Measuring natural abundance of ¹³C in respired CO₂: Variability and implications for non-invasive dietary analysis

1. Three experiments were performed, using laboratory mice (Mus musculus) as a model species, to evaluate the potential of using measurements of carbon isotope ratios in expired CO₂ for tracing diets. 2. Breath ¹³C signatures of mice fed a constant diet (-21.4‰ ± 0.35) reflected their diet, but were depleted by on average -5.7‰. Body mass, sex and age were independent and significant factors correlated with the variability of ¹³C enrichment in respired CO₂. 3. Breath ¹³C signatures from starved mice (7 h) were lower than unstarved mice by 2.0‰. Subsequently when starved mice were fed a small meal of a new diet, breath ¹³C signatures approached those of the new diets within 15 min, returning to preingestion levels after 105 min. 4. After a permanent diet switch ¹³C values of breath were not asymptotic within 6 days, possibly because of use of fat reserves during the daytime carrying an isotopic memory of the previous diet. Hence, individual breath ¹³C signatures may vary according to nutritive state and previous dietary history. 5. Interindividual variability was measured at 3.3‰. The implications are that large samples of individuals will be required to distinguish between diets of different populations where the isotopic difference between their diets was small - for example, that expected between herbivorous and carnivorous diets. However, breath would be suitable for distinguishing between dietary intakes of individuals for food types that are isotopically more distinct - such as between C₃ and C₄ plants.