Moving from a complete energy balance towards substrate oxidation: use of stable isotopes

Substrate oxidation can be estimated from gas exchange in combination with urinary nitrogen excretion measurements. The estimates are net rates of oxidation, because several metabolic processes, such as lipogenesis and gluconeogenesis, may affect the respiratory quotient, resulting in under- or overestimation of absolute rates of substrate oxidation. If these limitations, as well as accumulation of ketone bodies, lactate and urea, are taken into consideration, indirect calorimetry can be an effective method to quantify substrate oxidation in animals and humans. Methane should be excluded from the equations for calculating oxidation rates. Using 13C labelled substrates provides additional possibilities in oxidation studies. Whole body nutrient oxidation can be measured when tracer amounts of the nutrient of interest are continuously infused and when 13CO2 expiration and 13C enrichment of the precursor for oxidation are measured in blood plasma. Oxidation of exogenous substrates can be distinguished from that of endogenous substrates by using 13C enriched (or depleted) nutrients in the diet. In addition, labelling of individual monosaccharides, fatty acids or amino acids can provide insight in the metabolic fate of the tracee irrespective of availability of other substrates from the same macronutrient class (i.e. carbohydrate, fat, protein). Finally, selection of appropriate isotopomers may allow quantification of pre-identified metabolic processes (e.g. lipogenesis). Fluctuations in background enrichment, isotope administration strategy, sequestration of CO2 and isotopic gradient in the body are among the factors that should be considered when designing and interpreting oxidation studies with labelled nutrients. Modelling approaches should be in place or be developed to maximise the learnings from these studies. This appears especially challenging when animals are studied in a non-steady physiological state. In conclusion, using 13C labelling in combination with indirect calorimetry provides unique opportunities to quantify the contribution of individual substrates (endogenous as well as exogenous) to whole body energy metabolism.