The current public-health recommendation for physical activity is for individuals to participate in 30 minutes of moderately intense physical activity on most days of the week.31 The reason that people do not lose weight with increased exercise alone is because they usually also increase their food intake. Therefore, exercise combined with energy restriction is recommended based on the rationale that physical activity will result in an increase in total energy expenditure. It appears that the key factor that explains the relationship between exercise and weight is the adoption of an active lifestyle to prevent weight gain and weight regain.
Exercise has a powerful effect on insulin sensitivity. Studies in Pima Indians and persons of European ancestry demonstrated that physical fitness was as powerful a modulator of insulin resistance as was body weight; each variable accounted for approximately 25 percent of the differences in insulin-mediated glucose disposal in nondiabetic persons.32 Obese individuals with type 2 diabetes mellitus had an increase in insulin sensitivity following low-intensity bicycle riding.33 In nonobese, insulin-resistant relatives of type 2 diabetic subjects, moderate-intensity exercise had a 40 percent increase in insulin sensitivity.34 The Diabetes Prevention Program found that intensive exercise in combination with a change in diet could lower the risk of progressing to diabetes by 58 percent.35
Despite recommendations of exercise for prevention of weight gain and improvement of cardiovascular fitness and insulin sensitivity, the major challenge is adoption of a regular exercise pattern. Recent studies on the effectiveness of intermittent exercise (multiple 10- to 15-minute exercise sessions daily) suggest that intermittent exercise is a successful strategy for increasing the adoption of exercise in overweight individuals who are sedentary.36 The long-term, cumulative effect of small changes in activity level can be beneficial. By walking about 2000 extra steps a day, 100 extra calories can be burned a day. It appears that the key factor that explains the relationship between exercise and weight is the adoption of an active lifestyle to prevent weight gain and weight regain. Evidence from the National Weight Control Registry, a group of 1047 individuals who lost at least 30 pounds (13.6 kg) and maintained that loss for at least one year, supports this claim. In an analysis of successful weight maintainers, one hour or more of moderate to vigorous physical activity per day was the factor that led to a successfully maintained weight loss over an average of 6.9 years.37
Understanding how powerfully the environment influences behavior is necessary to understand why some people gain more weight than others in the same environment. Considerable interindividual differences in the trainability of cardiorespiratory endurance traits have been observed after exposure to identical training programs, suggesting genetic diversity accounts for the variability in weight gain.38 In twin studies, Bouchard et al. showed that the effect of exercise may be influenced by genetic differences between individuals. In his elegant studies, there was a 6.8 times greater change in body weight between pairs than within pairs of twins that were engaging in the same type and amount of physical activity.39 Energy expended during physical activity is highly variable, and it is the component of total energy expenditure over which an individual has the most control. It may represent 15 percent to 50 percent of the total 24-hour energy expended, depending on the activity of the individual. Increasing intensity and duration of activity will increase energy expenditure.
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