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Intuitively we know that exercise should be a treatment for overweight and obesity because changes in body weight are a result of the difference between caloric intake and energy expenditure. To lose weight we must either decrease caloric intake, increase energy expenditure, or both. Despite the obvious logic of this line of thinking, evidence-based treatment of overweight and obesity requires that there is a sound scientific basis for the mechanisms that lead to exercise increasing fat loss, that there are well-controlled clinical trials demonstrating the effectiveness of exercise in weight loss and weight maintenance programs, and that a realistic plan is available for patients to follow for weight loss and weight maintenance. This chapter is structured to cover these three points.

There have been several important symposia that have addressed the topic of exercise and obesity. The proceedings of those meetings have been published and references to reviews have been used in this chapter rather than the extensive original articles that they represent.

HYPOTHESES AS TO HOW EXERCISE MAY ENHANCE WEIGHT LOSS AND WEIGHT MAINTENANCE

Increased energy expenditure

The energy that we expend each day (total energy expenditure or TEE) can be portioned into three parts; resting energy expenditure (REE which is also sometimes referred to as the basal metabolic rate), the energy expended after a meal (thermic effect of a meal or TEM), and the energy of physical activity (activity energy expenditure or AEE). The resting energy expenditure (REE) is the energy necessary to run the basic processes of the body such as the energy to maintain electrochemical gradients, generate heat, synthesize proteins, etc. The REE can easily be measured by indirect calorimetry and generally accounts for approximately 60 to 70% of the total energy expenditure. For the average adult the resting energy expenditure (REE) is fairly close to 1 kcal/kg body weight/ hr (or about 1,680 kcal/ day for a individual weighing 70 kg or 154 lbs).

The thermic effect of a meal (TEM) can be measured by determining the increase in oxygen consumption after a meal. Although TEM varies with the composition of the diet, it is estimated to be roughly 10% of the total energy expended (TEE). The cause of TEM is not known with certainty but may be due to the energy necessary to digest and process the food that we ingest.

The most variable component of TEE is the energy of physical exercise (AEE). In sedentary people it can be as little as 10 to 15 %, while in active people it can be 30 to 40 % of the total energy expended each day. In athletes competing in exceptionally rigorous competition, such as the tour de France, AEE can be as much as 400% of the average daily energy expenditure of non-exercise days (1).

There are basically 2 ways that exercise can increase energy expenditure: (1) energy is expended while exercise is being performed and (2) resting energy expenditure (REE) is increased for a period after the finish of exercise. The energy that is expended during an exercise bout is dependent on many factors but some general calculations can be made to help a patient understand the concept. A 75 kg (165 lb) person requires approximately 100 kcal to walk a mile. Since 1 lb of body fat yields about 3500 kcal of energy, this means that 35 miles must be walked to lose a pound of fat.

Energy expenditure is dependent on the size and weight of an individual and therefore it is useful to use a unit called MET (multiples of resting energy expenditure) to calculate energy expenditure. Table 1 shows the energy expenditure in METs of some common types of exercise (2). From the measured or estimated resting energy expenditure (REE) the energy expenditure for an exercise can be calculated as the product of REE and MET value. Light activity would be defined as 1.5 METs, while 4 METS would represent moderate activities, such as brisk walking, and 6 METs would be representative of vigorous activities. In a moderately fit individual, 7 METs corresponds to approximately 70% of the maximal oxygen consumption capacity (1). A moderately fit individual is not able to expend more than 7 METs continuously for more than about 30 min. Exercise of 2-5 METs is usually advisable for obese individuals who are in a weight loss program. As they become more fit, they can increase the intensity, and thus the energy expenditure.

Calculating how much exercise is required to lose weight is a discouraging "exercise" but there may be good news for exercisers who are fit and can exercise at higher intensities (at least 70% VO2max for at least 30 min.). A number of studies have demonstrated that a single bout of exercise may increase the resting energy expenditure (REE) by 5-15% for 24-48 hours. The sustained increase in metabolism after exercise may be a result of elevations of serum norepinephrine (3). Athletes have been shown to have REEs that are 5-20% higher than those of sedentary individuals, even after adjusting for fat free mass. An increase of 10% in the REE would amount to 160 kcal/day for a 70 kg individual, which would mean the loss of a pound of fat in about 20 days.

Decreased appetite

Individuals who are dieting are often concerned that exercise may increase their appetite and thus make reducing food intake more difficult. There is no objective evidence to support this fear and the opposite may be the case (4). After very strenuous exercise there is a clear temporary decrease in appetite, which may be partially a result of diversion of blood from the viscera. There are also other exercise responses, such as elevation of catecholamines, which are known to depress the desire to eat.

Long-term regulation of caloric intake occurs by a very complex mechanism. At very high levels of energy expenditure, such as in lumberjacks, there seems to be a close link between caloric intake and expenditure so that body weight is kept constant. However, at low activity levels this mechanism seems to break down with resultant weight gain. Cross sectional studies in men show that at moderate, heavy and exceptionally heavy activity levels there were no differences in BMI, suggesting a link between caloric intake and expenditure. However, in the light and very light activity groups there was a significant inverse relationship between activity level and BMI, suggesting that appetite and food intake did not respond appropriately to physical exercise, or more correctly, to the lack of exercise (5). Likewise, in a longitudinal study previously obese women with a TEE/REE ratio above 1.75 had little or no weight regain, while women with low ratio (below 1.75) demonstrated an inverse relationship between weight gain and activity level. These data suggest that increasing activity from very light or light to moderate or heavy does not cause appetite to increase. Thus, energy expenditure can be increased without a compensatory increase in food intake.

Increased fat oxidation

Since obesity results from the storage of excess fat, the question arises whether obesity is linked to an inability to burn fat. In Pima Indians, weight gain was significantly correlated with the resting respiratory quotient (RQ) (6). Individuals in the 90th percentile of RQ (high carbohydrate, low fat oxidation) were at 2.5 times higher risk of gaining at least 5 kg body weight in 10 years than those in the 10th percentile RQ. Leg balance studies confirm that obese individuals have lower fasting fat oxidation in muscle than lean subjects (7). These studies suggest that individuals who burn less fat are at higher risk of becoming obese. If this is true, then anything that increases fat oxidation may be important for preventing obesity, improving weight loss during dieting, and maintaining weight loss.

At rest, approximately half of the energy expenditure is covered by fat oxidation with the remainder coming primarily from carbohydrate. Less than 5% of the energy comes from protein metabolism. Fat continues to make a significant contribution to energy expenditure at low intensities of exercise, but carbohydrate oxidation is proportionately increased with exercise intensity and contributes 100% at very high work rates (such as in sprinting). This relationship between fat oxidation and exercise intensity has led to the recommendation by some physicians that low intensity exercise was best for weight loss.

One of the important adaptations to endurance exercise training is an increase in the respiratory capacity of skeletal muscle that accompanies an increase in the number of mitochondria. This adaptation increases the ability to oxidize fat during exercise (1,8). Thus, being fit provides 2 advantages for weight loss and maintenance; (1) an individual can exercise at a higher intensity and for a longer period before becoming fatigued (thus expend more energy), and (2) fat oxidation is higher at all intensities of exercise than in an unfit individual.

The practical question for the physician and patient is how much exercise is needed to get the adaptation in fat oxidation. Luckily, adaptations in fat oxidation seems to occur relatively rapidly. With only 10 days of exercise training (approximately 1 hr/day), an increase in resting whole-body lipid oxidation was observed in lean volunteers (9). In elderly subjects, 8 weeks of training 3-4 days/wk also increased whole body fat oxidation at rest (10). Thus, it appears that a realistic exercise prescription can induce sufficient changes in fat oxidation. Characteristics of this exercise prescription are presented later in this chapter.

EVIDENCE THAT EXERCISE IS AN EFFECTIVE TREATMENT FOR WEIGHT LOSS AND WEIGHT MAINTENANCE

In an attempt to get a consensus opinion on the published studies, the American College of Sports Medicine (ACSM) sponsored a scientific roundtable on the role of physical activity in the prevention and treatment of obesity (11,12). The proceedings of this conference provide the most thorough examination of the literature that the authors could find and they are recommended for those readers who would like a more in depth review of the evidence.

A review of the literature makes it clear that there are problems inherent in conducting studies of physical activity. Some studies were conducted in the laboratory (i.e. a room calorimeter) but those results cannot be readily extrapolated to free-living individuals. In contrast, studies with free-living subjects have limited control of conditions that may influence the outcome and thus the data are variable and difficult to interpret. For example, in one study weight loss was less than predicted from the energy expended during the organized exercise program. However, a compensatory reduction in physical activity outside of the program may have influenced the outcome. Despite the many shortcomings of the various studies, the consensus reached in several published symposia proceedings seem to provide some useful recommendations, which are presented below.

Weight loss in adults

There are 3 related questions concerning the use of exercise in the treatment of obesity: (1) Does exercise alone produce weight loss? (2) Does exercise in combination with diet produce greater weight loss than diet alone? (3) Does exercise maintain lean tissue during dieting? Relative to the first question, there was one well-controlled study of exercise and weight loss reported by Bouchard and his coworkers (13). The study was conducted in a metabolic ward where participants exercised twice a day, 6 d/wk at 55% of VO2max and their intake was held constant. Subjects lost 8 kg over the 100-day study demonstrating that exercise alone can produce weight loss when other variables (such as caloric intake) are held constant.

The consensus opinion concerning studies of the effectiveness of exercise in weight loss programs (14) seems to be that "exercise alone produces modest weight losses". The mean weight loss over 12 studies was 2.4 kg (approximately 5.3 lb or a BMI difference of 0.7 kg/m2). Food intake and unorganized physical activity were not measured in these studies and the possibility exists that there were compensatory changes in these parameters that moderated the weight loss.

In a recent review Ross et al (15) took issue with some of the conclusions of the consensus conference. They calculated the energy expenditure of the studies that were reviewed and found that the modest weight loss was that expected for the amount of exercise performed. For example, 5 hr/wk of brisk walking may only produce an energy deficit that should produce a wt loss of about 0.2 kg/wk (16). Ross et al (15) did a study in which the energy deficit (700 kcal/day) for caloric restriction and exercise were equal and the two programs produced nearly identical results of 7.5 kg in response to a 12-week treatment program. However, for the amounts of exercise that most obese people will be willing and/or able to perform, there seems to be small, but consistent, weight losses.

Most of the studies of exercise in the treatment of obesity have used aerobic types of exercise, such as walking or biking. Several studies investigated whether resistance exercise, such a weight lifting, is an effective treatment for obesity. After a review of these studies Votruba et al (16) concluded that resistance exercise is not effective in reducing weight.

In response to the question whether diet plus exercise is more effective than diet alone in causing weight loss, the conclusion of the consensus panel was as follows (14):

The expert panel stated that the lack of statistical power (number of subjects) and the short length of the trials might have limited the ability to draw firm conclusions. However, the conclusion from these studies is similar to those of exercise alone; the effect of exercise on weight loss is rather small.

Although the weight loss due to exercise in an obesity treatment may be quite modest, there may be an important benefit of exercise that is not measured by body weight. A review of studies that measured body composition changes during weight loss demonstrates that more fat and less lean tissue is lost when exercise is combined with dieting (16). Men losing 10 kg by dieting alone lost 2.9 kg of fat free mass (mostly muscle mass) while a 10 kg loss by dieting plus exercise led to 1.7 kg decrease of fat free mass (17). Sparing of lean tissue is especially true when resistance exercise is combined with caloric restriction. The hypertrophic stimulus of resistance exercise may spare muscle from loss when there is an energy deficit.

Changes in visceral fat may be another positive facet of using exercise in obesity treatment. Visceral fat deposits have been associated with insulin resistance and increased cardiovascular disease, so that a decrease in visceral fat would have profound health benefits. Ross et al (15) reviewed the studies in which the effects of exercise and dieting plus exercise on visceral fat were investigated. They conclude that exercise has positive effects on reducing abdominal obesity. Even when there was no loss of weight, exercise led to a decrease in the dangerous visceral fat.

Weight maintenance in adults

There are two aspects of weight maintenance that must be considered; (1) maintenance of a normal weight (prevention of obesity), and (2) weight maintenance after successful treatment of obesity. Evidence to support an important role for exercise in prevention of obesity can be found in relationships among the prevalence of obesity, energy intake, and inactivity. Caloric intake is rather constant among social groups and across various age groups, but there is a striking relationship between inactivity and obesity (18). Likewise, obesity incidence increased in the United Kingdom between 1980 and 1997 but the Nutrition Food Survey showed a decrease in corrected household food intake. By inference, the increase in obesity must be due to a decrease in energy expenditure. There are also suggestive relationships between surrogates of inactivity, such as TV watching, and the incidence of obesity.

Longitudinal studies also support a relationship between physical activity and maintenance of body weight (19). In a study of 9,000 adults it was found that sedentary people who increase activity minimize the risk of major weight gain, compared to people who decrease their activity. Also, individuals who were fit at the start of the survey gained less weight over time than those were less fit. However several studies found that improvements in fitness were necessary to maintain weight over time.

Exercise also seems to be an effective treatment modality for maintenance of weight loss after successful treatment of obesity. The expert panel of the American College of Sports Medicine summarized their consensus as follows (14):

In a study of police officers, those who did not exercise during the follow-up period gained 92 % of their weight back by 18 months post treatment. There were no significant gains in body weight at 18 months for those who exercised through the follow-up period. (20).

The most impressive evidence for a positive role of exercise in weight maintenance comes from the National Weight Control Registry (21). The registry is a data base for a large number of men and women who have lost an average of 30 kg and maintained a required minimum weight loss of 13.6 kg for 5 years. Members of this group are highly active and only 9% of registry subjects reported maintaining weight loss without regular physical activity (22) Members of the registry reported expending an average of 2,800 kcal/week through physical activity. The American College of Sports Medicine recommends a minimum weekly exercise goal of 1,000 kcal and a recommended energy expenditure of 2,000 kcal/wk as an optimal physical activity level. Seventy-two percent of the registry sample met or exceeded the minimum recommendation and 52% met or exceeded the recommended energy expenditure for optimal physical activity level.

The expert panel (14) pointed out that there are some questions about correlational studies and those of the National Weight Control Registry. It is possible that the better weight maintenance is not due to exercise per se, but that exercise is just part of a constellation of weight controlling behaviors (or a marker for this constellation); i.e., people who are motivated to keep their weight off may also be motivated to exercise. However, the evidence available seems to suggest that long-term weight maintenance after successful treatment of obesity is improved when exercise is included in the program.

Weight loss and maintenance in children

Obesity among children is increasing at an alarming rate and presents a particular health concern because there has been a rapid increase in obesity related diseases, such as type 2 diabetes, in children. The cause of the increase in obesity has been reviewed and there is considerable evidence that inactivity plays a significant role (23). Time spent watching television may be a surrogate measure of inactivity and there is a high correlation between TV watching and childhood obesity.

With regard to the use of exercise in treatment of obesity in children and adolescents, a substantial number of studies were reviewed by the expert panel of the American College of Sports Medicine. Their report (24) concluded that "diet plus exercise improves short-term obesity treatment by almost one-half standard deviation above and beyond the contribution made by diet alone". Thus, increasing physical activity is important for both the prevention and treatment of childhood obesity.

Since most children spend a substantial portion of their day in school, the methods of increasing physical activity can be quite different than those used with adults. Several model programs based on school and family activities have been developed and tested. From these studies it is clear that increasing physical activity has a positive influence on prevention of obesity. The review by Goran et al (23) is recommended for readers who wish more information about the prevention and treatment of childhood obesity.

EXERCISE PRESCRIPTIONS FOR WEIGHT LOSS AND WEIGHT MAINTENANCE

It is essential that patients understand the importance of exercise in a weight loss and weight maintenance plan. However, it is equally important that they have realistic expectations. A majority of the studies presented above prescribed 30 to 60 minutes/day of moderate intensity exercise for 3 to 6 days per week. This level of exercise led to an average weight loss of approximately 5 pounds. This degree of weight loss is very discouraging to a patient who may need to lose 50 pounds. However, it is important that patients adopt the view that miracles will not occur overnight but the effort will be worth the long-term benefits. Weight maintenance after a diet program will not likely succeed without exercise as part of the plan. Thus, including exercise as part of the diet plan not only adds to the caloric deficit, it also begins to achieve a level of fitness that improves patient health and allows him/her to exercise at intensities and duration that will assure weight maintenance. For long-term weight stability, life style changes must be made that include exercise as part of a daily routine.

Patients with cardiovascular disease, diabetes, or other chronic diseases should undergo a medical evaluation before initiating any exercise program more vigorous than walking To set appropriate guidelines for exercise participation, many overweight and obese individuals will benefit from receiving exercise advice and direct supervision during training from an experienced professional with an exercise science degree and certification from the American College of Sports Medicine.

Adherence to formal exercise programs is typically poor among obese individuals but physician counseling can play an important role in motivating patients to exercise. In one study, physicians delivered one brief (5 to 7 minutes) counseling session and health educators made a brief "booster" call 2 weeks later. At the 6 week follow-up, patients who received counseling were walking significantly more than those who were advised to exercise but given no counseling (25). It is beyond the scope of this chapter to discuss exercise counseling but the interested reader is referred to two excellent papers on motivating patients to exercise (25,26).

How much exercise is enough?

The prescription of exercise in a weight loss program will be dictated to a large extent by what a patient can perform with their physical limitations. It is important to start at an exercise level that can be maintained without discouragement or injury. As fitness improves and weight loss occurs, increases in intensity and duration will provide greater benefits.

How much exercise is enough, is really a question of how much exercise does it take to maintain weight loss. Based on the studies cited in the previous section some recommendations can be made. The position of the American College of Sports Medicine and the Surgeon General (27, 28, 29) is that the minimum energy expenditure in physical activity to maintain fitness is 1,000 kcal/week. However, for optimum fitness and health they recommend that individuals should expend 2,000 kcal/week in physical activity. Individuals in the National Weight Control Registry expended an average of 2,817 kcal/wk in physical activity (21). Votruba et al (16) recently recommended that 2,500 kcal/week be expended in physical activity for maintenance of weight. Thus, it appears that the recommendation of the American College of Sports Medicine of 2,000 kcal/week in physical activity for optimal fitness is also a reasonable recommendation for maintenance of weight loss.

Patients have little concept how much exercise is required to expend 2,000 kcal/week. The Compendium of Physical Activities (2) provides an excellent source of information for extrapolation of energy expenditure into exercise intensity and duration. Table 1 has a limited number of common activities with their associated MET values so that some calculations can be made without reference to the original compendium. A sample calculation is made for sake of illustration. Activities of moderate intensity (such as brisk walking) are rated at approximately 4 METs. That means that the rate of energy expenditure is 4 times the resting energy expenditure (REE, which is about 1 kcal/hr/kg). For a 70 kg (154 lb) individual this would be an expenditure rate of 280 kcal/hr. To expend 2,000 kcal/week, approximately 7 hours (420 minutes) of brisk walking per week would be required. This amount of exercise may be a minimum for maintaining weight loss because Votruba et al (16) recently recommended 500 to 600 minutes/week of brisk walking to maintain weight loss.

What intensity and duration of exercise is best?

In many cases a discussion of exercise intensity with an individual starting a weight loss program is irrelevant because many are not fit enough to perform more than low intensity exercise. If possible, it is advisable to perform an exercise stress test prior to initiating an exercise program. Such a test serves two functions for the obese individual. First, it is likely that an obese individual would possess several risk factors for coronary artery disease, such as a family history for heart disease, an unfavorable lipid profile, hyperglycemia and/or hyperinsulinemia, hypertension, and other related conditions. Thus, an exercise stress test which monitors EKG tracings is suggested to detect possible ischemia and clear the subject to initiate an exercise training program. Secondly, a properly administered maximal test will result in the attainment of a maximal heart rate. The maximal heart rate can then be used to calculate a suitable exercise prescription, which will incrementally increase duration and intensity over time.

After assessment of physical fitness, a type and intensity of exercise should be recommended that can be sustained at least 20 min. However, as fitness improves and weight is lost, it is important to increase intensity and duration to produce the most favorable results. The reason to reach the highest intensity that can be sustained for at least 20 minutes relates to the earlier discussion of the scientific basis for the benefits of exercise. To induce the increase in resting energy expenditure, a decrease in appetite, and the adaptation of fat oxidation, an exercise intensity must be reached that is high enough to "trigger" the physiological response (3, 30). Thus, it is important that the individual be increasing the exercise intensity to the highest level that they can be sustained without injury or noncompliance.

Related to intensity is the duration of exercise. Is it better to exercise for several short bouts of exercise at higher intensity (and perhaps spaced throughout the day) or for a longer period at lower intensity? One argument for the lower-intensity, longer-period regimen is that fat oxidation is a more prominent energy source at moderate intensity exercise. On the other hand, the changes in resting energy expenditure and appetite are most favorably influenced by higher intensity exercise. The deciding factor in what intensity and duration to prescribe may relate to the regimen that produces the best compliance. In an experimental study, women in one group were assigned to several short bouts of exercise while another group was assigned one long exercise session (at the same exercise intensity). The group assigned to short bouts exercised for a longer total time than those in the long exercise bout group (16). This result may be related to the fact that programs that make exercise enjoyable and easy to adhere to improve compliance.

What is the best form of exercise?

The best form of exercise is dependent on the physical ability and the likes/dislikes of the individual. There is no use prescribing a running regimen for someone with arthritic knees. Likewise, if a person is bored by stationary cycle exercise, compliance will likely be a problem. The choice between an organized exercise program and a home based-exercise regimen may also be important. Participation in an organized exercise program may provide motivation from others in the group. However, obese individuals are often self-conscious about their appearance in situations that may contrast their physique with that of slim members of the group. In addition, several studies have shown that over a period of time individuals tend to drop out of organized programs (or health clubs) and not continue to exercise. Weight maintenance is a long-term goal that will require life style changes and home-based programs may have advantages. Thus, it is essential that an interest inventory be conducted to help individuals decide what form of exercise they would enjoy and continue to do for the rest of their lives.

The most often prescribed form of exercise is walking. It requires a minimum of equipment (good walking shoes are essential) and can be performed by most individuals without foot or leg problems. In poor weather conditions walking in the shopping mall has become popular or a treadmill can be used in the home. The exercises listed in Table 1 provide some examples of commonly prescribed exercises and the energy that is expended. A mixture of different exercises may provide the best compliance, since boredom can be a danger of repeating the same exercise on consecutive days.

Resistance exercise, or weight lifting, can be a valuable form of exercise for some individuals. The advantage to be gained from resistance exercise is an increase in muscle mass that can occur as an adaptation to training. Since energy is required to build muscle and lean tissue has higher basal energy requirement than fat tissue, there can be long-term benefits.

The National Weight Control Registry (21) poled those in the registry to determine the exercises in which individuals with successful weight loss and maintenance participate. Of those who participate in medium-intensity activities, the top four activities included stationary or road cycling, aerobics, walking or running, and hiking or backpacking (in that order). Weightlifting, running, stair stepper use and step aerobics were the most frequently reported heavy-intensity exercises.

The Surgeon General's report on Physical Activity and Health (29) makes the point that increasing the physical activity of daily living can alleviate some of the time spent in special exercise activities. Walking the stairs instead of riding the elevator, walking from a distant parking lot, walking down the hall instead of emailing, can all contribute toward the goal of expending 2,000 kcal/week in physical activities. However, patients need to consciously plan these extra energy expenditures because the "usual" approach is to take the easiest way. The energy expenditures of some daily living activities are listed in Table 1 so that patients can make estimates. Walking down the hall to another office for 2 min/hr instead of emailing could expend enough energy to make a difference of approximately one pound per year. Thus, it is clear that small lifestyle adjustments can be very important in weight maintenance.

The health bonus of exercise

Obese individuals are at increased risk for cardiovascular disease and diabetes. Sedentary lifestyle is also associated with increased risk for these diseases. Studies from the Cooper Clinic (25) determined that obese but fit men had death rates considerably lower than normal-weight men who were unfit. Higher levels of fitness provided protection against cardiovascular disease and all-cause mortality at all levels of overweight and obesity. Considering the health benefits of exercise, it is possible that the treatment of overweight and obese individuals should focus on developing healthful behaviors.