in the urine, while the non- 

 nitrogenous moiety is rapidly oxidised to carbon dioxide and water. How- 

 ever much protein is ingested, so long as the digestive powers of the animal 

 are not overtaxed, all that is not required for replacing tissue waste undergoes 

 this fate, and we can therefore attain nitrogenous equilibrium on a diet 

 containing 50 grm. or 150 grm. of protein daily. The amount of protein 

 taken in the food, and digested and oxidised in the body by any given 

 individual, affords no clue to the amount which is absolutely necessary for 

 the maintenance of life and for the normal discharge of the bodily functions. 

 It is therefore not surprising to find the greatest possible divergences between 

 various classes of men in the quantity of protein taken in their daily food. 

 An average individual in affluent circumstances, eating three meat meals a 

 day, probably takes in from 100 to 160 grm. of protein daily, corresponding 

 to a nitrogen content of 16 to 25 grm. On the other hand, there is no doubt 

 that an individual can lead a perfectly normal existence with a nitrogen 

 intake as low as 5 or 6 grm. a day. It is not possible to explain these differ- 

 ences as determined by individual idiosyncrasies, nor is the appetite of the 

 individual to be taken as a safe guide to the relative composition of the foods. 

 The average diet of any race has been determined up to the present not so 

 much by the physiological requirements of the body as by the nature of the 

 food available. Hence, whereas the races living in tropical climates are 

 mainly herbivorous or frugivorous, the northerners, who have developed their 

 intellectual and bodily superiority in their harder struggle for food amidst 

 more inclement surroundings, have been perforce obliged to satisfy their 

 energy requirements at the expense of animal food. In these days when the 

 products of all climes are at the disposal of civilised man, his food-supply 

 is no longer dependent on the country in which he lives, and it is possible to 

 regulate the composition of his food according to the results of physiological 

 experience. Since the proteins represent the most costly constituents of the 

 food, it becomes important to inquire how much of this class of food-stuffs is 

 essential to the maintenance of health and whether any advantage is given 

 by taking proteins in excess of the physiological minimum. The fact that 

 those nations which hold the highest place in the world are mainly flesh- 

 eaters cannot be regarded as any proof of the advantage of a flesh diet over 

 a diet poorer in protein. It is the hard struggle for existence which in the 

 northern races Jias eliminated the weakling and resulted in the production of a 

 superior race. The fact that he is a flesh- eater can also be ascribed to the 



