UTILISATION OF FOOD AND ENERGY 107 



daily work was light or increased threefold. The 

 food rich in carbohydrates had sufficed for the pro- 

 duction of the energy required for the hard work. 



II. The same animal was given a ration rather 

 poor in protein and afterwards the same ration 

 to which one kilo of starch was added. Whilst on 

 these rations the animal had to perform hard work, 

 which was gradually decreased until the excretion 

 of nitrogen remained constant and did not decrease. 

 In this way the maximum work which could be per- 

 formed on the two rations without calling upon 

 body protein was determined. The results showed 

 that by the addition of starch the animal was able 

 to perform considerably more work. 



III. A similar experiment was carried out with 

 the addition of linseed oil to the basal ration, and 

 it was found that the food to which oil had been 

 added yielded more energy than food without. 



These investigations show that carbohydrates and 

 fats are a source of muscular energy, and also under 

 what conditions the animal draws upon its body 

 protein to obtain energy. This happens only when 

 the total quantity of nutrients in the food, together 

 with the body fat, do not suffice to yield the neces- 

 sary energy. In such a case the animal is not 

 getting enough food and draws upon its own tissues. 

 Under the usual conditions of feeding the nitrogen- 

 free nutrients (carbohydrates and fats) of the food 

 are the chief sources of energy, the protein only 



