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 d(31)-palmitic acid and H(2)(18)O allows the assessment of dietary fatty acid oxidation from cumulative (2)H recovery in urine and the estimation of the total body water pool (TBW) from (18)O dilution. Continuous-flow isotope ratio mass spectrometry (CF-IRMS) coupled to either equilibration or high-temperature conversion (HTC) techniques permits (2)H- and (18)O-enrichment measurements in biological fluids. Thus it was of great interest to compare these methods applied to the determination of dietary fatty acid oxidation. The linearity, accuracy and correlation between CF-equilibration and CF-HTC were first checked using (2)H- and (18)O-enriched water and urine samples. Urine samples from 14 subjects were then measured with both methods. The (2)H and (18)O raw data were normalised against calibration lines. The final aim was to study the impact of the normalised raw results on physiological data (i.e. TBW and d(31)-palmitate recovery). No significant difference was observed between the (18)O- and (2)H-enrichment measurements depending on the analytical method used. The TBW volumes calculated from the (18)O enrichments measured either with CF-equilibration or CF-HTC were not significantly different: respectively, 45.1 ± 1.0 L or 45.7 ± 1.0 L (mean ± sem, p = 0.09). The palmitic acid oxidation results obtained from the (2)H-enrichment measurements and the TBW from CF-equilibration vs. CF-HTC were not significantly different (p ≥ 0.26): with δ(2)H values of, respectively, 16.2 ± 1.6% vs. 16.2 ± 1.1% at 8 h, 18.7 ± 2.0% vs. 17.6 ± 1.3% at 12 h 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 the study of dietary lipid oxidation in man and generate similar results.