BIOLOGICAL ENERGETICS 425 



by placing the subject in a large calorimeter and detcrniining the heat 

 output as described above for the bomb calorimeter. This actually 

 has been done in many cases, but is so cumbersome and expensive that 

 a shorter, indirect method has been devised. This method depends 

 on the fact that heat output is closely related to oxygen consumed and 

 carbon dioxide liberated by the subject. Thus for the oxidation of 

 glucose, 



CeHioOg + 60o -» 6CO2 + 6H2O + 678 Cal. 



it is apparent that six moles of oxygen are used for the oxidation of one 

 mole of glucose and the production of 678 Cal.^ Six moles of a gas 

 occupy 6 X 22.4 or 134.4 1. at standard temperature and pressure. There- 

 fore each liter of oxj^gen used in this reaction results in the production 

 of 678 -^ 134.4 or 5.047 Cal. For fat oxidation the relations are some- 

 what different. Taking tristearin as an example, 



2(Ci7H35COO)3C3H5 + 1630o-^ 114C0o + llOHoO + 17,060 Cal. 



it is seen that in this case one liter of oxygen produces 17,060 -f- (163 X 

 22.4) or 4.67 Cal. Oxidation of average mixed food fats yields 4.69 

 Cal. and of mixed proteins, 4.82 Cal. per liter of oxygen consumed. 



An indication of which type of foodstuff is being oxidized by the body 

 at a given time is provided by the respiratory quotient (R.Q.), which 

 is the ratio of carbon dioxide given off to oxygen used : 



_ CO2 given off 

 O2 used 



The amounts of the two gasses may be expressed in moles or in volumes 

 (measured at the same temperature and pressure) . It follows from the 

 above equations that the R.Q. for oxidation of glucose is 1.00 and of 

 tristearin 0.70 (114-^163). In general, the R.Q. for average mixed 

 carbohydrates is 1.00, for fats 0.71, and for proteins 0.81. It has been 

 found that the R.Q. for both animal and human subjects under the con- 

 ditions of the basal metabolism test is close to 0.82. For this R.Q. 

 value each liter of oxygen consumed produces 4.825 Cal.- All that is 

 necessary, therefore, to find the metabolic rate is to measure the liters 

 of oxygen consumed per unit time and multiply by 4.825. 



The basal metabolism is a fundamental characteristic of the living 

 animal. Its magnitude depends on the body weight, body surface area, 

 sex, age, and other factors. However, for healthy individuals of a given 



^ The current value.s of heat output per liter of oxygen are based on 678 Cal. as the 

 heat of combustion of glucose, although 073 Cal. probably is more nearly correct. 



" Calculated on the assumption that only fats and carbohydrates are being oxidized. 

 The amount of protein oxidized, wliich can be estimated from the urinary nitrogen 

 excretion, does not have to be considered since proteins also yield 4.82 Cal. per liter 

 of oxygen. 



