Ventilation and its control during incremental exercise in obesity.

BACKGROUND: In obesity, the addition of mass loading of the chest wall by adipose tissue decreases compliance, but its ventilation does not seem to be a limiting factor to physical performance. Plasma K(+) and lactic acid are considered important determinants of ventilation during exercise. Obesity is characterized by insulin resistance.

OBJECTIVES: The aim of this study was to assess ventilatory adaptations to sustained effort and the effects of lactic acid and potassium in young obese subjects.

METHODS: Twelve obese subjects with a body mass index of 40 (mean age 27 years, 6 males) and 12 normal subjects with a body mass index of 22 (aged 28 years, 6 males) performed a progressive cycloergometric test with increases of 20 W every 4 min to exhaustion while minute ventilation, oxygen consumption, carbon dioxide production, end-tidal oxygen pressure, and end-tidal carbon dioxide pressure were measured. Blood samples were collected at the end of every step to determine plasma K(+). Lactic acid was measured at rest, 40, 80, 120 W and peak exercise (or only at peak exercise if <120 W). Before each exercise, we tested insulin sensitivity using the quantitative insulin sensitivity check index.

RESULTS: Obese subjects had lower insulin sensitivity (0.318 vs. 0.345, p < 0.01). Peak exercise was not significantly different between both groups (125 W in the obese group vs. 137 W in the control group), but the ventilatory threshold was at lower power output in the obese group compared to the controls (76 vs. 107 W, p < 0.05). Ventilation increased less in the obese group but oxygen saturation of hemoglobin remained within normal limits up to exhaustion in both groups. Ventilation was appropriate for the CO(2) increase but less appropriate for the increased O(2) consumption. Both K(+) and lactic acid increased less in the obese group.

CONCLUSIONS: In our obese subjects, ventilation was not a limiting factor during exercise. Its lower increase may be due, in addition to the characteristics of their chest walls, to insulin resistance which may limit the increase in lactic acid during effort, and to the hypertrophy of muscle fibers previously noted, which may be linked to a lower increase in plasma K(+) during physical exercise.