636 PHYSIOLOGY 



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 Kubner 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 

 stimulating 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. 

 The administration of a large meal of carbohydrates to a starving animal 

 may raise the respiratory exchanges from 20 to 30 per cent., and the increase 

 may last four hours after the meal. This stimulating influence on metabolism 

 is, however, much less than that observed on the administration of large 

 doses of protein. If carbohydrate be given in excess of the daily energy 

 requirements the greater proportion of it remains in the body, being stored 

 up to a small extent as glycogen, but mainly in the form of fat. 



