CHAPTER XXII. 



FEEDING WORK ANIMALS AND GROWING ANIMALS. 



Work requires energy, which is produced by the oxidation of 

 organic matter within the tissues. It has been known for a long 

 time that animals use up more oxygen and give off more carbon 

 dioxide while they are doing work than when they are at rest. 

 A considerable amount of investigation has been required to 

 ascertain the value of different nutrients for producing work. 



Nutrients Oxidized During Work. The effect of work upon 

 the proteids of the body can be studied by determining the quan- 

 tity of nitrogen in the urine during periods of rest and periods 

 of work. If the work necessarily involves a destruction of pro- 

 teids, an increased elimination of nitrogen will result. Experi- 

 ments have given contradictory results. For example, a dog fed 

 on meat eliminated the following quantities of nitrogen, during 

 three consecutive periods : 



Nitrogen in urine 

 grams per day 



No work 54.9 



Working 48.6 



No work 55.0 



In other experiments, the proteid metabolism was considerably 

 increased. The results depend on the ration being fed. If the 

 ration supplies sufficient carbohydrates and fat to furnish energy 

 for the work, proteids will not be oxidized, but if the ration is 

 deficient in this respect, proteids will be oxidized to furnish the 

 necessary energy. 



Animals at work exhale increased amounts of carbon dioxide, 

 even though there be no increase in the quantity of nitrogen 

 eliminated. This is evidence that fats or carbohydrates are be- 

 ing used for the production of work. Further evidence that 

 starch and other non-proteids can be used for the production of 

 work, is afforded by experiments, such as the following: 



A working animal was fed on a ration poor in protein. In 

 a second period the same ration was fed with the addition of 

 starch and the amount of work was then increased until the 



K was 



