Effect of substrate intake and physiological state on background 13CO2 enrichment.

The natural enrichment of 13C in energy substrates varies, and this variation must be taken into account when stable isotopic tracers are used in metabolic studies. This is conventionally accomplished by measuring background samples taken before the tracer infusion begins and subtracting these values from postinfusion values. Whereas this approach is satisfactory if no perturbation occurs between the collection of the background samples and the collection of postinfusion sample, the data presented in this paper show that any change in the metabolic state can significantly alter the background enrichment of expired CO2. This study not only confirmed that the introduction of natural energy sources may alter the background enrichment of CO2, but we also found that changes in substrate oxidation induced by different physiological states, such as exercise, can cause significant changes in expired CO2 enrichments. Conclusions from studies in which oxidation of substrates were measured by means of a 13C tracer but potential changes in background enrichments were not accounted for must, therefore, be reassessed.