Recent surveillance data from the general US population suggest a continued increase in the prevalence of overweight and obesity that is consistent with weight gain trends observed globally among industrialized countries (12). A myriad of environmental, behavioral, physiologic and genetic factors contribute to the development of human obesity; however, the common underlying feature is a positive energy balance. Attenuated metabolic responses to environmental exposures combined with predisposing factors and overall low energy expenditure may contribute to this positive energy balance. Although exercise is most effective in the prevention of obesity, it can also contribute to weight loss and to weight maintenance over the long term. Numerous intervention studies have evaluated the role of exercise training of various modes and intensities on the reduction of body weight and adiposity and there is little doubt about the established benefits of increasing physical activity to the attainment and the maintenance of healthy body weight throughout the life span. Moreover, since exercise itself improves metabolic, respiratory and cardiovascular function independent of weight loss, it has special significance for obese individuals who are at increased risk for obesity-related chronic conditions. The purpose of this chapter is to describe the importance of exercise to the prevention and treatment of obesity, as well as to the prevention of weight regain. In addition, this chapter will address the contributions of the built environment to the onset and possible reversal of obesity at the population level.
Inactivity and obesity are closely linked conditions accounting for a large burden of chronic disease and impaired function. Over the past several decades, ever-decreasing levels of daily energy expenditure, along with a constant food supply, have resulted in a marked disruption to energy regulatory systems, which are still genetically programmed for the subsistence efficiency of our late-Paleolithic ancestors (6). As stated previously, the underlying agent in the etiology of obesity is a long-term positive energy balance; however, the relative importance of excess energy intake over low energy expenditure to this imbalance is controversial. In any case, the pathway determining the rate and extent of weight gain with a positive energy balance is complex, and the unique and combined contributions of heredity, physiology, and behavior to the development of obesity are not understood completely --especially since the influence of any one these primary factors is usually modified by a constellation of other secondary factors endemic to our current obesogenic environment (Figure 1).
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| Figure 1. Public health model illustrating the multi -factorial etiology of obesity. The traditional public health model of disease transmission applied to obesity etiology. In this model, the impact of the agent (positive energy balance) can be modified by a number of host (specific to the individual) and environment (specific to collective behaviors or conditions) factors. In addition, a variety of vehicles/vectors are responsible for transmitting the causal agent. |
The 2008 Physical Activity Guidelines for American (30) recommends 150 min·week-1 of moderate-intensity physical activity (e.g., brisk walking) or 75 min·week-1 of vigorous activity (e.g., running) for the improvement of cardiovascular and metabolic function, and the consequent reduced prevalence of important conditions such as dyslipidemia, hypertension and insulin resistance (28). Consensus maintains, however, that these population guidelines are actually more effective for health promotion and the primary prevention of chronic disease risk factors than for the reversal of already established chronic conditions (5, 26).
Indeed, the treatment or reversal of many established conditions more than likely requires a higher dose of exercise and frank obesity is one such chronic condition for which these PHS recommendations may not be sufficient. For example, there is now epidemiologic evidence that the physical activity dose related to the successful prevention of excess weight gain (10) is far less than that needed to reverse obesity (23, 25) or to sustain weight loss following obesity (27) (Table 1). Although population-and laboratory-based data are limited, it appears that about 45-60 min·day-1of moderate activity is necessary to prevent the transition from normal weight to obesity, at least for a large part of the population (26). Note that this level of activity is significantly higher than the Public Health Service recommendations. Moreover, there is substantial individual heterogeneity with regard to a person's responsiveness to an exercise regimen, and this responsiveness may vary by age, sex, degree of obesity, adipose tissue distribution, and even adipocyte size (21). Thus, the benefits of increased physical activity to cardiovascular and metabolic health notwithstanding, its effectiveness per se for substantial weight loss and in the reversal of obesity may be less so.
Table 1
The ACSM (18) and the 2008 Guidelines (30) define a clinically-significant weight loss to be approximately 5% of body weight with a recommend weight loss rate of 1-2 pounds (0.5-1 kg) per week as safe and effective. Weight loss at this rate, however, would require a negative energy balance of ~500-1000 kcal·day-1 over an extended period of time (18, 23, 25). Such an energy deficit is difficult to achieve by lowering energy intake alone, and more importantly, such drastic decreases in caloric intake could result in nutritional deficiencies, as well as the loss of lean mass lowering of the metabolic rate (22). Moreover, adherence to such a degree of caloric restriction is unlikely to be maintained over long periods of time, and therefore this strategy does not generally result in long term weight loss (13). Most research now supports the contention that the addition of exercise to any caloric restriction regimen will not only increase the overall net caloric deficit induced by the weight loss program, but will markedly attenuate the loss of fat free mass that is concomitant with overall body mass loss.
Whether exercise alone (without coincident caloric restriction) can significantly alter body weight in obese people is debatable. As stated previously, what confirmatory data there are suggest that the exercise dose necessary to reverse obesity is substantial. In 2002, the Institute of Medicine (IOM) issued stronger recommendations of 60 min·day-1 of moderate-intensity activity as necessary for meaningful weight loss (5, 13), which (assuming no change in caloric intake and a walking rate of 3.5 mph) would result in an energy expenditure of ~474 kcal·day-1 for men and 384 kcal·day-1 for women (1). These guidelines are consistent with those provided by the 2008 Guidelines and by ACSM; however, they may underestimate the actual caloric expenditure necessary to lose weight at the recommended rate of 1-2 pounds per week (~0.5 - 1.0 kg·week-1). In fact, weight loss at this rate would require a caloric deficit of about 500 - 1000 kcal·day-1 (18). Thus, a 90 kg man may need to perform 68-136 min·day-1 of brisk walking (7.9 kcal·min-1) and an 80 kg woman may have to perform 72-145 min·day-1 of the same (6.4 kcal·min-1) on 7 days/week in order to achieve these recommended weight loss goals. Further, although this walking rate (3.5.mph) is consistent with a comfortable walking rate for overweight women during a 20-min session, it may overestimate what obese women would actually be able to perform over a longer period (11). Whereas these calculations ignore the small (yet prolonged) increases in the post-exercise metabolic rate (16), as well as the increase in both resting and exercise metabolic rate resulting from gains in lean mass, it nonetheless seems clear that the volume of exercise necessary for the reversal of obesity far exceeds the recommended levels for maintaining overall health. The majority of evidence now suggests that both exercise and caloric restriction are necessary components of a successful weight loss program. Despite the modest impact of exercise alone during the initial weight loss period, it is essential to the maintenance of lean mass in the face of fat loss. The volume (i.e., intensity, frequency and duration) of exercise necessary for meaningful weight loss in the absence of caloric restriction, however, is unlikely to be achieved by most obese individuals, at least at the beginning of a weight loss program.
Unfortunately, many people who are successful in losing substantial amounts of body weight either through diet, exercise, medication or even surgery often quickly regain it, indicating that adaptations to caloric deprivation may also contribute to the obesity epidemic. Laboratory findings report that the level of daily energy expenditure necessary to prevent the regain of body weight following obesity is also quite high relative to the modern-day lifestyle (27). The current consensus statement from the International Association for the Study of Obesity (26) recommends 60-90 min·day-1 of moderate-intensity activity or about 35 min·day-1 of vigorous activity for successful weight maintenance with the reversal of obesity, which (again) is twice that of the PHS recommendations (30).
As stated previously, caloric restriction without exercise may result in a loss of lean mass along with adipose tissue, thereby resulting in a drop in the metabolic rate and setting the stage for relapse. Both aerobic and resistance exercise will preserve lean tissue during weight loss, however, and this is primarily a function of the amount of exercise performed over the weight loss period (20). Finally, as is the case through the period of dynamic weight loss, those who combine caloric restriction with exercise, especially exercise that will maintain lean mass, are more successful in maintaining a lower body weight over the long term compared with those using exercise alone (19, 20).
Not surprisingly, one of the major difficulties with the reliance of exercise alone for weight loss is that adherence to prescribed exercise dose is more than often, quite low. This is especially so in obesity compared with normal weight people, as exercise is often perceived as being more difficult to accomplish and thus, self-selected intensities are lower (11). Evidence suggests, however, that total daily accumulated energy expenditure is the strongest predictor of weight loss in obese individuals (17, 20). Therefore, an alternative to the typical recommendation of continuous exercise may be intermittent exercise, which can result in a similar weight loss in obese subjects but with improved adherence over the long term. Finally, the integration of more physical activity as part of an overall lifestyle change (e.g. more walking and stair climbing as part of the daily routine) may be as successful in promoting weight loss as is a structured exercise program. Given the high degree of negative energy balance required for weight loss, however, high levels of lifestyle activity combined with caloric restriction are now prescribed for both initial and long-term weight loss in obese and overweight individuals.
The Physical Activity Level (PAL) has become the standard method of expressing total daily energy expenditure (TEE) in multiples of the resting metabolic rate (RMR: PAL=TEE/RMR), and thus far, only one study has examined this issue at the population level. Data from men in the Aerobics Center Longitudinal Study cohort suggest that a daily PAL >1.60 METs· 24-h-1 (i.e., an average daily TEE 60% above RMR) is optimal for preventing meaningful weight gain (~0.82-0.91 kg·y-1 (14) through middle-age (10). Moreover, increasing daily activity from the low PAL category (<1.46 METs · 24 h-1) to the moderate (1.46 1.60 METs · 24 h-1) or high (>1.60 METs · 24 h-1) categories resulted in a slight weight loss over time in this cohort (Figure 2).
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| Figure 2. Predicted weight change over time by PAL change category among men in the Aerobic Center Longitudinal Study (ACLS) cohort. PAL=average daily physical activity level expressed as the ratio of total energy expenditure to the resting metabolic rate (TEE/RMR). Models adjusted for age, sex, height, baseline weight, and smoking. DiPietro, et al. Int J Obesity. 28:1541-1547, 2004. |
In order to accomplish this average level of daily activity, exchanging passive or very low intensity activities (i.e., those involving sitting) for moderate intensity activities that have energy requirements of about 4-6 METs might be the most useful strategy. Moderate-intensity activities may have a substantially greater impact on the PAL than vigorous activities, since vigorous activity is usually performed for very short periods of time and then may be compensated for by reduced volitional activity throughout the remainder of the day (31). Therefore, the best way to increase the average daily PAL from sedentary (1.4 METs · 24 h-1) to active (1.6 METs · 24 h-1) is to add about 45-60 min of brisk walking or cycling to the daily routine. As described above, using either a continuous or intermittent exercise routine is equally effective in increasing overall TEE.
In recent years, the issue of the relative importance of physical inactivity or low cardiorespiratory fitness and excess body fat to chronic disease risk (i.e., the "fitness versus fatness" debate) has become quite controversial. A number of observational studies have reported a strong inverse relation between physical activity or fitness and disease risk that persists in normal weight, overweight, and even in obese men and women (2-4, 7). In contrast, however, other studies describe the greater relative impact of body mass index compared with physical activity or fitness on outcomes such as diabetes and mortality. Several factors may contribute to the difficulty in disentangling the unique and combined contributions of low activity or fitness and excess body fat to increased disease or mortality risk and these factors are discussed in three recent editorials (3, 9, 15). First, both inactivity and obesity are strongly correlated with each other and each has a similar pattern of association with various disease or mortality outcomes that are biologically plausible, temporally consistent and dose-dependent. Second, the methods used to determine level of physical activity or body habitus will affect the strength of the etiologic relation between both of these conditions and disease risk. Studies that use an objective measure of cardiorespiratory fitness as a marker for habitual physical activity generally report a stronger relation to disease risk compared with those that rely on self-reported measures of physical activity due to the greater precision of measurement. Also, factors other than physical activity (i.e., heredity) may influence cardiorespiratory fitness and disease risk through shared biological pathways. Similarly, the body mass index is a gross measure of overall body composition that may not fully explain the contribution of fatness (specifically, excess abdominal adiposity) to the pathophysiology of chronic disease. This is especially so in the middle ranges of BMI (i.e., 23-27 kg·m-2) where the combination of muscle and fat is highly varied among the population and when body weight and height are self-reported rather than measured (15). Thus, the lack of precision with the measurement of self-reported physical activity or body weight, combined with the lack of specificity with the BMI and the differential reporting of body weight and physical activity by actual weight status will underestimate the true magnitude of association between either of these two factors and disease risk. Finally, excess adiposity usually lies in the biological pathway between physical inactivity and disease risk, and therefore, controlling statistically for this important modulating variable will diminish the relative contribution of inactivity or low cardiorespiratory fitness.
Future epidemiologic work to determine more specific contributions of either fitness or fatness to disease risk should rely on the most precise and valid measures of these variables as possible; regardless, however, this debate may never be resolved. The relative contributions of fitness and obesity to overall health and function more than likely varies by age and the disease outcome of interest, and, according to Blair & Church (3) may be nothing more than an academic issue. Increased physical activity results in predictable increases in fitness in most of the general population and is a primary component of successful weight loss and weight loss maintenance. Thus, physical activity is the common denominator for the clinical treatment of low fitness and excess body weight (3), and the promotion of a physically active lifestyle should be central to clinical therapy and future public health policy.
There are few surveillance data on physical activity patterns over many years in representative populations that use consistent methods of data collection. Data from consumer groups and national monitoring and surveillance systems among persons living in the United States generally show a stable pattern of both leisure time and sport activity (26), but a decrease in work activity over the past 5 decades (8). These types of data are useful at the ecologic level in order to describe lifestyle trends among the population and to provide background data for community-based interventions that eventually affect public policy. Environmental interventions that promote change in risk conditions at the community level have a greater public health impact than attempting to change risk factors at the individual level. Environmental strategies more directly related to promoting an active lifestyle involve altering the built environment in which people spend the majority of their time - the community, the workplace, and the school. The specific characteristics of the built environment most closely related to physical activity remained to be determined, however.
A recent report from the Transportation Research Board (TRB) and the IOM outlines a number of recommendations pertaining to physical activity and the built environment (29). These recommendations state the primary need for multidisciplinary and inter-agency research (particularly longitudinal research and "natural experiments") linking specific aspects of the built environment with different types of physical activity. Ecological studies that can geocode physical activity and health data from surveillance systems such as the Behavioral Risk Factor Surveillance System (BRFSS) or from the National Health and Nutrition Examination Survey (NHANES) could provide useful information on the environment and the specific locations of where low activity and/or high prevalence of overweight is occurring. Similarly, statistical tools such as Geographical Information Systems (GIS) can provide more detailed information on the built environment (land use, sidewalks, green space) to link with surveillance data on physical activity patterns and various health indicators like obesity within a community. These data are also quite useful in tracking how changes to the environment affect changes in behavior and in subsequent health outcomes.
The Health Impact Statement historically has been used in environmental risk assessment to inform the public of the health consequences of various actions (e.g., the building of a new manufacturing plant in the community) and generally they are effective at involving inter-agency action and public consensus. Since available evidence suggests that the built environment plays a major facilitating role in promoting an active lifestyle, urban planners, local zoning officials, those responsible for the construction of residences, developments, and supporting transportation systems, and members of the community must work together in the design of more activity-friendly environments.
The majority of research to date suggests that exercise is far more effective in the prevention of overweight and obesity than it is in its reversal. It therefore seems important to focus future research efforts on the prevention of excess weight gain over the life span. In addition to the behavioral and intervention studies of the past several decades, an understanding of the regulatory processes governing energy intake, energy storage, and energy expenditure and how the reinstatement of exercise can correct the disruption of these neural pathways is vital to the future of obesity research. Molecular and clinical studies that can identify candidate genes and other biomarkers of energy regulation responding to exercise should link with large epidemiologic studies to determine the relations among these biological markers, physical activity patterns and long-term weight gain among various populations. Controlled intervention trials should continue to test the dose-response relation between physical activity duration (min/week), volume (kcal/week), and/or intensity and various functional endpoints as rigorously as do pharmacological trials. Finally, public health science needs to link with public health practice to better enable the translation of this knowledge into policies that can alter the environment in a way that promotes an active lifestyle for all.
This work was supported in part by grants from the National Institutes of Health, National Institute of Aging (AG.17163 and AG .027470 to LDP) and National Heart Lung and Blood Institute (HL071159 to NSS).