Exercise and Physical Activity

The term 'physical activity' refers to bodily movement produced by skeletal muscle that results in energy expenditure; it thus includes activities of daily living, as well as leisure activity from sport and exercise. The term 'exercise' refers to planned or structured bodily movements, usually undertaken in leisure time in order to improve fitness (e.g., aerobics), while 'sport' is physical activity usually in structured competitive situations (e.g., football). Physical activity at recommended levels (moderate intensity for 30 min for 5 days each week) is associated with many health benefits; these include lower all-cause mortality rates, fewer cardiovascular events such as myocardial infarction and stroke, and a lower incidence of metabolic disorders including non-insulin-dependent diabetes mellitus and osteoporosis. Levels of activity have been falling in Westernized societies largely because of a decrease in physical activity at work (from increasing mechanization) and increasingly sedentary leisure-time pursuits (such as television viewing). The Allied Dunbar National Fitness Survey of the UK showed that 70% of the population are insufficiently active, and a separate UK government survey showed that 1 in 3 adults could be classified as sedentary, i.e., taking less than half an hour of continuous moderate-intensity physical activity each week (Figure 2). Both cross-sectional data and prospective studies confirm an inverse relationship between physical activity and weight gain. The finding that in many countries such as the UK, average energy intake has fallen over the time that obesity has been increasing, emphasizes the importance of inactivity as a cause of obesity. These secular changes of inactivity are most marked in children who now spend much of their leisure time watching television or in other sedentary pursuits. Health authorities in many countries now

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Figure 2 Percentage of adults in England by age and sex (1990-1991) with a sedentary life style; dark bars, men; light bars, women. Data from Fentem and Walker (1995) Setting targets for England: challenging, measurable and achievable. In: Killoran A (ed.) Moving On: International Perspectives on Promoting Physical Activity. London: Health Education Authority.

advocate an increase in physical activity as a means of preventing obesity and improving health and fitness. While there is agreement that such measures may be useful in preventing obesity, the role of exercise in treating obesity is less clear. Potential mechanisms linking exercise and activity with weight loss and weight loss maintenance are shown in Figure 3. Like dietary change, increasing time spent on exercise and activity can be seen as part of a generalized behavioral change, which can be self-reinforcing.

Exercise and activity raise energy expenditure over and above the resting metabolic rate. Under some circumstances, such as prolonged vigorous exercise in trained individuals, rates of energy expenditure remain elevated for some time after the cessation of exercise. Logically, therefore, exercise should be a useful way to treat obesity. However, the amounts of exercise-induced energy expenditure are small in comparison with potential changes in energy intake.

The energy cost of activity and exercise can be expressed as a multiple of resting metabolic rate, termed a MET; the term 'physical activity level' (PAL) represents the total daily energy expenditure divided by the resting energy expenditure; it typically averages 1.5. The energy costs of walking are about 2.0 MET - for a 70 kg individual this is about 0.5 MJh-1 (120 kcal h-1) - while gentle running costs about 8 MET or 2 MJh-1 (480 kcal h-1). A moderately fit individual would only be able to maintain a level of exercise of 7 MET for about 30 min, representing an additional energy expenditure of about 1.5 MJ (360 kcal) resulting, if energy intake were maintained, in a weight loss of about 0.3 kg per week.

Energy expenditure remains above baseline for some time after exercise has stopped; this is termed 'post-exercise energy expenditure.' The effect is small and only produced by very high levels of activity, capable of achievement only by elite

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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Figure 4 Relationship of total weight loss to fat-free mass loss in women (A) and men (B) undertaking a diet with exercise (solid squares, solid line) or without exercise (open squares, broken line). Data from 21 randomized controlled studies, collated by Garrow JS, Summerbell CD (1995) Meta-analysis: effect of exercise, with or without dieting, on the body composition of overweight subjects. Eur J Clin Nutr 49: 1-10.

the groups that had included exercise were maintaining losses of 2.2-2.7kg while those on diet alone had only managed to maintain a 0.9 kg loss. Similar findings have been seen in dieters from commercial slimming groups.

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