500 ANIMAL BIOCHEMISTRY 



methods have been sought. AH these methods are based on the fact 

 that there is a close relationship between the heat produced and the 

 weights of the materials concerned. In the oxidation of glucose as an 

 example, 6 moles of oxygen is required and 6 moles of carbon dioxide 

 formed. Therefore, a determination of either gas will be a measure of 

 the quantity of glucose metabolized when this sugar is fed to an ani- 

 mal, and the heat produced can be calculated at the rate of 3.75 

 kcal./g. of glucose. More highly reduced foods involve different 

 proportions of oxygen and carbon dioxide, but the calculation is the 

 same in principle. Since most natural food materials are mixtures of 

 classes of foods, it is necessary to measure both oxygen and carbon 

 dioxide in order to estimate the relative extents to which these classes 

 are contributing energy. The general procedure is considered in more 

 detail on pages 504 to 506. 



Direct calorimetry with known foods permits a comparison of the 

 heat actually obtained and that expected after correction for digestive 

 and excretory losses. Any differences indicate that food intake and 

 heat production are out of balance. When the observed heat is lower 

 than the anticipated value, the animal is not oxidizing the food com- 

 pletely. Either it is not digested with the usual efficiency, or free 

 energy is being conserved and not converted to heat. This latter situa- 

 tion is common during the growth of young animals, recovery after 

 injury or illness, or the deposition of fat. Whenever the heat meas- 

 ured exceeds that expected, the food ingested must be inadequate in 

 quantity or composition, requiring the animal to draw on its reserves 

 and metabolize body materials with a loss in weight. Likewise, in- 

 direct calorimetry with foods of known composition reveals differences 

 between observed and expected oxygen consumption and carbon 

 dioxide evolution. Such results indicate gain or loss in weight. 



For the initial study of the heat contents of foods by animal calorim- 

 etry, the animal must be in metabolic equilibrium with a constant 

 body weight. When rations are studied under these conditions, values 

 like those of Table 21-2 are obtained for the heat contents, called 

 caloric values in most nutrition tables. Data of this type are used to 

 compile diets meeting the levels of energy and nutrients required by 

 particular individuals. The Appendix contains a more extensive 

 table of the same sort. A comparison of values from the two tables for 

 given foods reveals numerous differences. For one thing these varia- 

 tions reflect the inhomogeneity of most foods. Differences in experi- 

 mental techniques, efficiency of digestion, and differences between in- 

 dividuals account for some of the variations. As a result, the values 

 reported can be considered only as approximations and used as such. 



