The purpose of this study was to test the hypothesis that in obese children: (1) ventilatory efficiency (VentE) is decreased during graded exercise; and (2) weight loss through diet alone (D) improves VentE, and (3) diet associated with exercise training (DET) leads to greater improvement in VentE than by D. Thirty-eight obese children (10 +/- 0.2 years; BMI > 95th percentile) were randomly divided into two study groups: D (n = 17; BMI 30 +/- 1 kg/m2) and DET (n = 21; 28 +/- 1 kg/m2). Ten lean children were included in a control group (10 +/- 0.3 years; 17 +/- 0.5 kg/m2). All children performed maximal treadmill testing with respiratory gas analysis (breath-by-breath) to determine the ventilatory anaerobic threshold (VAT) and peak oxygen consumption (VO2peak). VentE was determined by the VE/VCO2 method at VAT. Obese children showed lower VO2peak and lower VentE than controls (p < 0.05). After interventions, all obese children reduced body weight (p < 0.05). D group did not improve in terms of VO2peak or VentE (p > 0.05). In contrast, the DET group showed increased VO2peak (p = 0.01) and improved VentE (VE/VCO2 = 6.1 +/- 0.9; p = 0.01). VentE is decreased in obese children, where weight loss by means of DET, but not D alone, improves VentE and cardiorespiratory fitness during graded exercise.

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