Use of a new gas chromatograph isotope ratio mass spectrometer to trace exogenous 13C labelled glucose at a very low level of enrichment in man

Abstract

The use of ¹³C labelled glucose in human metabolic studies has been limited by the high cost of the tracer and the problems of measuring low ¹³C isotopic abundance in plasma glucose. In the present work we describe a new gas chromatograph-isotope ratio mass spectrometer allowing the measurement of a 0.001 atom % increase in ¹³C abundance over baseline, on a nanomole glucose sample. Studies were performed in rats and in human subjects. The rate of glucose appearance in 24 h fasted rats using D-[1-¹³C] glucose as tracer and analysed by this new method was found to be 10.4±0.7 mg·kg⁻¹· min⁻¹, a value 21% lower than that found using D-[6,6-²H₂] glucose as tracer (13.1±1.1 mg· kg⁻¹· min⁻¹) analysed by classic gas chromatography-mass spectrometry. The new method was also used to trace, in combination with D-[6,6 ²H₂] glucose, the metabolic fate in human subjects of two oral glucose loads (0.5 g· kg·⁻¹), 1 g· kg ·⁻¹) labelled with 0.1% D-[U-¹³C] glucose. During the six hours following the glucose load, it was found that total glucose appearance was 0.97±0.04 g· kg·⁻¹ and 1.2±0.04 g· kg·⁻¹, exogenous glucose appearance was 0.51±0.02 g· kg·⁻¹ and 0.84±0.04 g· kg·⁻¹, endogenous glucose production was 0.44±0.04 g· kg·⁻¹ and 0.35±0.06 g·kg·⁻¹ after the 0.5 and 1 g·kg·⁻¹ load respectively. These values are similar to those reported using glucose labelled with radioactive isotopes. These results show that reliable kinetic parameters of glucose metabolism can be determined, without health hazard, in humans, at low cost, using ¹³C labelled glucose analysed with a new gas chromatograph-isotope ratio mass spectrometer.