THE EFFECT OF FOOD ON METABOLISM 



635 



increased metabolism which rises in proportion to the nitrogenous income. 

 The limit to this increase is set by the powers of the alimentary canal to 

 digest the protein. The rise in metabolism consequent on protein food is 

 very rapid and affects the gaseous exchanges as well as the output of nitrogen. 

 Magnus Levy and Falk found that a large protein meal might increase the 

 respiratory exchanges 40 per cent., an increase which lasted seven hours. 



The nitrogenous output also rises immediately after a protein meal, so that 

 50 per cent, of the nitrogen of the ingesta may appear in the urine within 

 seven hours after the meal. 



The whole of these results cannot be strictly applied to omnivorous 

 animals, such as man. In these it is impossible to supply all the energy 

 requirements of the body on a pure protein diet. Even if a man eats as 

 much meat as he can, he will be unable to obtain sufficient energy for his 

 daily requirements. Whereas the average daily requirements of a man 

 amount to about 3000 calories, 1 Ib. of meat would yield only about 400 

 calories, and even if he took 4 Ib. of meat daily, an amount which is im- 

 possible for most individuals, he would only be obtaining about 1600 calories. 

 The cures for obesity, in which a large protein diet plays an important part, 

 owe their efficiency to this fact. They are in all cases practically equivalent 

 to a state of semi-starvation. Many experiments have been made on the 

 influence of variations in the quantity of protein in a mixed diet. Within 

 wide limits the output of nitrogen is strictly proportional to the intake. 

 A normal adult man seems to be unable to store up protein in any form, and 

 differs in this respect from carnivora, such as the dog or cat. The only way 

 in which protein can be laid on in the body is by furnishing a physiological 

 stimulus to the growth of muscle, i.e. by constant exercise. Without this it 

 is not possible to produce growth of the muscles of the body, however much 

 protein we may give in the diet. The conditions are, however, different 

 when dealing with an individual in whom from some cause or other the 

 muscular tissues have not attained their full development. Thus in growing 

 individuals a certain amount of the protein of the food is -always retained in 

 the body and laid on as tissue-protein. In convalescence after severe fever, 

 during which a great wasting of the muscles has taken place, forced feeding 

 with large amounts of protein has been found to give rise to a considerable 

 retention of protein in the body. This process only goes on until the muscles 

 have attained their normal condition of development. When the tissues 



