THE NORMAL DIET OF MAN 731 



energy output of a man on a given diet as affording a criterion of 

 the amount of food necessary for him per day, since it is possible that 

 with a smaller amount of food the stimulating effect on metabolism 

 might be wanting and that the functions of the body might be 

 normally performed with a greater economy of material. The stimu- 

 lating effect of fats and carbohydrates on metabolism has not, how- 

 ever, been universally observed, whereas in the case of proteins every 

 worker has noted an increased metabolism in proportion 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 carbohydrates, 

 for meeting the energy requirements of the body. The amino- 

 acids produced by the disintegration of the proteins in the alimentary 

 canal are rapidly absorbed and apparently undergo deamination 

 in the wall of the gut itself as well as in the liver. 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. However 

 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 nitro- 

 genous 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 differences 

 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 frugi- 

 vorous, the northerners, who have developed their intellectual and 



