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Figure 3 Mechanisms linking exercise with weight loss and weight loss maintenance.

athletes. The mechanism for this effect is unknown. Moderate intensity exercise programs, of the sort prescribed to the obese, are unlikely to raise energy expenditure by more than about 0.2 MJ (50kcal) per exercise session.

Regular exercise does, however, elevate long-term energy expenditure by its effect on altering body composition. Resting metabolic rate is proportional to the fat-free mass. Exercise increases muscle development and bone mass, so directly raising metabolic rate. The purpose of weight loss is to reduce fat mass, with as little loss of fat-free mass (FFM) as possible. The loss of fat to meet the extra energy requirements of regular exercise will decrease the ratio of fat to FFM and thus indirectly favor an increase in resting metabolic rate for any given body weight. These effects are modest, and mainly only seen from the sort of high-intensity excercise achieved by athletes. Even endurance-level training over periods of up to 12 weeks increases nonexercising daily energy expenditure by less than 0.8 MJ (190kcal).

The effects of exercise are thus quantitatively small. The relatively small potential for exercise to reduce body weight is borne out by the results of trials of exercise in obesity treatment, which suggest that exercise programs achieve weight losses of less than 0.1 kg per week, and that total weight loss averages about 3 kg. In one meta-analysis of five controlled trials of exercise without dietary restriction, mean weight loss in 95 men was 2.6 kg over 30 weeks, compared with a gain of 0.4 kg in the control group.

Programes that combine dietary and exercise interventions can be more successful, but it is often difficult to separate the effects of one from the other. In order to explore the effect of exercise on the composition of weight loss during dieting, Garrow analyzed data from 21 randomized, controlled studies. All trials that combined exercise and diet and included information about weight and FFM loss were included (Figure 4). A small reduction in the percentage of FFM lost is observed if exercise is included with the dietetic intervention. Thus, for example, in a woman losing 15 kg, exercise would reduce her FFM loss from 3.6kg (24%) to 3.0kg (20%). Similar but quantitatively greater benefits are seen in men: for a 15 kg weight loss, exercise reduced FFM loss from 3.6kg (24%) to 2.5kg (17%).

Activity and exercise are strong predictors for successful weight loss maintenance. A number of studies have shown that obese women who have lost weight and continue to undertake regular exercise are 3-4 times more likely to maintain their weight loss over a follow-up period of 2-3 years. The amount of exercise also correlates with the degree of success. In one study of about a hundred obese men and women who had lost about 27 kg, those with high levels of exercise were maintaining an average of 18 kg loss at 3 years, compared with 9 kg in the moderate exercise group and no weight loss in the nonexercisers. The importance of exercise and weight loss maintenance is demonstrated by a 2-year study of obese subjects treated by either diet, exercise, or a combination of the two. Weight loss in the diet group at 1 year was 6.8 kg, in the exercise group 2.9 kg, and 8.9 kg in the combination treatment group. However, after 2 years

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