Comparison of high-temperature conversion and equilibration methods for the determination of d31-palmitic acid oxidation in man using continuous-flow isotope ratio mass spectrometry

Abstract : During nutritional interventions, the ingestion of d31-palmitic acid and H218O allows assessment of dietary fatty acid oxidation from cumulative 2H recovery in urine and estimation of the total body water pool (TBW) from 18O dilution. Continuous flow – isotope ratio mass spectrometry (CF-IRMS) coupled to either equilibration or high temperature conversion techniques (HTC) permits 2H and 18O enrichment measurements in biological fluids. Thus it was of great interest to compare these methods applied to the determination of dietary fatty acid oxidation. Linearity, accuracy and correlation between CF-equilibration and CF-HTC were first checked using 2H- and 18O- enriched water and urine samples. Urine samples from 14 subjects were then measured with both methods. 2H- and 18O- raw data were normalised against calibration lines. The final aim was to study the impact of normalised raw results on physiological data (i.e. TBW and d31-palmitate recovery). No significant difference was observed between δ18O‰ and δ2H‰ enrichment measurements depending on the analytical method used. Volumes of TBW calculated from δ18O‰ enrichments measured either with CF-equilibration or CF-HTC were not significantly different with respectively 45.2±1.0L or 45.7±1.0L (mean±sem, p=0.09). Palmitic acid oxidation results obtained from δ2H‰ enrichment measurements and TBW from CFequilibration vs CF-HTC were not significantly different (p≥0.26) with respectively 16.2±1.6% vs 16.2±1.1% at 8h, 18.7±2.0% vs 17.6±1.3% at 12h and 21.7±1.9% vs 21.5±1.3% at 3 days post-dose (mean ± sem). Thus, even if CF-HTC was preferred because it was more practical to carry out, both methods allow study of dietary lipid oxidation in man and generate similar results