Naturally Labeled ¹³C-Glucose Metabolic Studies in Human Diabetes and Obesity

The ¹³C/¹²C ratio in most commercial preparations of glucose used in clinical investigation is much higher than that of expired air. Variations in expired air ¹³C, after glucose load, are indicative of the catabolism of this exogenous glucose. The changes in the ¹³C/¹²C ratio of CO₂ in expired air after oral administration of glucose were determined by mass spectrometry. Results in eleven healthy volunteers and seven obese subjects with normal oral glucose tolerance tests (OGTT) are reported. In all cases, the administration of glucose resulted in a marked rise in the ¹³C/¹²C ratio of expired CO₂, reaching its maximum at the fourth hour and then declining progressively. In seven obese patients with chemical diabetes and in five insulin-dependent diabetics, the ¹³C/¹²C ratio of expired CO₂ during OGTT was significantly reduced, in comparison with the results obtained from the control groups. This study demonstrates the potential of this procedure using “naturally labeled” ¹³C glucose for the metabolic studies in man as anticipated from previous studies of Duchesne and his coworkers.