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Deriving heat production from gaseous exchange: validity of the approach

Abstract : The use of indirect calorimetry as a means to quantify heat production (Q) and net substrate oxidation has increased rapidly since the pioneering work of Lavoisier, and today, indirect calorimetry is often used as a reference for other measures of Q. Simple equations were developed and widely adopted to calculate the production of heat from the measurement of the production of CO2 and CH4, the consumption of O2 and urinary nitrogen loss. The coefficients in these equations were derived from stoichiometry of the complete oxidation of carbohydrates, fat and protein. In this chapter, taking the Brouwer equation as an example, these calculations are explained and their validity to compute Q is discussed. Particular attention is paid to anaerobic fermentation and de novo lipogenesis from carbohydrates. It is concluded that Q can be predicted satisfactorily from the O2 consumed and CO2 produced using factors derived from oxidation of carbohydrates, fat and protein. Caution is required for corrections for CH4, particularly in ruminants, although quantitatively, the impact of these corrections on the estimates of Q seems limited. The approach is also valid when de novo lipogenesis from carbohydrates is substantial. As it is assumed that O2 and CO2 pools within the body do not change within a measurement period, a critical attitude towards this approach is required whenever pushing a biological system with e.g. a diet or infusate while using it to estimate short term variation in Q. Examples of these include strenuous exercise or an environmental challenge, e.g. heat stress, during which energy containing intermediates can be temporarily stored in the body and/or the bicarbonate pool fluctuates. Lastly, the measurement of retained energy based on indirect calorimetry in growing subjects is compared with other techniques. Comparison with the comparative slaughter technique in growing subjects reveals that measurement based on indirect calorimetry tends to lead to higher estimates of retained energy, but treatment differences within studies are quantified accurately. It can be argued that the bias reflects a true difference in Q, related to differences in e.g. housing conditions. Comparison of indirect calorimetry with the carbon-nitrogen balance technique and with direct calorimetry, reveal a very close match between these techniques. In conclusion, although a critical attitude is appropriate, Q can be accurately derived from gaseous exchange measurements using the simple equations developed in the previous century, with coefficients based on the complete oxidation of carbohydrates, fat and protein.
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W.J.J. Gerrits, J.J.G.C. van den Borne, Etienne Labussière. Deriving heat production from gaseous exchange: validity of the approach. Indirect calorimetry, Wageningen Academic Publishers, 2015, 978-90-8686-261-0. ⟨10.3920/978-90-8686-261-0_1⟩. ⟨hal-02799191⟩



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