THE NORMAL DIET OF MAN 655 



simply to increase the total metabolism of the body without serving any 

 useful physiological purpose other than heat production.* Many ailments, 

 especially of middle age, have been ascribed to an excess of protein in the 

 food. It has been thought that the kidneys and other organs may suffer 

 from the strain of eliminating excess of nitrogenous waste products. But the 

 energy metabolism of proteins results almost entirely in the formation of urea 

 an innocuous substance which can have little harmful effect on the kidneys, 

 even if we assume (an assumption hardly justifiable) that these organs (unlike 

 other organs of the body) suffer as a result of their normal functional activity. 

 There is no doubt that many disorders of middle life may be put down to 

 over-feeding and lack of muscular exercise, but there is just as much reason 

 to ascribe these evils to the carbohydrates as to the proteins of the diet. It 

 is indeed possible that an almost exclusive protein diet might be more 

 suitable than carbohydrates for a sedentary life, where the normal stimulus 

 to oxidation of the food- stuffs, viz. muscular exercise, is absent, and that the 

 ' stimulant ' effect of proteins on metabolism might have a real value to the 

 organism. 



The limitation of protein diet to a minimum is only justifiable in adult 

 healthy men. Where from any cause there has been a loss or destruction 

 of nitrogenous tissue, as after infective diseases, the body possesses the power 

 of storing up nitrogen in the form of flesh or muscular tissue, and it is 

 important under such circumstances to give in the food an excess of protein 

 which can be used for this purpose. Moreover, when a rapid growth of 

 muscle is going on, as during training, an excess of protein in the food is 

 also desirable, though nothing is gained in pushing this administration of 

 protein to an inordinate extent. 



The fact that in many cases greater economy and efficiency of nutrition 

 are attained by diminishing the protein of the diet affords no argument for 

 accepting or rejecting any given class of foods. Thus the normal requirements 

 of the individual can be obtained by the administration of a diet containing 

 meat, eggs, vegetables, and cereals, or by a diet derived entirely from the 

 vegetable kingdom. A purely vegetable diet is rendered possible in civilised 

 countries by the ease with which the products from warmer climates can be 

 obtained. A diet composed only of the products of temperate climates 

 would tend to be deficient in fats and oils. In such climates it is therefore 

 necessary to import the oily fruits grown in warmer lands, or to supplement 

 the diet with such animal food as milk or butter or eggs. In drawing up a 

 purely vegetarian diet it is important to remember that its constituents, 

 especially its protein, are digested with greater difficulty than the corre- 

 sponding ingredients in animal food. A larger quantity therefore has to be 

 given in a vegetable diet in order to allow for the greater loss by the faeces. 

 Any general reform of diet which may be indicated by recent physiological 

 experiments would seem to lie rather in the direction of limitation of the 



* But heat production is a very important function of the food, and on the 

 Chittenden diet tends to be deficient ; so that individuals on this regime ' feel the 

 cold ' more than they did when on an ordinary diet. 



