In this study we have investigated a hypothesis that proposes the reverse of the so-called "glucose-fatty acid cycle, " i.e., that accelerated carbohydrate metabolism directly inhibits fatty acid oxidation. We studied normal volunteers in the basal state and during a hyperinsulinemic, hyperglycemic clamp (plasma insulin = 1,789 +/- 119 pmol/l, plasma glucose = 7.7 +/- 0.2 mmol/l). We quantified fat oxidation using indirect calorimetry and stable isotopes ([1-13C]oleate). Plasma oleate enrichment and free fatty acid (FFA) concentration were kept constant by means of infusion of lipids and heparin. Glucose oxidation increased from basal 6.2 +/- 0.8 to 22.3 +/- 1.4 mumol.kg-1.min-1 during the clamp (P < 0.01). Total (indirect calorimetry) and plasma fatty acid oxidation (isotopic determination) decreased from 2.6 +/- 0.2 to 0.4 +/- 0.3 (P < 0.01) and 2.2 +/- 0.2 to 1.4 +/- 0.1 mumol.kg-1.min-1 (P <0.05), respectively. We conclude that under the conditions of the present experiment, glucose and/or insulin directly inhibits fatty acid oxidation. Our findings suggest that, contrary to the prediction of the glucose-fatty acid cycle, the intracellular availability of glucose (rather than FFA) determines the nature of substrate oxidation in human subjects.