Meeting Abstract

98.8  Monday, Jan. 6 15:15  Isotope labeled metabolic tracers are not created equal: comparing the oxidative kinetics of free- versus biochemically-integrated dietary nutrients MCCUE, M D*; CARDENTEY, A; St. Mary’s University mmccue1@stmarytx.edu

Clinicians and researchers are increasingly using ¹³CO₂-breath-testing to diagnose metabolic disorders, especially those related to protein and lipid metabolism. Breath testing involves giving an oral dose of a ¹³C-labeled nutrient and tracking the rate at which it is oxidized by collecting exhaled CO₂– and measuring the recovery of ¹³C atoms. In this study we examined how the extent to which the tracer atom is biochemically integrated into larger macromolecules affects the outcome of the breath tests. Specifically, we compared the oxidative fates of 1) an amino acid tracer (¹³C-Leucine) that was dosed as a free amino acid and the same tracer molecule integrated into ¹³C-ovoalbumin protein or 2) a fatty acid tracer (¹³C-1-palmitic acid) that was dosed as a free fatty acid and the same tracer molecule integrated into a triacylglyceride (i.e., ¹³C-1,1,1-tripalmitin). Although the time courses of tracer oxidation were species-specific and differed between the protein and lipid tracers, we found clear differences in oxidative kinetics among all three species of animals. In every case, the recovery of ¹³C-atoms in the breath was substantially lower (e.g., by an order of magnitude) from metabolic tracers that were integrated into larger macromolecules than when the tracers were dosed as monomers of macromolecules. We conclude that 1) characterization of the oxidative kinetics of small tracer molecules may not be representative of ‘typical’ dietary nutrients when these monomers are biochemically integrated into larger macromolecules and 2) direct comparisons of studies that examine the oxidative kinetics between integrated and nonintegrated tracer molecules will yield misleading outcomes.