To determine the time course of metabolic dysfunctions in recent active obesity, we studied basal energy expenditure and lipid and glucose oxidation in 31 obese children (duration of obesity 1-11.5 yr), compared with 14 lean age-matched control subjects. Using indirect calorimetry in basal overnight fasting conditions, we found that obese children produced 15% more energy than control subjects. Obese children oxidized twice as much lipid (56 +/- 4 mg/min) as normal children (25 +/- 5 mg/min, P < 0.0005), so that lipid oxidation provided 61 +/- 6% of overall energy production (vs. 33 +/- 3% in control subjects, P < 0.0005). This increase of lipid oxidation was already present in the earlier stages of obesity. Glucose oxidation was diminished in the obese (93 +/- 6 mg/min) compared with the control children (136 +/- 6 mg/min, P < 0.0005) and accounted for only 39 +/- 3% of energy production (67 +/- 6% in control subjects, P < 0.0005). This decrease was not present initially and appeared after approximately 4 yr and worsened with obesity duration (r = 0.72, P < 0.0005). The results were similar when lipid and glucose oxidation were normalized to body surface area or lean body mass. We hypothesize that increased lipid oxidation is one of the earlier dysfunctions observed in recent-onset obesity and that lipid oxidation may induce a progressive decrease of glucose oxidation, insulin resistance, and increased fasting insulin secretion.