FOOD REQUIREMENTS 351 



Food Requirements 



Instead of basing the feeding standard on the amount and propor- 

 tion of digestible nutrients (nutritive ratio), which is still the prevalent 

 custom, a more scientific and trustworthy feeding standard is based 

 on the amount of digestible protein in the ration and the chemical 

 energy or fuel value of the nutrients. The body may be likened to a 

 machine and the protein is necessary for the repair of the machine and 

 to supply additions to the machine, as in growing animals, and in 

 pregnant animals, and in those giving products rich in protein — as 

 milk. Therefore, there is a standard for maintenance and another for 

 growing animals and another for milch cows, etc. 



Animals at work do not require more protein than those at rest, 

 as the machine is running all the time and requires repairs in either 

 case, but more fuel is required. The carbohydrates and fat represent 

 the fuel, which is burned in the body to create heat or chemical energy, 

 and runs the machine. It is true proteids in excess or as waste pro- 

 ducts also act as fuel, but being much more expensive than carbo- 

 hydrates or fat are not desirable for fuel. Now the units of heat value 

 for food are termed the calorie (or great calorie) and therm. The 

 calorie is that amount of heat necessary to raise 1 kilogram (2.2 lbs) 

 of water 1 degree centigrade. The therm is equal to the quantity of 

 heat required to raise 1,000 kilograms (2,204.6 lbs.) of water i° C. 

 It is comparatively easy to estimate the heat value of food by burning 

 it outside the body but it takes very elaborate experiments on animals 

 to determine the heat or energy value in the body. As the result of 

 such investigations the following tables * of the heat or energy value 

 of food stuffs for the animal have been made. By these experiments 

 the actual proportion of the food which undergoes combustion in the 

 body is estimated, as part of the food escapes combustion by way of 

 the bowels and urine, part of the energy value is used in separating 

 the real fuel which the body can use from the useless portion, and 

 part is used in converting fuel materials into special products, as flesh 

 and milk. 



* Bull.- 346, U. S. Agricultural Dept. 



