FOODS. 145 



existing between them, indicate the amounts of proteid, fat, and 

 carbohydrate which are required to cover the loss. 



Metabolism on a Purely Proteid Diet. Notwithstanding 

 the chemic composition of the proteids and the possibility of their 

 giving rise to both fat and a carbohydrate during their metabolism, 

 it has been found extremely difficult to maintain the normal nutrition 

 for any length of time on a pure proteid or fat-free flesh^diet. This, 

 however, has been accomplished with dogs'. IfWSWSuRci, however, 

 that, in order to maintain the equilibrium, it was necessary to increase 

 the proteids from two to three times the usual amount. Thus, a dog 

 weighing 30 to 35 kilograms required from 1500 to 1800 grams of 

 flesh daily in order to get the requisite amount of carbon to prevent 

 consumption of its own adipose tissue. Under similar circumstances, 

 a human being weighing 70 kilograms would require more than 2000 

 grams of lean beef an amount which, from the nature of the digestive 

 apparatus, it would be practically impossible to digest and assimilate 

 for any length of time. Even the slight habitual excess beyond the 

 amount normally required is imperfectly assimilated and gives rise 

 to the production of nitrogen-holding compounds which, on account 

 of the difficulty with which they are eliminated by the kidneys, ac- 

 cumulate within the body and develop the gouty diathesis, with all 

 its protean manifestations. 



Metabolism on a Fat and Carbohydrate Diet. As nitrogen 

 is an indispensable constituent of the tissues, it is evident that neither 

 fat nor carbohydrates can maintain nutritive equilibrium except for 

 very short periods. On such a diet the tissues consume their 

 own proteids, as shown by the continuous excretion of urea, though 

 the amount is less than during starvation. An excess of fat retards 

 the metabolism of proteids. The same holds true for the carbohy- 

 drates. 



Thus, in any well-arranged dietary there should be a combina- 

 tion of proteids, fats, and carbohydrates in amounts sufficient to 

 maintain nutritive equilibrium; in other words, to repair the loss of 

 tissue and to furnish the requisite amount of heat in accordance with 

 work done, as well as with climatic and seasonal variations. 



COMPOSITION OF FOODS. 



The food principles essential to the maintenance of the nutrition 

 of the body are contained in varying proportions in compound sub- 

 stances termed foods; e. g., meat, milk, wheat, potatoes, etc. Their 

 nutritive value depends partly on the amounts of their contained 

 food principles and partly on their digestibility. The dietary of 

 civilized man embraces foods derived from both the animal and 

 vegetable worlds. 



