Chapter 10: Converting carbon dioxide production to energy expenditure

10.1 Introduction
10.2 Estimating Eeq_CO2 for different oxidation mixtures
10.3 Adjusting Eeq_CO2 for nutrient imbalance in the subject
10.4 References

Contributor: Marinos Elia

10.1 Introduction

The fact that the doubly-labelled water method estimates carbon dioxide production rather than oxygen consumption is something of a drawback since the energy equivalent of carbon dioxide (Eeq_CO2) varies to a much greater extent than the energy equivalent of oxygen. The variation is governed by the substrate mixture being oxidised and hence the respiratory quotient (RQ). This is illustrated in Tables 10.1 and 10.2 which show, for example, that the Eeq_CO2 for carbohydrate oxidation is 30% lower than for fat, and 10% lower than for alcohol. The Eeq_CO2 for protein oxidation is intermediate between that of fat and carbohydrate.

As a result of this variation it is necessary to have quite an accurate estimate of the mean RQ (or better still the mean Eeq_CO2 over the whole of a DLW measurement period in order to convert CO₂ production to energy expenditure). The earliest human studies assumed an average value for the RQ of 0.85 for all subjects, but acknowledged that this was an approximation. In 1986 Black et al ¹ suggested a more sophisticated approach based on a knowledge of the macronutrient content of each subject's diet. This can be summarised in the form of a food quotient (FQ = calculated CO₂ produced/O2 consumed during oxidation of foods). Under conditions of perfect nutrient balance the FQ must equal the RQ. Black et al provided methods for converting FQs into RQs under conditions of imbalance, and demonstrated that under most circumstances it was possible to reduce the potential error from this source to below 2%.

This chapter provides a more detailed analysis of the issues involved.

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