THE EFFECT OF FOOD ON METABOLISM 681 



requirements. Whereas the average daily requirements of a man amount to 

 about 3000 Calories, 1 Ib. of lean meat (i.e. entirely devoid of fat) 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 practi- 

 cally 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 gave 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 goes on only 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 the nitrogen output during starvation, 

 carbohydrate food causes a diminution in the nitrogen output, and thus 

 exercises a marked sparing effect on the proteins of the body. Voit found 

 that during starvation or with an insufficient protein diet addition of fat 

 to the food increased the total metabolism. When sufficient protein was 

 being supplied, the addition of fat caused no increase in the total metabolism, 

 the whole of the fat in the food being laid on as fat in the body. The stimu- 

 lating effect of fat on metabolism is but slight. Magnus Levy found that 

 the increase in the metabolism on the administration of fat to a starving 

 animal was minimal and never exceeded 10 per cent. 



Carbohydrates have a somewhat greater influence on metabolism. This 



