Exercise and obesity

Obesity is a condition of excess body fat. While genetic factors can influence the body fatness of an individual, environmental factors, promoting a high energy intake and low energy expenditure (EE), appear to be driving the high prevalence of obesity in many countries (Danforth, 1985; Brownell & Wadden, 1992; Van Itallie, 1996). A low level of physical activity can contribute to a low total EE and is thought to promote positive energy balance and weight gain (Flatt, 1987; James, 1995). This is supported by crosssectional data which show that individuals and groups characterized by a low level of physical activity have a greater body weight and body fat content compared with their more active counterparts (Schulz & Schoeller, 1994; Roberts, 1995; Ryan et al. 1996). A low activity level, however, could be a consequence rather than a cause of obesity. Nevertheless, longitudinal data also suggest that a low level of physical activity is associated with an increased risk of weight gain (Roberts et al. 1988; Griffiths et al. 1990; Williamson et al. 1993). Thus, regular physical exercise appears to be important in helping maintain a normal body weight and body composition. Both metabolic and behavioural factors play a role in the development of obesity (Hill et al. 1994). The relative contributions of these factors to weight gain probably vary between individuals. Both food intake and physical activity have behavioural and metabolic components. With respect to physical activity, the behaviour relates to the quantity and type of physical activity an individual chooses to engage in, whereas the metabolic factors relate to the amount of energy expended, the pattern of fuels utilized and the hormone and metabolite changes resulting from the exercise. As yet, we have a poor understanding of how the metabolic effects of physical activity affect exercise and/or food intake behaviour. However, the metabolic response to exercise is more clearly understood. The present paper, therefore, will review the metabolic effects of exercise, specifically with respect to energy and fat metabolism, and how this may relate to the long-term control of body weight.