Obesity is a significant public health concern in the United States and other developed countries. Obesity has been linked to many chronic diseases including type 2 diabetes mellitus, heart disease, and various forms of cancer. The last 20 to 30 yrs have resulted in a significant increase in the prevalence of overweight and obesity. It is now estimated that at least 60% of adults in the United States are classified as overweight, which is defined as a body mass index (BMI) ≥25.0 kg·m−2 (2). Moreover, the greatest increase in prevalence has occurred in individuals classified as obese (BMI ≥kg·m−2), and it is now estimated that approximately 30% of adults in the United States are obese (2). Therefore, although it is important to consider interventions to prevent weight gain and the onset of obesity, it is equally important to continue to develop interventions that lead to successful long-term weight loss. The goal of this review is to examine the role of a component of total physical activity, specifically planned exercise, in the treatment of overweight and obesity.
Exercise can be an important component of weight loss interventions, and therefore is commonly included as part of a comprehensive weight management program. However, the effects of exercise on weight loss and changes in body composition in short-term treatment programs (<6 months) is minimal compared with the impact of reductions in energy intake. For example, in a 12-wk study of women and men, Hagan et al. have reported weight loss of 0.3–0.6% from exercise, with changes in diet and diet plus exercise resulting in weight loss of 5.5–8.4% and 7.5–11.4%, respectively (3). The exercise performed in this study was 30 min·d−1 on 5 d·wk−1. This translated into 10.0 ± 2.4 miles·wk−1 for men and 9.5 ± 1.8 miles·wk−1 for women, which equates to approximately 200–300 kcal·d−1. These results suggest that changes in diet may have the greatest impact on initial short-term weight loss. However, the magnitude of the benefits of exercise on weight loss and body composition changes may be improved if the dose of exercise is equivalent to the larger energy deficits typically observed with modifications to energy intake (500–1000 kcal·d−1) (13).
Of importance is the growing body of literature supporting the role of exercise in improving long-term maintenance of weight loss. Data from the National Weight Control Registry has shown that maintenance of exercise is associated with maintenance of weight loss (12). Moreover, Jakicic et al. (11) have shown that exercise participation is associated with long-term weight loss, with more exercise resulting in greater weight loss. However, although exercise may be predictive of long-term maintenance of weight loss, the maintenance of weight loss may not be attributed to exercise alone. It has been shown that maintaining changes in both eating and exercise behaviors is important for improving long-term weight loss (9).
Exercise may also be important for eliciting health benefits that are independent of reductions in body weight (15). For example, it has been reported that higher levels of cardiorespiratory fitness are associated with reductions in mortality from cardiovascular disease, and overweight adults with the highest level of fitness may have lower risk of cardiovascular disease than unfit individuals who are at an optimal body weight. This may be a result of the exercise improving cardiovascular disease risk factors (e.g., blood pressure, diabetes, etc.) independent of changes in body weight. Thus, even individuals who have difficulty reducing their body weight can benefit from adopting periods of regular exercise, provided that the volume and intensity of the exercise are sufficient to elicit these health benefits.
When developing exercise interventions for overweight adults, it may be important to address three important factors that impact the prescription and patient adoption of exercise behavior. These factors can be summarized as the following: assessment of current exercise behaviors, considerations for exercise dose, strategies for exercise adoption, and maintenance. As shown in Figure 1, all of these factors interact to impact the adoption and maintenance of exercise behavior. Each of these areas are described and addressed below.
ASSESSMENT OF CURRENT EXERCISE BEHAVIORS
In clinical weight management programs it is beneficial to assess the level of exercise participation of patients. Before beginning an intervention, information on the current exercise level can be used to develop an appropriate exercise prescription for individual patients. Moreover, constant monitoring of exercise participation throughout the intervention can provide valuable information related to compliance and adherence, and this could potentially trigger the implementation of additional intervention strategies to improve exercise participation in overweight patients. However, the ability to accurately assess exercise participation can be challenging.
The most common method of assessing exercise participation is typically self-report of exercise using exercise diaries or logs. Although this may allow for assessment of exercise at a relatively low cost, the information that is available through self-report may be misleading. For example, Jakicic et al. (6) demonstrated that individuals do misrepresent their self-reported exercise behaviors. This study also demonstrated that using objective monitoring techniques to identify individuals who misrepresent their exercise may assist in explaining poor weight loss in overweight patients. Therefore, based on these findings, there may be advantages for including objective measures of exercise into clinical weight loss programs.
There are a number of options for objectively monitoring exercise under free-living conditions, and these may include pedometers, portable heart rate monitors, and accelerometers. However, each of these devices has limitations that may limit their use in clinical weight loss programs. A component of our research has focused on the development of objective techniques for assessing exercise participation in overweight adults. Although these efforts continue, to date our published research has focused primarily on the use of accelerometry. Results of these studies have shown that accelerometers can provide a reasonable estimate of energy expenditure compared with indirect calorimetry during periods of level walking and running (≤1.0 kcal·min−1 difference). However, accelerometers tend to underestimate energy expenditure for walking and running as the incline of these activities increases (10). Moreover, accelerometers appear to underestimate energy expenditure during other modes of activity such as stationary cycling, stair stepping, and activities involving lateral movements (10).
Although there are limitations to accelerometry when considering energy expenditure, accelerometry may be beneficial because it can differentiate between periods of activity and inactivity. For example, accelerometers have been used to verify patterns of exercise in clinical weight loss and exercise trials (8,11). In these exercise trials, accelerometers were used to verify participation in either intermittent or continuous periods of exercise, and were also used to confirm the number of days that a patient engaged in periods of exercise that were at least 10 min in duration (8,11). Because it appears that accelerometers can differentiate between periods of activity and sedentary behavior, the use of accelerometers may provide valuable objective information on exercise patterns during intervention periods. Accelerometers have been used to determine if individuals underestimate or overestimate their exercise participation (6). Results from a 20-wk study indicated that overweight adults who overreported their exercise participation based on the comparison of accelerometry and self-report had less weight loss (6.3 ± 3.6 kg) compared with individuals who underreported their exercise participation (9.4 ± 5.2 kg) (6). Thus, there may be advantages for including objective measures of exercise, such as accelerometry, in clinical weight control programs to identify potential subjects for whom exercise adherence may be problematic.
CONSIDERATIONS FOR EXERCISE DOSE
Although it is important to engage in regular exercise to elicit desired reductions in body weight and improvements in health, the optimal level of exercise for overweight adults is a point of debate. Until very recently, the current public health recommendations for exercise and physical activity were to engage in a minimum of 30 min of moderate-intensity activity on most days of the week (14). This is commonly interpreted as a minimum of 150 min·wk−1 of exercise and physical activity. However, recently the Institute of Medicine suggested that 60 min·d−1 of exercise may be necessary for controlling body weight, and this recommendation is approximately double the amount previously recommended (4).
Despite the limited research that has compared the impact of 30 versus 60 min·d−1 of exercise on weight loss, the evidence that is available does appear to support the recommendation of 60 min·d−1 of exercise to improve long-term weight control (11,12). Data from the National Weight Control Registry revealed that individuals that had maintained an average weight loss of 30.0 ± 15.5 kg for approximately 5.5 yrs were also averaging 2500 kcal·wk−1 in energy expenditure from leisure-time physical activity (12), which is approximately twice the minimal public health recommendation (14). As reported by Jakicic et al. (11), exercise averaging >280 min·wk−1 during an 18-month intervention, which again is twice the minimal public health recommendation, was associated with the greatest magnitude of weight loss (see Fig. 2). These results may suggest that overweight adults need to eventually progress to this magnitude of exercise to maintain a significant weight loss long-term. The pathway by which higher levels of exercise may improve long-term maintenance of weight loss may be purely from a metabolic perspective, because greater exercise can create a greater energy deficit by increasing overall daily energy expenditure. However, higher levels of exercise have also been negatively correlated with energy intake in individuals during periods of weight loss, with both factors significantly contributing to improvements in long-term weight loss (9). Thus, exercise may contribute to the energy deficit during periods of weight loss by impacting both energy expenditure and energy intake.
Although it does appear that the dose of exercise necessary for enhancing long-term weight loss may be somewhat greater than the minimal level of activity necessary to improve other health parameters, support for these findings is provided primarily from results of correlational analyses. Therefore, there is a need for randomized clinical trials to support these findings. There is currently a National Institutes of Health–funded study underway to examine different doses of exercise combined with changes in eating behavior on weight loss across a 24-month period (John M. Jakicic, P.I., HL64991). The Active L.I.F.E. (Lifestyle Interventions for Exercise) Study is comparing different intensities (moderate vs vigorous) and energy expenditures (1000 vs 2000 kcal·d−1) of exercise on long-term weight loss in overweight adults. This study will also examine the effects of this intervention on cardiorespiratory fitness, body composition, and a variety of psychosocial factors.
Intensity is an important factor to consider when prescribing exercise for overweight adults. Exercise that is prescribed at a relatively high intensity may increase the risk of exercise participation in sedentary overweight adults, especially in the presence of additional cardiovascular disease risk factors. Moreover, because some individuals may not enjoy participating in vigorous forms of exercise, promoting exercise only at a vigorous intensity may have a negative impact on exercise adoption and maintenance in overweight individuals. Therefore, prescribing exercise at an optimal level to meet the needs of the individual is critical.
Ideally, prescribing exercise based on relative oxygen update or heart rate would provide optimal quantification of exercise intensity. It has been shown that the assumed relationship between oxygen update and heart rate is present in obese adults (5). Because assessing oxygen update during exercise is not feasible in many clinical programs or programs promoting exercise in free-living environments, these data suggest that heart rate can provide an accurate estimate of exercise intensity in overweight adults. However, even the regular assessment of heart rate during exercise in many situations can be challenging for patients, which may minimize compliance to this recommendation. Under these circumstances, it may be advantageous for clinical weight loss programs to consider using the rating of perceived exertion (RPE) to monitor exercise intensity. There is research that shows that a relationship is present between RPE and both oxygen uptake and heart rate in obese adults (5), and these findings provide support for the use of RPE in clinical weight loss programs. These data suggest that a RPE range of 11–13 corresponds to 60–70% of both oxygen consumption and heart rate reserve, and this is representative of moderate-intensity exercise.
STRATEGIES FOR EXERCISE ADOPTION AND MAINTENANCE
Despite the importance of exercise in the weight loss process, maintaining exercise participation in overweight adults may be less than desirable, with attrition rates exceeding 50% in some long-term studies. Therefore, although it is important to understand the level of exercise for producing the greatest long-term weight loss, it is equally important to understand the factors that prohibit the adoption and maintenance of adequate levels of exercise in overweight adults. As illustrated in Figure 3, as exercise barriers are reduced, exercise participation will increase, and it is hypothesized that this will result in reductions in body weight. Although there are numerous barriers to exercise, this review will focus specifically on environmental factors that may influence the convenience of exercise, and factors that may impact the perceived lack of time for exercise.
Environmental factors may impact the adoption and maintenance of exercise by influencing the convenience of exercise participation. If true, manipulations to the environment in which an individual functions (e.g., home, work, neighborhood, etc.) may be important for modifying exercise behavior. This may assist in making exercise opportunities more convenient, which may increase participation. An example of an environmental manipulation that is common in the general population is the addition of exercise equipment to the home. A cross-sectional study reported that the amount of exercise and sports equipment in the home was significantly associated with levels of leisure-time physical activity (7). The type of equipment that was associated with physical activity differed by gender, with team sports equipment (e.g., softball) associated with physical activity in men and both home exercise equipment (e.g., a treadmill) and individual sports equipment (e.g., tennis) associated with physical activity in women. Jakicic et al. (11) have also shown that the introduction of a treadmill into the home may improve exercise participation and weight loss in overweight women. These results suggest that modifications to the environment may impact exercise participation, which may in turn influence changes in body weight.
Perceived Lack of Time
Perceived lack of time has also been identified as a barrier to exercise adoption and maintenance. For sedentary adults, a strategy for overcoming this barrier may be to focus on accumulation of exercise throughout the day rather than attempting to achieve a sufficient amount of exercise in one continuous exercise session (30 to 40 min per session). The effectiveness of encouraging the accumulation of exercise in 3–4 exercise sessions that were each a minimum of 10 min in duration on adherence and weight loss in overweight women has been examined (8). Results of this study showed that this strategy can be effective at increases in the initial 6-month exercise participation rates in previously sedentary adults, and these results are illustrated in Figure 4. When combined with a dietary intervention, there was also a trend for this strategy to have a greater impact on initial weight loss when compared with exercise that was recommended to be performed in a continuous session one time per day.
Encouraging the participation of exercise at a specific time or under specific supervision may negatively influence exercise adoption and maintenance, and this may be a result of these sessions being inconvenient and participants perceiving that they do not have adequate time to attend these sessions. Thus, providing individuals with behavioral strategies that allows them the flexibility to develop an exercise program to overcome barriers contributing to a lack of time may be effective. Dunn et al. (1) reported that training individuals in behavioral strategies to adopt and to maintain exercise was as effective as requiring individuals to attend supervised, facility-based exercise programs (e.g., exercise classes). This strategy should be considered when individuals are resistant to participation in more traditional, structured, and supervised forms of exercise.
Obesity is a serious public health concern in the United States and other developed countries, and exercise should be included as an important component of any weight loss intervention. Although exercise may have a modest impact on initial short-term weight loss, exercise may have its greatest effect on long-term weight loss and prevention of weight regain. Moreover, the magnitude of exercise required to facilitate successful long-term loss may be greater than what is necessary to elicit significant improvements in health, and this requires further investigation. When making exercise recommendations to overweight adults it is important to consider assessment, dose prescription, and intervention strategies that can impact adoption and maintenance.
Dr. Jakicic is supported by research awards from the National Heart, Lung and Blood Institute (HL64991) and the National Institute of Diabetes and Digestive and Kidney Diseases (DK58002) of the National Institutes of Health.
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