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This study describes a rapid and simple method to determine short‐chain fatty acid (SCFA) concentrations and their isotopic enrichments (M0+ 1 and M0+ 2) in human plasma. Sample preparation involves SCFA extraction and derivatization with 1‐(tert‐butyldimethylsilyl)imidazole. Gas chromatography/mass spectrometry was performed using chemical ionization with ammonia as the reagent gas. Outstanding resolution, excellent linearity and good detection limits were obtained. Inter‐assay and intra‐assay repeatability was below 10% and 3% respectively for SCFA concentration. Inter‐assay repeatability was below 5%, 4%, 6%, and 14% for isotopic enrichment determination of [1‐13C]acetate and [1,2‐13C2]acetate, [1‐13C]propionate and [1‐13C]butyrate respectively, with intra‐assay being below 6%. Such SCFA concentrations and isotopic enrichments were determined in the plasma of rats infused with a 13C‐labeled SCFA. The turnovers of acetate, propionate and butyrate in rats were 19 µmol kg−1min−1, 2.6 µmol kg−1min−1, 0.3 µmol kg−1min−1respectively. Copyright © 2001 John Wiley & Sons, Ltd.

Glutamine is an essential fuel for tissues with high rates of cell replication, such as enterocytes and lymphocytes. Infusion of ¹³C-labeled glutamine tracers allows for measurement of the rates of production, utilization and oxidation of glutamine’s carbon skeleton in vivo. The use of this tracer, however, has been limited by its high cost and/or the difficulty in measuring low enrichments in biological fluids using conventional gas chromatography–mass spectrometry (GC/MS) techniques. We have developed a method using gas chromatography–combustion–isotope ratio mass spectrometry (GC–C–IRMS) that allows for the determination of low ¹³C enrichments (down to 0.06 mol.% excess) with a precision of 2% or better, and a within-day and between-day variability better than 5%, in plasma free glutamine. The method was applied to measuring the incorporation of ¹³C in plasma glutamine over the course of infusion of ¹³C-labeled acetate in a human subject. © 1997 John Wiley & Sons, Ltd.