Measurement of the rate of metabolic heat production by indirect calorimetry depends on two assumptions:

1. (1) It is assumed that the end result of all the biochemical reactions which occur in the body amounts effectively to the combustion or synthesis of three substances – carbohydrate, fat and protein.

2. (2) It is assumed that for each of these substances, when it is oxidised in the body, there are fixed ratios between the quantities of oxygen consumed, carbon dioxide produced and heat produced.

By any standards these are gross oversimplifications. The first completely ignores the metabolism of minerals, which represent nearly 7% of the total bodyweight. Although the mineral status of the body may be nearly stable in unproductive adults, major changes occur during phases of skeletal growth, pregnancy and lactation. The second assumption implies a uniformity in the properties of fat and protein that would seem improbable in view of their extremely varied chemical compositions. The only real justification for the assumptions is that over many years of practical application it has been found that indirect calorimetry is remarkably consistent and in close agreement with direct calorimetry.

The first step in evaluating a theoretical basis for indirect calorimetry must be to establish the values of the calorific factors for carbohydrate, fat and protein. These are determined from the results of combustion of the materials in a bomb calorimeter.

Bomb calorimetry

Adiabatic bomb calorimetry

As the name of the calorimeter implies, the energy content of a sample is determined by combustion in a closed system which can neither gain heat from, nor lose heat to the outside environment.