Role of physical activity and energy expenditure in the development of obesity
Although it is a popular belief that reduced levels of energy expenditure and physical activity lead to the development of obesity, this hypothesis remains controversial and has been difficult to prove. There are certainly good examples of an inverse relationship between physical activity and obesity (e.g., athletes are lean and nonobese individuals), as well as good examples of the positive relationship between obesity and physical inactivity (obese individuals tend to be less physically active). However, not all studies provide supporting evidence. For example, several studies suggest that increased television viewing (as a marker for inactivity) increases the risk of obesity, whereas others do not. Similar to the results for physical active ity, some studies suggest that a low level of energy expenditure predicts the development of obesity, and others do not support this hypothesis.
Physical activity is hypothesized to protect people from the development of obesity through several channels. First, physical activity, by definition, results in an increase in energy expenditure owing to the cost of the activity itself, and is also hypothesized to increase RMR. These increases in energy expenditure are likely to decrease the likelihood of positive energy balance. However, the entire picture of energy balance must be considered, particularly the possibility that increases in one or more components of energy expenditure can result in a compensatory reduction in other components (i.e., resting energy expenditure and activity energy expenditure). Secondly, physical activity has beneficial effects on substrate metabo-lism, with an increased reliance on fat relative to carbohydrate for fuel utilization, and it has been hypothesized that highly active individuals can main-tain energy balance on a high-fat diet.
Cross-sectional studies in children and adults have shown that energy expenditure, including physical activity energy expenditure, is similar in lean and obese subjects, especially after controlling for differ-ences in body composition. Children of obese and lean parents have also been compared as a model of preobesity. Some studies show that children of obese parents had a reduced energy expenditure, including physical activity energy expenditure, whereas another study did not. A major limitation of the majority of studies that have examined the role of energy expen-diture in the etiology of obesity is their cross-sectional design. Because growth of individual components of body composition is likely to be a continuous process, longitudinal studies are necessary to evaluate the rate of body fat change during the growing process. Again, some longitudinal studies support the idea that reduced energy expenditure is a risk factor for the development of obesity, whereas others do not. Finally, intervention studies have been conducted to determine whether the addition of physical activity can reduce obesity. These studies tend to support the positive role of physical activity in reducing body fat.
Several possibilities could account for such dis-crepant findings. First, the ambiguous findings in the literature may be explained by the possibility that differences in energy expenditure and physical activ-ity and their impact on the development of obesity are different at the various stages of maturation. This hypothesis is supported by previous longitudinal studies in children, showing that a reduced energy expenditure is shown to be a risk factor for weight gain in the first 3 months of life, but not during the steady period of prepubertal growth. Secondly, there could be individual differences in the effect of altered energy expenditure on the regulation of energy balance. Thus, the effect of energy expenditure on the etiology of obesity could vary among different sub-groups of the population (e.g., boys versus girls, dif-ferent ethnic groups) and could also have a differential effect within individuals at different stages of devel-opment. It is conceivable that susceptible individuals fail to compensate for periodic fluctuations in energy expenditure. Third, explanations related to the meth-odology can also be offered because of the complexity of the nature of physical activity and its measure-ment. The success of controlled exercise interven-tions in improving body composition indicates an extremely promising area for the prevention of obesity. However, further studies are required to elu-cidate the specific effects of different types of exercise on the key features of body weight regulation.