Optimizing Fat Oxidation Through Exercise in Severely Obese Caucasian Adolescents

Stefano Lazzer; Carlo Busti; Fiorenza Agosti; Renzo Pozzo; Alessandro Sartorio

Disclosures

Clin Endocrinol. 2007;67(4):582-588. 

In This Article

Summary and Introduction

Summary

Objective: To measure the contribution of substrate oxidation to energy expenditure during cycling at different workloads and to identify the exercise intensity that elicits the maximum fat oxidation rate in groups of severely obese or nonobese Caucasian adolescents.
Design: A total of 30 severely obese subjects (mean body mass index, BMI = 34·7 kg/m2; fat-mass = 39·9%) and 30 nonobese sedentary adolescents (mean BMI = 22·7 kg/m2; fat-mass = 21·8%) aged 14-16 years, participated in this study. Body composition was assessed by bioelectrical impedance. Peak oxygen uptake (VO2peak) and maximal fat oxidation rate were determined with indirect calorimetry by using a graded exercise test on an electromagnetically braked cycle ergometer.
Results: Predicted VO2max were expressed in absolute (l/min) and relative (ml/kg FFM/min) values, and maximal work rates were not significantly different between obese and nonobese adolescents, but were significantly higher in boys than in girls. No significant differences in fat oxidation rates were found in obese and nonobese sedentary adolescents during the graded exercise test. Maximal fat oxidation was observed at an exercise intensity corresponding to (mean ± SD) 41 ± 3% VO2max or 58 ± 3% HRmax. At this exercise intensity, fat oxidation rates were higher in boys than in girls (0·32 ± 0·02 g/min vs. 0·25 ± 0·02 g/min, P < 0·001).
Conclusions: Severely obese and sedentary nonobese adolescents reached maximal fat oxidation rates at 41% VO2max, which corresponds to 58% HRmax. At this exercise intensity, fat oxidation rates were higher in boys than in girls probably due to higher VO2max and absolute workload during the exercise steps for boys compared with those for girls.

Introduction

The prevalence and severity of obesity in adolescents, and the comorbidities of obesity, are increasing in industrialized countries.[1] Many studies have shown that obesity represents a risk factor for the development of metabolic complications such as insulin resistance, diabetes, hypertension, ischaemic heart disease and sleep apnoea.[2,3,4] Physical activity attenuates these risks[5] and is recommended as an adjunct to a weight-reducing diet for the treatment of obesity. Thus, it is necessary to develop physical activity programs designed to optimize fat oxidation rates and to reduce fat mass (FM).

In obese subjects defects in skeletal muscle fat metabolism have been found in postabsorptive conditions[6] and b-adrenergic stimulation.[7] Moreover, carbohydrate and fat metabolism defects may interact, especially through substrate competition.[8] Thus, this blunted capacity to oxidize fatty acids may play an important role in the development of a positive fat balance and may increase fat storage in obese subjects. One of the underlying mechanisms for the positive effects of exercise training in obesity may be related to its effects on fat utilization. Physical activity increases energy expenditure[9] and promotes fat use by skeletal muscle[10] and exercise training clearly enhances fatty-acid oxidation during physical activity.[11] Cross-sectional studies reported that the exercise intensity which corresponds to maximal fat oxidation was 42% of VO2max[12] in obese adults and 40% of VO2max[13,14] in obese children. However, little is known regarding patterns of substrate utilization during exercise in severely obese adolescents compared with normal-weight sedentary adolescents. In addition, given the potential effects of both obesity and physical fitness on substrate selection during exercise, it is of particular importance to examine whether or not substrate utilization during exercise is altered in obese adolescents, irrespective of their physical fitness.

Therefore, the objectives of the present study were to measure the contribution of substrate oxidation to energy expenditure during cycling at different workloads[1] and to identify the exercise intensity that elicits the maximum fat oxidation rate in severely obese and sedentary nonobese Caucasian adolescents.[2]

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