HATCH, K.A.*; PINSHOW, B.; SPEAKMAN, J.R.: Stable sotopes in exhaled CO₂ and the assessment of substrate metabolism in flying birds

It is difficult to measure metabolic substrate use in flying birds. Indirect calorimetry can be used for short periods of time in wind tunnels but is not easily used in the field. Blood metabolite concentrations have become increasingly popular as indicators of metabolic substrates in freely flying birds. However, measuring blood metabolites does not directly measure metabolic substrate use. We suggest that the application of stable isotope analysis offers a more direct method of measuring metabolic substrates, particularly when applied in concert with the measurement of blood metabolites. Stored body lipids are depleted in ¹³C relative to the diet, whereas carbohydrates are not. Consequently, breath CO₂ reflects the metabolic substrate at sampling time. We have demonstrated in trained tippler pigeons that naturally occurring ¹³C /¹²C ratios in exhaled CO₂ provide a measure of the relative contribution of carbohydrates and lipids to flight metabolism. Manipulating isotope signatures in the diet promises to increase the usefulness and resolution of this method.