Short Communication Anaerobic Heat Production Measurements: a New Perspective

Heat production measurements can be performed in two ways: direct and indirect calorimetry. Direct calorimetry is the measurement of heat loss, which under normal steady-state conditions equals the heat production (H). Indirect calorimetry is the calculation of heat production based on parameters related to the heat production, such as respiratory gas exchange (O2 and CO2) and nitrogen excretion (urea, uric acid or ammonia). A simplified form of indirect calorimetry is the calculation of heat production by means of an oxycalorific value. The calculation of this oxycalorific value is rather complex (Gnaiger, 19836). Furthermore, it is only possible if the ratio of the amounts of the three substrates for oxidation (carbohydrate, fat and protein) is known. Both forms of indirect calorimetry are based on a standard composition of the substrates for oxidation. Direct calorimetry is a difficult and expensive technique. Indirect calorimetry, which is a relatively simple technique, is therefore used more frequently. Direct calorimetry, however, is independent of aerobic or anaerobic conditions, whereas indirect calorimetry, based on respiratory gas exchange and nitrogen excretion, is restricted to aerobic conditions. Thus direct calorimetry is the only available method for both aerobic and anaerobic heat production measurements. However, if the endproducts of incomplete oxidation can be measured accurately, the indirect calorimetric formula for the calculation of heat production can be adjusted, as Brouwer (1965) did for the methane production of ruminants. It should be noted that the excreted protein-nitrogen, in the form of urea, uric acid or ammonia, is also an endproduct of incomplete oxidation; correction of the formula is made by a reduction of the caloric value of protein, depending on the nitrogen excretion product(s). The excreted nitrogen is used to calculate the amount of oxidized protein. It is obvious that the oxycalorific value is useless under anoxic conditions. Indirect calorimetry not only involves a formula for the calculation of heat production, but also formulae for the calculation of the amounts of oxidized carbohydrate and fat. The aim of this paper is to describe general formulae for indirect calorimetry and to discuss their usefulness for anaerobic heat production measurements.