In obese patients submaximal brisk walking exercise may be very difficult to sustain, because of the extra metabolic burden imposed by the excess body mass. Parameters of maximal aerobic exercise performance may be strikingly reduced when expressed per kg body mass. The aim of the present study was to analyse whether cardiorespiratory exercise function is truly impaired in obese children and adolescents, when parameters of aerobic exercise function are used which are independent of body mass. Therefore the kinetics of oxygen uptake (×{special}O₂) at the onset of exercise were studied by analysis of the normalised oxygen deficit.

The patients underwent square wave exercise testing on a treadmill. The speed was set at 5 km h^-1 and the inclincation at 4 %. The oxygen deficit was calculated by subtracting the ×{special}O₂ measured at the onset of exercise from the steady-state ×{special}O₂ obtained at the end of the exercise. These differences were cumulated and expressed as a percentage of the cumulated oxygen cost for the 6 min exercise test. All data are expressed as means and standard deviation. Differences between groups were calculated by Student’s unpaired t test. The local medical ethical committee approved the study. The subjects were 17 obese patients (mean age: 11.2 ± 2.6 years), body mass was 70.7 ± 21.4 kg, body mass index averaged 28.9 ± 2.8 and the percentage overweight was 53.9 ± 16.5 %. The patients were compared to a group of 18 normal controls of comparable age: 11.6 ± 2.2 years (P > 0.25 patients vs.controls), body mass was 40.4 ± 10.6 kg (P < 0.001, patients vs. controls).

In the obese patients, the oxygen deficit amounted to 7.2 ± 1.9 % and was not significantly (P > 0.25) different from the value obtained in normal controls: 6.9 ± 1.0 %. However obese patients exercised at a higher percentage of the maximal heart rate (79 % in the obese subjects vs. 70 % for normal controls). Due to a less efficient walking economy during treadmill exercise, ×{special}O₂ (expressed per kg body mass) during submaximal exercise was slightly higher in the obese (22.3 ± 2.7 ml O₂ min^-1 kg^-1) compared to the normal controls (20.2 ± 2.4 ml O₂ min^-1 kg^-1) (P < 0.05).

The similar values for O₂ deficit at the onset of exercise in obese patients compared to normal controls shows that there is no evidence of a cardiovascular limitation of exercise capacity in obese patients. Due to a less efficient walking pattern, ×{special}O₂ during submaximal exercise was higher in the obese patients. Therefore a same absolute work intensity is perceived as more strenous in obese subjects compared to normal controls.

This study was supported in part by the Foundation for Research in Pediatric Cardiology.