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Introduction: Respiratory exchange ratio (RER) is the ratio between CO2 produced and O2 used for body metabolism. It indicates the type of fuel that is metabolized in the body. This study aimed to measure and compare the RER in non-obese and obese sedentary young adults in rest, moderate-intensity, and vigorous-intensity exercise.
Methods: This cross-sectional study was conducted with 23 non-obese and 24 obese sedentary young adults. Resting RER was measured with 12-h fasting with 15-min rest with an automated gas analyzer. Then, RER was measured during steady moderate-intensity and vigorous-intensity exercise on a cycle ergometer. RER was compared between male and female, non-obese and obese in resting, in moderate-intensity, and vigorous-intensity exercise by t-test.
Results: The Mean age of the non-obese and obese group was 19.35±1.11 and 19.79±0.78 years, respectively. The overall male showed higher RER (in resting and in moderate-intensity exercise) than females. In resting (0.802±0.018 versus 0.821±0.022, P = .001), moderate-intensity exercise (0.812±0.013 versus 0.83±0.02, P <.001), and vigorous-intensity exercise (0.853±0.43 versus 0.914±0.032, P <.001), the obese group showed higher RER in comparison to non-obese group. In relation to resting value, RER significantly increased during moderate- and vigorous-intensity exercise in both the non-obese and obese groups.
Conclusions: Obese young adults use relatively more carbohydrates as fuel than non-obese in both resting conditions and during exercise. When the intensity of exercise is increased, both obese and non-obese showed higher RER which indicates that sedentary young adults use relatively more carbohydrates as fuel in a higher grade of exercise.

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