Carbohydrate utilization in obese subjects after an oral load of 100 g naturally-labelled [13C] glucose

1. Total carbohydrate (CHO) and ingested glucose oxidation was measured in five obese subjects with normal glucose tolerance after an oral load of 100 g naturally-labelled [13C]glucose using indirect calorimetry and mass spectrometry respectively. 2. CHO utilization rate (107 +/- 14 mg/min in the post-absorptive state) increased 30 min after the glucose load to reach a plateau (245 +/- 25 mg/min) between 90 and 120 min. It then decreased to basal values at 330 min. Cumulative CHO oxidation over 480 min was 66 +/- 7 g and the CHO oxidized above basal levels was 26 +/- 7 g. 3. Enrichment of expired carbon dioxide with 13C began at 45 min and maximum values were observed between 210 and 300 min. At 480 min, cumulative oxidation of the ingested glucose was 24 +/- 2 g. 4. Compared with controls, the obese subjects exhibit an impairment of CHO utilization which precedes glucose intolerance. This impairment can be explained by an increased availability of free fatty acids which favours lipid oxidation at the expense of ingested [13C]glucose oxidation.