Assessment of Energy Expenditure

The study of energy metabolism in humans has recently raised a great interest in the regulation of these processes thanks to advances in the construction of open-circuit ventilated hood indirect calorimeters and comfortable respiration chambers.

With the only measurement of VO2 (in liters of O2 STPD (standard temperature (0 °C), pressure (760mmHg), and dry) per min), metabolic rate (M), which corresponds to energy expenditure, can be calculated (in kilojoules per min) as follows:

The number 20.3 is a mean value (in kJl-1) of the energy equivalent for the consumption of 11 (STPD) of oxygen. The value of the energy equivalent of oxygen depends on the composition of the fuel mixture oxidized. However, the error in using equation 1 instead of an equation that takes into account the type of fuels oxidized (equations 2 and 3, see below) is no greater than ±1 to 2%.

To take into account the heat released by the oxidation of the three macronutrients (carbohydrates, fats, and proteins), three measurements must be carried out: oxygen consumption (VO2), carbon dioxide production (VCO2), and urinary nitrogen excretion (N).

Simple equations for computing metabolic rate (or energy expenditure) from these three determinations are written under the following form:

The factors a, b, and c depend on the respective constants for the amount of O2 used and the amount of CO2 produced during oxidation of the three classes of nutrients. An example of such a formula is given below:

where M is in kilojoules per unit of time, VO2 and VCO2 are in liters STPD per unit of time, and N is in grams per unit of time. As an example, if VO2 = 6001 day-1, VCO2 = 5001 day-1 (respiratory quotient, or RQ = 0.83), and N = 25 g day-1, then M= 12 068 kJ day-1. With the simpler equation (1) the results give a value of 12180 kJ per day.

Slightly different factors for the amounts of O2 used and of CO2 produced during oxidation of the nutrients are used by other authors, and the values for the factors a, b, and c are modified accordingly. The difference in energy expenditure calculated by the various formulae is not greater than 3%.

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