CHAPTER XX. 



UTILIZATION OF FOOD. 



Food is used by animals to maintain the body activities and 

 restore waste of material. It is also used for the production of 

 new material in growth and fattening, for milk production, and 

 for energy to produce work. Whenever energy or heat are to be 

 generated, oxygen unites with the substances, forming carbon 

 dioxide and water from fats, organic acids, sugars, etc., and 

 carbon dioxide, water, and urea or other nitrogenous waste pro- 

 ducts, from proteids. The carbon dioxide is eliminated by the 

 lungs, and the nitrogenous waste passes off in the urine. The 

 oxidation does not take place at one time, but a number of inter- 

 mediate products are formed. 



The following is the average composition of ten kinds of 

 animals, according to analyses made by Lawes and Gilbert, at 

 Rothamsted : 



Per cent. 



Protein 13.5 



Fat 28.2 



Water 49.0 



Ash 3.2 



Contents of stomach and intestines 6. i 



Total IGO.O 



The ash consists of approximately 86 per cent, calcium phos- 

 phate, and 12 per cent, calcium carbonate, with small quantities 

 of fluorides, chlorides, iron, potash, and magnesia. These 

 materials must all be supplied by the food. 



Different nutrients of food have different values for the pur- 

 poses above stated. The first bodily activity with respect to food 

 is its mastication and digestion. This consumes food material, 

 which although derived from food previously eaten, must be re- 

 placed by the food being eaten. Different kinds of food require 

 different amounts of energy in mastication and digestion. 1 



The food material remaining after deducting the losses due to 

 mastication, digestion, and undigested residues, may be used for 

 1 Hagemann, Exp. Sta. Record 10, p. 906. 



