MAINTENANCE RATIONS OF FARM ANIMALS. 



19 



AVAILABILITY OF ENERGY. 



Since the chief function of the feed, aside from a minimum of 

 protein, is to supply energy, it would be natural to suppose that the 

 quantity of energy liberated in the body by the oxidation of any 

 given substance (i.e., its metabolizable energy) would be the measure 

 of its nutritive value. If one gram of starch, for example, can liberate 

 4.2 calories of energy in the body and a gram of fat 9.5 calories, 

 apparently the relative values of the two should be in proportion to 

 these figures. But while the metabolizable energy of the feed rep- 

 resents the maximum amount of energy which can be extracted from 

 it by the organism, it does not follow that all of it can be utilized for 

 maintenance. Energy is not something which can be fed into the 

 organism regardless of its source, like fuel under a boiler. Whatever 

 energy is in essence, so far as the animal is concerned it is carried 

 as chemical energy by the compounds of the feed, and these must be 

 such as can take part in the actual chemical changes occurring in the 

 cells if their energy is to be utilized. The body can not, like a heat 

 engine, avail itself of energy in the kinetic form. It is quite con- 

 ceivable that a compound might be resorbed from the digestive tract 

 and then simply oxidized to get rid of it without its entering into 

 the cell metabolism. Its energy- would be metabolized, that is, con- 

 verted into the kinetic form, but it would be simply a source of heat 

 and not of other forms of energy. Somewhat similar is the case of 

 the chemical changes occurring in the digestive tract. Some of these, 

 notably the fermentations of the feed, set free energy as heat, yet 

 this energy plays no part in the actual metabolism of the tissues. It is 

 clear, then, that we are not warranted in concluding that because, 

 for example, a fasting animal breaks down body substance equivalent 

 to 10 therms per day, therefore a ration containing 10 therms of 

 metabolizable energy will suffice to maintain the animal. That will 

 depend upon how completely the body is able to use the 10 therms of 

 metabolizable energy supplied to it. In other words, the energy must 

 not only be present, but it must be available energy. 



If the metabolizable energy were all available to protect body 

 tissue from oxidation, then giving feed to a fasting or partially 

 fasting animal would be practically the substitution of one kind of 

 fuel for another, and the total heat production would remain the 

 same. It is, however, an observation as old as the time of Lavoisier 

 that the consumption of feed tends to increase the heat production 

 of an animal. That investigator observed the oxygen consumption 

 of man to increase materially (about 37 per cent) after a meal, and 

 subsequent experiments by a large number of investigators have fully 

 confirmed these earlier results, so that the fact of an increased metab- 

 olism consequent upon the ingestion of feed is undisputed. It is 

 especially to the investigations of Zuntz and his associates that we 



