The Effects of Physical Activity on Obesity

Transcription

1 QUEST, 2004, 56, 1-11 PHYSICAL ACTIVITY AND OBE\SITY National Association for Physical Education in Higher Education The Effects of Physical Activity on Obesity William H. Dietz The epidemic of obesity has resulted from an imbalance of energy intake and energy expenditure. Further weight gain of individuals and populations can be prevented if energy intake balances energy expenditure. For those who are overweight, reduction in weight can only be achieved by a negative energy balance in which energy expenditure exceeds intake. In this manuscript, we will describe the epidemic of obesity and explore the evidence that reduced energy expenditure accounts for obesity, the role that physical activity plays in weight reduction, and the steps necessary to achieve energy balance within the population. The Obesity Epidemic Obesity in adults is defined as a body mass index (BMI) > 30. According to this definition, 30% of U.S. adults are obese (Flegal, Carroll, Ogden, & Johnson, 2002). Five percent of the adult population, and 15% of African American women have class 3 obesity, defined as a BMI > 40 (Flegal et al., 2002). Overweight in children and adolescents is defined by a BMI > 95 th percentile of the CDC growth charts for children of the same age and gender. Although a BMI > 95 th percentile in a young adult corresponds to a BMI of 30, the term we will use to describe children and adolescents with a BMI > 95 th percentile is overweight. Approximately 15% of children and adolescents meet this criterion (Ogden, Flegal, Carroll, & Johnson, 2002). Between the second National Health and Nutrition Examination Survey (NHANES 2, ) and the most recent NHANES survey ( ), the prevalence of overweight in 6-11 year-old children and obesity in adults doubled (Flegal et al., 2002; Ogden et al., 2002), but the prevalence of overweight year-old adolescents tripled (Flegal, et al., 2002; Ogden et al., 2002). Diseases frequently associated with obesity are shown below. Although most of these diseases are uncommon in overweight children and adolescents, type 2 William H. Dietz is Director, Division of Nutrition and Physical Activity at the National Center for Chronic Disease Prevention and Control, Centers for Disease Control and Prevention in Atlanta, GA wcd4@cdc.gov. 1

2 2 DIETZ diabetes now accounts for 8-45% of all new cases of diabetes in some centers (Fagot-Campagna, Pettit, Engelgau, Burrows, & Geiss, 2000). Furthermore, over 60% of overweight 5-10 year-old children have at least one additional cardiovascular disease risk factor and 25% have two or more (Freedman, Dietz, Srinivasan, & Berenson, 1999). The costs associated with adult obesity have been estimated at over $100 billion/year (Finkelstein, Fiebelkorn, & Wang, 2003), or approximately 10% of the national health care budget. Among children and adolescents, recent data indicate that rates of hospitalization for common problems associated with obesity increased rapidly between 1979 and 1999 (Wang & Dietz, 2002), and the costs of hospitalizations for obesity-related diagnoses increased three-fold. Because substantial proportions of cardiovascular disease, diabetes, and cancer are attributable to obesity, successful control of the obesity epidemic will also reduce the prevalence of these other chronic diseases. Diseases Frequently Associated With Obesity Arthritis Cancer Cardiovascular disease: hypertension, hyperlipidemia Type 2 diabetes mellitus Gall bladder disease Gastroesophageal reflux Hepatic steatosis Polycystic ovary disease Sleep apnea Changes in Food Intake and Energy Expenditure Substantial changes in food intake and physical activity have occurred over the last 20 years in the United States. Changes in food intake are better documented than changes in physical activity. For example, almost 50% of a family s income spent on food is spent on food consumed outside the home (Lin, Frazao, & Guthrie, 1999). There has been a decline in family meals and an increase in meal skipping by adolescent girls (Devaney & Stuart, 1998). Soft drink consumption, which consists of both sodas and 10% juice, increased by over 60% between 1972 and 1992 (U.S. Department of Health, 1995) and accounts for over 10% of the average adolescent s daily caloric intake (NHANES 3, unpublished data). An increased variety of products is available in supermarkets (Food Institute Report, 1998). Portion sizes of foods consumed outside and at home have increased substantially (Young & Nestle, 2002; Nielsen & Popkin, 2003). Although each of these shifts has been associated with an increase in food intake, none have unequivocally been linked to the onset of obesity. During this same time period, physical activity levels have also shifted. Although each change alone may have a relatively trivial impact on energy expenditure, together their impact on energy expenditure may be quite substantial. Calculations of the caloric content of adipose tissue suggest that an excess of 50 Kcal/ day may produce as much as five pounds excess weight gain per year. Therefore, modest uncompensated changes in energy intake or expenditure over years may

3 PHYSICAL ACTIVITY AND OBE\SITY 3 produce obesity. For example, national data suggest that only 30% of children who live within a mile of school walk to school (U.S. Department of Health and Human Services, 2000). Comparable figures for Georgia indicate that less than 20% of children who live within a mile of school walk to school (Bricker, Kanny, Mellinger- Birdsong, Powell, & Shisler, 2002). Furthermore, according to the 1995 Nationwide Personal Transportation Survey, 25% of the trips we make are less than one mile, but over 75% of these trips are by car (Sandra Ham, unpublished analyses). Although we lack accurate historical data, a comparison of physical activity of Australian adults who simulated the lifestyle of Australians of approximately 150 years ago with control subjects indicated that their activity levels were over twice those of the controls (Egger, Vogels, & Westerterp, 2001). Anecdotal data suggest that physical activity was more common years ago than it is today. For example, labor saving devices abound, such as escalators, remote controls for television sets, electric garage door openers, other home technologies, and self propelled lawn mowers. Energy Expenditure Prior to Weight Gain The advent of the doubly labeled water technique made possible estimates of total energy expenditure (TEE) in free-living populations. If resting metabolic rate (RMR) is measured, the energy spent on physical activity (PAEE) can be calculated from the difference between TEE and RMR or expressed as a ratio known as PAL (physical activity level; TEE/RMR). Although the impact of the thermic effect of food (TEF) is ignored in these calculations, TEF accounts for less than 10% of total daily energy intake and are less variable between individuals than PAEE. Reduced resting metabolic rate may be associated with an increased risk of weight gain (Ravussin et al., 1988). However, because resting metabolic rate is familial (Bogardus et al., 1986), it is clearly not under voluntary control. Furthermore, it is not clear what people should do if they were able to know their RMR. One of the earliest studies of the effect of TEE on weight gain suggested that infants with lower TEE have an increased risk of subsequent overweight (Roberts, Savage, Coward, Chew, & Lucas, 1988). Nonetheless, this study has not been replicated, and data to the contrary have been published from a larger cohort of infants followed for a longer period of time (Davies, Day, & Lucas, 1991). Furthermore, no other studies have yet demonstrated that reduced TEE is a risk factor for the development of obesity or increases in body fat (Davies et al., 1991; Stunkard, Berkowitz, Stallings, & Schoeller, 1999; Goran et al., 1998). However, few of these studies have been adequately powered to detect the impact of low energy expenditure either on obesity or on increases in body fat; most have been relatively short, and none have examined high risk periods for the development of obesity. One exception is a prospective study recently concluded in Cambridge, Massachusetts. In this study, preliminary data suggest that low RMR, TEE, or PAEE did not predict changes in body fatness among preadolescent girls studied at baseline and followed until four years after menarche (Bandini, Must, Phillips, Naumova, & Dietz, 2003). Therefore, available data do not support the hypothesis that reduced TEE is a risk factor for subsequent increases in body fat.

4 4 DIETZ Physical Activity and Weight Maintenance Weight maintenance represents the first step in control of the obesity epidemic. For example, if weight increases in the population stopped, incident cases would cease. Furthermore, the behaviors necessary to achieve weight maintenance are essential to prevent relapse after weight loss. Most of what we know about weight maintenance has been derived from studies of people who have lost weight, rather than populations prior to weight gain or obese populations prior to weight loss. For example, among individuals who lost > 30 kg and sustained those losses for over 5 years, the principal strategies employed included breakfast consumption, low fat diet, regular weight monitoring, and approximately 1 hour daily of moderate physical activity (Klem, Wing, McGuire, Seagle, & Hill, 1997). This dose roughly corresponds to the 80' /day of moderate physical activity necessary for weight maintenance estimated from doubly labeled water studies (Schoeller, Shay, & Kushner, 1997). Although the Dietary Guidelines (U.S. Department of Human Services and U.S. Department of Agriculture, 2000) and the Surgeon General s Report on Physical Activity and Health (U.S. Department of Health and Human Services, 1996) suggest that 30 minutes of moderate physical activity on most or all days of the week constitutes the minimal dose of physical activity necessary for health, no data are yet available to indicate that this dose is sufficient for weight maintenance among children, adolescents, or adults. A recent publication has suggested that moderately intense rather than vigorous physical activity may be the most important predictor of PAL (Levine, Eberhardt, & Jensen, 1999). More recently, a report from the Institute of Medicine (2003) suggested that one hour of physical activity per day is necessary for weight maintenance. This estimate was based on the amount of activity necessary to move an individual with a sedentary PAL (PAL < 1.4) to an active PAL (PAL > 1.6). However, as data from doubly labeled water studies reviewed by the IOM report show, obese and nonobese individuals already spend a comparable proportion of their total daily energy expenditure on physical activity, and, as emphasized above, the few prospective studies that have been done have failed to demonstrate that reduced energy expenditure at baseline is a risk factor for the development of obesity (Davies et al., 1991; Stunkard et al., 1999; Goran et al., 1998). The Impact of Physical Activity and Inactivity on Weight Gain A systematic review of 16 prospective studies disclosed that baseline physical activity levels did not demonstrate a consistent relationship with the risk of subsequent obesity (Fogelholm & Kukkonen-Harjula, 2000).). Although a more consistent relationship existed between increased physical activity and reduced obesity at follow up, these findings may only suggest that with the development of obesity, individuals become less active (Fogelholm & Kukkonen-Harjula, 2000). Inactivity may be a more robust predictor of risk of overweight in children than the level of physical activity. Because the average adolescent watches almost 5 hours of television per day (Gortmaker et al., 1996), television viewing may be a surrogate measure of inactivity. Obesity in children and adolescents (Gortmaker et al., 1996; Dietz & Gortmaker, 1985; Anderson, Crespo, Bartlett, Cheskin, & Pratt,

5 PHYSICAL ACTIVITY AND OBE\SITY ) and adults (Hu, Li, Colditz, Willett, & Manson, 2003) has been associated with television time. The more television children and adolescents watch, the greater the likelihood that they will be overweight. Furthermore, changes in the amount of television watched predicts changes in body mass index among children (Berkey, Rockett, Gillman, & Colditz, 2003). Children who increase the amount of television they watch increase their BMI more than expected for age and gender, and children who reduce the amount of television that they watch have decreased rates of BMI increase compared to children of the same age and gender. Although the mechanism(s) that link television time to obesity remain uncertain, displacement of more vigorous activity is one potential explanation. Although physical activity is generally thought of as activity associated with gross motor movements, new perspectives on obesity suggest that minor motor movements may distinguish persons with high and low energy expenditures in metabolic chambers. Several activities that may ordinarily be considered sedentary activities may actually increase metabolic rate above resting levels. For example, chewing gum increased RMR by almost 20% (Levine, Baukol, & Pavlidis, 1999), and playing a video game increased RMR by 80% (Segal & Dietz, 1991). In addition, differences in energy expenditure in a metabolic chamber are not explained by major motor movements (Ravussin, Lillioja, Anderson, Christin, & Bogardus, 1986; Zurlo et al., 1992), and nonexercise activity thermogenesis, which includes activities of daily living, fidgeting, or maintaining posture when not recumbent, may account for differences in weight gain associated with overfeeding (Levine et al., 1999). Energy Expenditure After Obesity Develops Although overweight persons tend to be less active (Johnson, Burke, & Mayer, 1956), the proportion of TEE spent on physical activity (PAEE and PAL) appears comparable to that of nonoverweight persons (Ravussin, Burnand, Schutz, & Jequier, 1982; Bandini, Schoeller, & Dietz, 1990).This paradoxical observation can be readily explained by the energy cost of physical activity among overweight individuals. Simple physics indicate that comparable movements by overweight and nonoverweight individuals will require more energy by the overweight individuals because of the increased mass that must be moved. The Role of Physical Activity in Weight Reduction Numerous studies have considered the role of increased energy expenditure on weight reduction. These studies suggest that physical activity alone has a limited impact on body weight among the obese (Miller, Koceja, & Hamilton, 1997) and the addition of physical activity to dietary therapy provides a modest increase in the amount of weight lost (Grundy et al., 1999; Jakicic et al., 2001; Ballor & Poehlman, 1994). In a careful study of the effect of diet plus either moderate or vigorous physical activity of differing duration, no significant differences in weight loss could be observed (Jakicic, Marcus, Gallagher, Napolitano, & Lang, 2003). Although a logical argument can be made that losses of fat free mass or the reduction in metabolic rate that accompany weight loss could be averted by physical activity, resistance training does not appear to affect either rates of weight loss or to preserve resting metabolic rate during weight reduction (Ballor & Poehlman,

6 6 DIETZ 1994; Donnelly et al., in press). Nonetheless, physical activity may play a much more important role in weight maintenance after weight loss. For example, participants in a weight loss registry consists of people who lost 30 kg or more and sustained these losses over a 5-year period (Klem et al., 1997; McGuire, Wing, Klem, Seagle, & Hill, 1998). In addition to weight monitoring and consumption of a low fat diet, these individuals also spent approximately 2500 Kcal per week on physical activity, mostly through walking. The Impact of Physical Activity on Obesity-Related Comorbidity Although physical activity may have a limited effect on weight loss, increased physical activity appears to protect against or reduce mortality among obese adults (Lee, Blair, & Jackson, 1999; Stevens, Cai, Evenson, & Thomas, 2002; Farrell, Braun, Barlow, Cheng, & Blair, 2002) and to reduce the risk of obesity associated comorbidities, such as hypertension, hyperlipidemia, or glucose intolerance (National Institutes of Health, 1998). Furthermore, increases in fitness appear to decrease the risk of mortality (Blair et al., 1995). A significant source of controversy is whether the risk of fatness on mortality is independent of fitness. For example, Blair s group (Lee, Jackson, & Blair, 1998) has argued that after fitness is controlled, fatness exerts no additional effect on mortality, a result observed in at least one other study (Farrell et al., 2002). Nonetheless, other data suggest that fatness independently affects mortality (Stevens et al., 2002). Several questions deserve further considerations in this debate. Because the unfit group in the Cooper Clinic studies (Lee et al., 1998; Farrell et al., 2002) was designated as those in the slowest quintile of treadmill time, and the fit group consisted of those in the other four quintiles, the fit group might be more appropriately characterized as not unfit. Furthermore, the classification of obesity by Lee et al. (1999) was based on hydrostatic weighing, skinfold thicknesses, or both. However, the mean BMI was 28.4 in what was designated as the obese, fit (or not unfit) group and 30.5 in the obese, unfit group. Because these weight categories are overweight (BMI ) and barely obese (BMI > 30), respectively by more widely accepted standards, the generalizability of these findings to more obese groups remains uncertain. In addition, although the determination of fitness was based on total treadmill time, body weight affects energy spent on a treadmill time (Institute of Medicine, 2003), and therefore fitness. As a result, the groups categorized as fat and fit may have been fat and superfit. Finally, although mortality offers a clear endpoint, it is not clear that fitness protects against other sequelae of obesity. Physical Activity Recommendations Physical activity recommendations are shown below. Beginning with the predecessor of the President s Council on Physical Fitness and Sports (PCPFS), recommendations have generally focused on the amount of physical activity necessary for health. In general, the guidelines have consistently recommended that 30' of physical activity was required for cardiovascular health. In 1995, a physical activity recommendation first appeared as a Dietary Guideline but focused on energy expenditure as a mechanism to balance dietary intake. As indicated above, no data yet justify a recommendation regarding the dose of physical activity necessary

7 PHYSICAL ACTIVITY AND OBE\SITY 7 to prevent obesity. Because synergies between obesity and its metabolic consequences appear to maintain weight at an increased level, it seems reasonable to assume that the dose of physical activity necessary to maintain weight after a loss may would be higher than required to maintain weight before weight gain. Physical Activity Guidelines 1965: PCPFS: 30' of physical activity 5 times/week 1995: ACSM/CDC: 30' moderate physical activity most or all days of the week 1995: Dietary Guideline: Balance the food you eat with physical activity; maintain or improve your weight. 1996: Surgeon General s Report: 30' moderate physical activity most or all days of the week Challenges in the Implementation of Physical Activity Recommendations Although substantial imprecision remains, physical activity clearly plays a role in weight maintenance, weight loss, weight maintenance after loss, and the reduction of obesity associated comorbidities. The most important challenge with respect to the role of physical activity and obesity is how to increase physical activity for the population. The latest data from the Behavioral Risk Factor Surveillance System suggest that approximately 30% of U.S. adults are sedentary (Macera et al., 2003). Within medical settings, increased counseling regarding the role of physical activity in the prevention of disease will surely play a role in efforts to increase physical activity. Nonetheless, the U.S. Preventive Services Task Force (1996) concluded that existing studies have not yet established the effectiveness of clinician counseling to promote physical activity. This finding reflects the lack of multiple and consistent trials that show a positive effect, not a determination from randomized clinical trials that such counseling is ineffective. Therefore, the Task Force concluded that counseling was warranted based on the impact of regular physical activity on many of the chronic diseases associated with obesity. In contrast, the physical activity chapter for the Guide for Community Preventive Services recommended 6 strategies to increase physical activity (see below; Kahn et al., 2002). These include informational strategies, such as point of decision prompts for stairwell use, behavioral and social interventions such as school-based physical education, or environmental and policy interventions such as access to and promotion of facilities for physical activity. A chapter for the Guide for Community Preventive Services on school, work site, and community strategies to address obesity will soon be forthcoming. Recommended Strategies to Increase Physical Activity From the Guide for Community Preventive Services 1. Informational Community-wide campaigns Point of decision prompts for stairwell use 2. Behavioral and social interventions School-based physical education programs

8 8 DIETZ Social support in community settings Individually-adapted health behavior change 3. Environmental and Policy Access to and outreach for facilities for physical activity An equally important and urgent problem is how to begin to implement the strategies set forth in the Community Guide. Translation of these recommendations into practical interventions for states and communities, and development of reasonable outcome measures to evaluate their effects, represent essential steps. For example, in 2002, Texas restored physical education programs to all their elementary schools. However, access and promotion of recreational facilities will require funds at a time when resources have rarely been scarcer. Although a clear argument can be made that the costs of inactivity are substantial (Pratt, Macera, & Wang, 2000), no data yet demonstrate that investments in physical activity are cost-effective. Nonetheless, awareness of the obesity epidemic offers a unique opportunity to emphasize the importance of physical activity, to strengthen the science base of clinical and public health strategies, to implement the programs needed to implement these strategies, and to develop the political will necessary to devote the resources required to achieve these goals. The Congress and the Department of Health and Human Services have initiated this process by providing the funds necessary to fund 20 States to develop nutrition and physical activity strategies to reduce obesity and its associated chronic diseases, and by providing funds through the STEPS to a Healthier U.S. initiative to communities to address obesity, type 2 diabetes, and asthma, and the risk factors of tobacco use, inactivity, and poor nutrition. These programs reflect a commitment to begin to address these problems through public health programs. The challenge throughout these efforts is to identify models of effective programs and to demonstrate their effectiveness in communities. References Anderson, R.E., Crespo, C.J., Bartlett, S.J., Cheskin, L.J., & Pratt, M. (1998). Relationship of physical activity and television watching with body weight and level of fatness among children. Journal of the American Medical Association, 279, Ballor, D.L., & Poehlman, E.T. (1994). Exercise-training enhances fat-free mass preservation during diet-induced weight loss: A meta-analytical finding. International Journal of Obesity, 18, Bandini, L.G., Schoeller, D.A., & Dietz, W.H. (1990). Energy expenditure in obese and non-obese adolescents. Pediatric Research, 27, Bandini, L.G., Must, A., Phillips, S.M., Naumova, E.N., & Dietz, W.H, (2003). Longitudinal changes in body mass index and body fatness in relation to energy metabolism during the premenarcheal period in girls. Obesity Research, 11, A65. Berkey, C.S., Rockett, H.R.H., Gillman, M.W., & Colditz, G.A. (2003). One-year changes in activity and inactivity among 10- to 15-year-old boys and girls: Relationship to change in body mass index. Pediatrics, 111, Blair, S.N., Kohl, H.W., Barlow, C.E., Paffenbarger, R.S., Jr., Gibbons, L.W., & Macera, C.A. (1995). Changes in physical fitness and all-cause mortality. A prospective study of healthy and unhealthy men. Journal of the American Medical Association, 273,

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11 PHYSICAL ACTIVITY AND OBE\SITY 11 Ravussin, E., Burnand, B., Schutz, Y., & Jequier, E. (1982).Twenty-four-hour energy expenditure and resting metabolic rate in obese, moderately obese, and control subjects. American Journal of Clinical Nutrition, 35, Ravussin, E., Lillioja, S., Anderson, T.E., Christin, L., & Bogardus, C. (1986). Determinants of 24-hour energy expenditure in man. Methods and results using a respiratory chamber. Journal of Clinical Investigation, 78, Ravussin, E., Lillioja, S., Knowler, W.C., Christin, L., Freymond, D., Abbott, W.G.H., Boyce, V., Howard, B.V., & Bogardus C. (1988). Reduced rate of energy expenditure as a risk factor for body-weight gain. New England Journal of Medicine, 318, Roberts, S.B., Savage, J., Coward, W.A., Chew, B., & Lucas, A. (1988). Energy expenditure and intake in infants born to lean and overweight mothers. New England Journal of Medicine, 318, Schoeller, D.A., Shay, K., & Kushner, R.F. (1997). How much physical activity is needed to minimize weight gain in previously obese women? American Journal of Clinical Nutrition, 66, Segal, K.R., & Dietz, W.H. (1991). Physiologic response to playing a video game. American Journal of Diseases of Children, 145, Stevens, J., Cai, J., Evenson, K.R., & Thomas, R. (2002). Fitness and fatness as predictors of mortality from all causes and from cardiovascular disease in men and women in the lipid research clinics study. American Journal of Epidemiology, 156, Stunkard, A.J., Berkowitz, R.I., Stallings, V.A., & Schoeller, D.A. (1999). Energy intake, not energy output is a determinant of body size in infants. American Journal of Clinical Nutrition, 69, U.S. Department of Health and Human Services.(1995). Third report on nutrition monitoring in the United States: Executive summary. Washington, D.C.: U.S. Government Printing Office. U.S. Department of Health and Human Services. (1996). Physical activity and health. A report of the Surgeon General. Washington DC: Author. U.S. Department of Health and Human Services. (2000). Healthy People 2010 (2 nd ed.). With understanding and improving health and objectives for improving health (2 vols).washington D.C.: U.S. Government Printing Office. U.S. Department of Health and Human Services, U.S. Department of Agriculture. (2000). Nutrition and your health: Dietary guidelines for Americans. Washington D.C.: U.S. Government Printing Office. U.S. Preventive Services Task Force. (1996). Counseling to promote physical activity. In Guide to clinical preventive services. Report of the U.S. Preventive Services Task Force (pp ). Washington, D.C.: Author. Wang, G., & Dietz, W.H. (2002).Economic burden of obesity in youths aged 6 to 17 years: Pediatrics. On-line. Available: 5/e81). Young, L.R., & Nestle, N.M. (2002). The contribution of expanding portion sizes to the U.S. obesity epidemic. American Journal of Public Health, 92, Zurlo, F., Ferraro, R.T., Fontvielle, A.M., Rising, R., Bogardus, C., & Ravussin, E. (1992). Spontaneous physical activity and obesity: Cross-sectional and longitudinal studies in Pima Indians. American Journal of Physiology, 263, E296-E300. Acknowledgment I am grateful to Bill Kohl for his review of a preliminary draft of this manuscript