THE EFFECT OF FOOD ON METABOLISM 711 



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 suffi- 

 cient 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 impossible 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 equi- 

 valent 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 have, so to speak, reached ' par,' the possibility of laying 

 on protein tissues ceases. On the other hand, protein food has in man, 

 as in animals, a specific stimulating effect on metabolism, so that the 

 respiratory exchanges are largely increased as a result of a heavy 

 protein meal. This effect has been named by Rubner the ' specific 

 dynamic effect ' of protein. We shall have occasion later to discuss 

 its significance. 



THE INFLUENCE OF FATS AND CARBOHYDRATES 



If either fats or carbohydrates be given to a starving animal 

 a certain sparing of the fat of the body takes place, but this effect, 

 according to some observers, is accompanied by a distinct increase 

 in the metabolic exchanges of the body. As regards the protein 

 metabolism, Cathcart finds that while administration of fat increases 



