Vital Energetics: A Study in Comparative Basal Metabolism

By Francis G. Benedict | Go to book overview

TECHNICAL CONDITIONS FOR COMPARABLE
MEASUREMENTS OF HEAT PRODUCTION

INSTRUMENTAL TECHNIQUE

As the most accurate index of vital activity is heat production, the method of measuring the heat production and the calculation of the results of such measurements are important. Ideally and theoretically direct heat measurements are desirable, but no calorimeters are sufficiently sensitive to permit measurements in relatively short periods and, as already emphasized, periods of over one, hour are for the most part with practically all animals contaminated by more or less activity. Consequently recourse must be had to indirect calorimetry which fortunately has, by repeated experiments with respiration calorimeters, been found to agree extraordinarily well with direct calorimetry. For this verification periods of complete repose are by no means necessary. Comparisons of the direct with the indirect method can be carried out without any reference whatsoever to the degree of repose, provided the period of measurement is sufficiently long. The Nutrition Laboratory has every confidence in indirect calorimetry. This has been particularly stressed by its recent experience with geese under surfeit feeding, when, with tremendous increases in the respiratory quotient,1 the results by indirect calorimetry were perfectly in accord with those by direct calorimetry. In indirect calorimetry measurements are made of the two gaseous factors of metabolism, carbon-dioxide elimination and oxygen consumption. As the caloric value of carbon dioxide per liter varies enormously, depending upon the character of the combustion, the measurement of the carbon- dioxide production has only a general use in heat calculations. Nevertheless when an animal is known to have a respiratory quotient characteristic of fat combustion, about 0.71, the measurement of the carbon-dioxide elimination alone is justifiable. In those cases, and they are numerous, where the post- absorptive respiratory quotient is not necessarily that of fat, since the caloric value of carbon dioxide varies greatly with the increased respiratory quotient, knowledge of the respiratory quotient is imperatively needed. With most of the animals studied the determination has been made of the oxygen absorbed as well as the carbon dioxide produced. In a few instances, however, the carbon-dioxide measurements alone have been made. Only in those cases where it was known as a result of other experiments that the respiratory quotient was essentially that of fat are the carbon-dioxide values used in our analysis. The greatest stress, therefore, is laid upon the calculations of the heat production based on measurements of the oxygen consumption. In many instances, especially with the spirometer form of apparatus, the oxygen consumption alone is measured. As the caloric value of oxygen varies only slightly whether the combustion is exclusively carbohydrate or exclusively fat, the probable caloric value of oxygen for the

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1
Benedict, F. G., and R. C. Lee, Carnegie Inst. Wash. Pub. No. 489, 1937, pp. 207 et seq.

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