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ESSENTIALS OF PHYSIOLOGY. 



In the experiment recorded in the foregoing table the animal on the 

 first day was in nitrogenous equilibrium. When it received three times 

 as much protein, it again reached nitrogenous equilibrium in the course 

 of the next seven days. 



If the food does not consist solely of protein, but also contains fat 

 and carbohydrate, most of the energy of the body is derived from the 

 latter, and nitrogenous equilibrium can be maintained on a comparatively 

 small amount of protein ; in this case the protein is used mainly to 

 repair tissue waste. 



The Sources of Muscular Energy. The muscles form about 40 per 

 cent, of the body weight and furnish the greater part of the energy set 

 free as heat or work in the body. This energy is derived from all the 

 food-stuffs, and the fact that the respiratory quotient remains almost 

 unaltered during muscular exercise shows that these food-stuffs must 

 be used nearly in the same proportions during exercise as during rest. 



That being the case, it would be expected that the breaking down of 

 protein in the muscles and the consequent excretion of nitrogen in the 

 urine would be increased by exercise, but observation shows that the 

 amount of nitrogen in the urine is practically unaffected, even by 

 severe exercise. The most probable explanation of this apparent 

 anomaly is that the nitrogenous moiety of muscle protein is resynthesised 

 in the muscle, and therefore does not appear as a waste product. 



Although the muscles make use of all the food-stuffs, they derive 

 their energy, both when resting and when active, principally from the 

 food-stuff which is most abundantly supplied to them in the blood. If 

 the diet consists mainly of fat or carbohydrate, these furnish the chief 

 source of muscular energy, and but little energy is derived from protein ; 

 when protein is the principal or sole food, it does serve as a source of 

 energy, and the nitrogen in the urine is increased. This has been clearly 

 shown in dogs which were made to do work when fed entirely on lean 

 meat ; their bodies contained hardly any fat or carbohydrate, so that 

 protein must have been used as the chief source of muscular energy. 



