On the eve of the Centennial Olympic Games held in Atlanta, Georgia, the U.S. Surgeon General released a landmark report on physical activity and health (DHHS 1996). In marked contrast to the fitness and physical achievements of the world’s Olympic athletes, the Surgeon General reported that 60 percent of American adults do not meet recommended levels of physical activity,1 and 25 percent are completely sedentary (DHHS 1996). Sedentary lifestyles are estimated to contribute to as many as 255,000 preventable deaths a year in the United States despite scientific evidence that regular physical activity—even at moderate levels, such as walking briskly for 30 minutes on most days—provides clear health benefits (Hahn et al. 1990 and Powell and Blair 1994 in DHHS 1996).
Concerned about the adverse health effects of physical inactivity, the Robert Wood Johnson Foundation (RWJF) and the Centers for Disease Control and Prevention (CDC) have undertaken environmental health initiatives to explore the causes of Americans’ increasingly sedentary lifestyles and identify opportunities to effect change through policies that would encourage greater levels of physical activity. The role of the built environment—in particular, decentralized land use patterns and reliance on the automobile—has come under scrutiny as one important potential contributor to reduced physical activity levels.
STUDY CHARGE AND SCOPE
In the above context, this study was requested by RWJF and CDC to examine the role of the built environment in physical activity levels. In particular, this report
Reviews the broad trends affecting the relationships among physical activity, health, transportation, and land use;
Summarizes what is known about these relationships and what they suggest for future policy decisions at all levels of government; and
Identifies priorities for future research.
The built environment is broadly defined to include land use patterns, the transportation system, and design features that together generate needs and provide opportunities for travel and physical activity.2 It refers to physical environments that have been modified by humans and comprises public spaces, parks, and trails, as well as physical structures (e.g., homes, schools, workplaces) and transportation infrastructure (e.g., streets, sidewalks).
A fairly extensive body of literature exists on the causal relationships between transportation policies and land use, although debate continues about both the direction and strength of those relationships (TRB 1995). Many of the adverse environmental and health effects of low-density development and reliance on automobile travel, such as poor air quality, diminished water supply and quality, and traffic injuries, have also been examined. The present study attempts to extend this understanding to examine the causal role of transportation and land use in increasingly sedentary lifestyles—a connection that has received much less research attention. The study can be viewed as a framing exercise whose objective is to sort out the complex relationships among transportation,
land use, physical activity, and health. Are there identifiable characteristics of built environments associated with different levels of physical activity? What are the strength and magnitude of any causal relationships? Do these relations differ by subgroups of the population or by type of physical activity? What implications for policy can be drawn from the current state of knowledge? What methods and data problems must be resolved to improve understanding in this area? What are priorities for future research?
The study is focused primarily on the U.S. experience. International studies and experience are reviewed where relevant. However, differences in land use and travel patterns as well as regulatory and institutional arrangements limit the applicability of foreign experience to the United States.
The remainder of this chapter provides a brief review of the importance of physical activity to health and energy balance, an overview of the committee’s approach to the study and key issues considered, and a summary of the organization of the report.
PHYSICAL ACTIVITY AND HEALTH: OVERVIEW
The primary reason for recent interest in the physical activity levels of the U.S. population, both adults and youths, stems from the clear connection between physical activity and health. The Surgeon General’s report of 1996 reviewed the existing literature on the role of physical activity in preventing disease. That review revealed an inverse association between physical activity and several diseases that is “moderate in magnitude, consistent across studies that differed substantially in methods and populations, and biologically plausible” (DHHS 1996, 145). The report concluded that the evidence is sufficiently strong to draw a causal relation between physical activity and health outcomes, including reductions in the risk of mortality from all causes, as well as reductions in cardiovascular disease (e.g., heart attacks, strokes), colon cancer, and non-insulin-dependent diabetes. Subsequent research has confirmed that endurance-type physical activity (e.g., walking, cycling)
also reduces the risk of developing obesity, osteoporosis, and depression (Saris et al. 2003; Landers and Arent 2001). In addition, physical activity may improve psychological well-being and quality of life (DHHS 1996).
Concern about physical activity levels also stems from economic considerations. According to CDC, direct medical expenses associated with physical inactivity totaled more than $76 billion in 2000 (CDC 2003; Pratt et al. 2000). This figure does not take into account indirect costs, such as lost productivity from the physical and mental disabilities to which sedentary behavior contributes. Research has shown that people who are physically active have, on average, lower annual direct medical costs and fewer hospital stays and physician visits, use fewer medications, miss fewer days of work, and are more productive at work than physically inactive people (Pratt et al. 2000). If 10 percent of adults began walking on a regular basis, an estimated $5.6 billion in heart disease costs alone could be saved (Pratt et al. 2000).
ENERGY BALANCE AND THE OBESITY CONNECTION
An important function of physical activity is to help maintain energy balance. Weight gain occurs when energy intake (calories consumed) exceeds total daily energy expenditure for a prolonged period (DHHS 1996). Total energy expenditure represents the sum of three factors: (a) resting energy expenditure to maintain basic body functions (approximately 60 percent of total energy requirements); (b) processing of food, which includes the thermic effect of digestion, absorption, transport, and deposition of nutrients (about 10 percent of total energy requirements); and (c) nonresting energy expenditure, primarily in the form of physical activity (about 30 percent of total energy requirements) (Leibel et al. 1995 in DHHS 1996). Energy balance tilts to weight gain when disproportionately more energy is taken in. For a typical person, about 1 pound (0.45 kilograms) of fat energy is stored for each 3,500 kilocalories of excess energy intake (DHHS 1996).
Obesity, a major public health problem, is frequently and mistakenly confused with inadequate levels of physical activity—a separate and critical public health problem. The recent marked rise in obesity levels among the U.S. population is due to an energy imbalance. According to the results of a 1999–2000 CDC-sponsored survey, nearly two-thirds of U.S. adults aged 20 and older are overweight or obese,3 and approximately 15 percent of children and adolescents aged 6 to 19 are overweight (Flegal et al. 2002; Ogden et al. 2002). Among U.S. adults, obesity levels doubled between 1980 and 2000—from 15 percent of the adult population to 31 percent. The percentage of children and adolescents defined as overweight has more than doubled since the early 1970s, with many adverse health consequences (CDC 2004). The 10- to 12-pound median weight gain of the U.S. population over the past two decades is the result of an estimated daily net caloric imbalance of about 100 to 150 calories, equivalent to drinking about two-thirds of a 12-ounce soda each day (Cutler et al. 2003).
Physical activity can play an important role in helping to restore and maintain energy balance. For example, increasing physical activity levels by walking briskly for 1 to 1.5 miles a day (e.g., a 15- or 20-minute mile) could offset the estimated net daily caloric imbalance of 100 to 150 calories (Cutler et al. 2003). Of course, the precise amount of caloric expenditure associated with physical activity is a function of the weight of the individual and the type and duration of the activity (Cutler et al. 2003). In general, however, relatively small changes in physical activity levels can play an important role in weight management and the reversal of obesity trends.
Theories abound concerning the causes of the recent rise in obesity levels—the extent to which it can be attributed to caloric intake
versus caloric expenditure and the role played by physical activity (Cutler et al. 2003). Resolving this debate, however, is not the primary purpose of this study. The focus is on understanding what effects the built environment may have in fostering sedentary behavior, the strength and magnitude of these effects, and opportunities and incentives to encourage greater physical activity.
STUDY APPROACH AND KEY ISSUES
The effects of the built environment on physical activity levels operate through a complex set of relationships. Figure 1-1 shows the committee’s conceptualization of the key connections. The starting point is the individual, with all the demographic characteristics, genetic components, lifestyle and other preferences, and time
constraints that influence the capacity and propensity to be physically active. For example, some individuals prefer a physically active lifestyle, choose to live in neighborhoods with bicycle lanes and walking trails, and use these and other activity-friendly facilities during their leisure time and, whenever possible, for utilitarian travel. Others are less physically active by choice or are constrained by limited time, income, or physical disabilities.
The figure shows that the individual is embedded in a built environment and in a larger social environment of economic, political, and societal forces that shape the available opportunities and choices for physical activity. For example, those inner-city neighborhoods with high crime rates, boarded-up store fronts, and poorly maintained infrastructure discourage walking or cycling even though the greater accessibility of many destinations, connectivity (directness of travel), and mix of land uses often found in inner cities are important correlates of physical activity (Saelens et al. 2003). Similarly, the character of communities in which individuals live, their daily activity patterns, and their opportunities for physical activity are affected by social norms, such as teenagers’ preference for driving to school; government policies, such as those affecting the availability of public transportation; and market forces, such as the demand for low-density living and the high cost of housing, that encourage the development of automobile-dependent communities far from city centers.
Figure 1-2 indicates the primary areas of investigation in this study, namely, the characteristics of the built environment and the various types of physical activity it may influence. Notably, the health box falls outside of this area—not because health is unimportant; indeed, it is the primary reason for the interest in physical activity—but because the link between physical activity and health is well established. As in Figure 1-1, the starting point is the individual, who operates at various geographic scales. The building or site, which has certain characteristics (e.g., stairwells, interior layout, access to and among other structures) that may affect physical activity levels, is the smallest unit of interest. The neighborhood is the next largest geographic unit of interest. It encompasses
residences, local retail and other commercial services, and schools. Several characteristics of neighborhoods may affect an individual’s propensity to be physically active, such as street layout (grid or culde-sac); the availability of parks, sidewalks, and bicycle paths; and land use mix (e.g., variety and numbers of destinations). The largest geographic unit for the purposes of this study—the region—is
generally defined by the commute area that captures most jobs associated with the resident population. In an urban context, the region is the metropolitan area. Here, the emphasis is on such characteristics as the size of the region, the distribution of jobs and commerce relative to residences, and the supply of transportation facilities (e.g., highways, public transit, bicycle paths) that influence individuals’ travel choices and physical activity levels. The focus of this study is primarily on the neighborhood and regional levels; very little is known about physical activity at the building or site level.
Characteristics of the built environment may provide opportunities for the individual to engage in a variety of physical activities. For purposes of this study, physical activity has been categorized into four types (see the rightmost box on Figure 1-2): (a) leisure-time recreation and exercise (e.g., bicycle riding, working out at a sports club or on a home treadmill), (b) transport or utilitarian travel (e.g., commuting, grocery shopping), (c) household production and home maintenance (e.g., housework, gardening, raking leaves), and (d) occupation-related physical activity (e.g., physically active jobs, stair climbing at work). The distinctions among these categories, however, are not always clear. For example, walking to run an errand could be counted as both exercise and utilitarian travel.
The diagram illustrates the complexity of the causal chain from the individual to the built environment to physical activity. For example, if a researcher focuses only on the link between the built environment and physical activity, the role of the built environment could be overstated. If, instead, the researcher steps back and controls for individual characteristics, including the possibility that the individual may choose or self-select an activity-friendly environment, the independent effect of the built environment on physical activity may be smaller. The diagram also shows several feedback loops. The built environment may influence the individual (for example, living in a neighborhood in which it is pleasant, safe, and easy to walk to stores may induce a more positive attitude toward utilitarian walking; living in a transit-rich area may increase one’s propensity to try transit). Physical activity itself may reinforce the propensity of an individual to be physically active.
An analysis of the linkages between the built environment and physical activity levels raises several key issues. First, scale plays an important role in determining which characteristics of the built environment are likely to affect individual decisions about travel mode and physical activity. For example, such neighborhood characteristics as amount of traffic and availability and proximity of facilities such as sidewalks, local parks, and paths are likely to be important to the decision to walk or cycle in the neighborhood. These characteristics, however, probably have little effect on whether an individual chooses to drive or take transit to work or to a shopping center. That decision is likely to be affected more by relative travel time—a function of distance between origin and destination and proximity and quality of service of available transport modes—as well as trip complexity (e.g., number of destinations, time constraints).
Second, the relationship between the built environment and physical activity operates through many mediating variables. For example, the actual or perceived safety of the environment could affect an individual’s propensity to be physically active in certain ways. Safety from crime may be the dominant concern in poor inner-city neighborhoods, while safety from traffic may be the major concern in middle- and higher-income suburban developments. Air quality is another mediating variable. Parents and school officials may limit the outdoor physical activity of school-age children in areas with poor air quality because of adverse health effects. For example, a recent study found that participation in multiple team sports and time spent outside were associated with the development of physician-diagnosed asthma in children of middle-income communities with high concentrations of ozone (McConnell et al. 2002).
Third, the trade-offs individuals make among their travel modes and the kinds of physical activity in which they engage are important in determining total levels of physical activity—the primary dimension of interest from the perspective of energy expenditure and health. For example, an individual may choose to commute to work by car rather than by a more physically active mode, such as
bicycling or walking to the bus or subway stop. By using the faster mode, however, that individual may have more time for exercise at a sports club or at home. Alternatively, those who choose to commute by bicycle or transit may forgo other recreational activity once they have reached their destination. Who will have expended the greater level of energy and achieved the greatest health benefits is not readily apparent.
Finally, time is a critical dimension in assessing opportunities for physical activity. Despite the tremendous growth in labor-saving technologies during the past century, particularly in the home, the demands of family, work, and travel limit the time available for physical activity for many individuals, at least during the workweek. Thus, it is important to consider how opportunities for physical activity can be fit into peoples’ daily routines at both work and home.
ORGANIZATION OF THE REPORT
The remainder of this report addresses the committee’s charge as outlined above. Chapter 2 provides a more complete discussion of the link between physical activity and health and presents data on the current physical activity levels of the U.S. population. In Chapter 3, historical data that may help explain the apparent long-term decline in total physical activity levels are examined in the areas of technological innovations in the workplace, at home, and in travel; decentralization of population and employment; and time use. Chapter 4 explores the contextual factors that affect physical activity levels—from the individual level; to the social context; to the institutional, regulatory, and political forces that have shaped the built environment in place today—and draws implications for intervention. Chapter 5 is concerned with issues in designing research for studying the relationship between the built environment and physical activity, particularly for examining causal connections, while Chapter 6 critically reviews the empirical research and findings to date. In Chapter 7, the committee provides its own findings, conclusions, and recommendations for policy and future research.
CDC Centers for Disease Control and Prevention
DHHS U.S. Department of Health and Human Services
TRB Transportation Research Board
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Physical Activity and Health
The evidence provided in this chapter demonstrates a strong and well-established scientific basis for linking physical activity to health outcomes. Current guidelines recommend either vigorous-intensity activity for at least 20 minutes per day for a minimum of 3 days a week or moderate-intensity activity for at least 30 minutes per day on all (or a minimum of 5) days of the week. The latter activity can accumulate over the course of a day in sessions of at least 10 minutes. Yet current survey data indicate that the majority of the U.S. population falls short of achieving these targets. More than half of adults report not meeting recommended levels of physical activity, and more than one-quarter characterize themselves as being completely inactive during their leisure time. Nearly one-third of high-school-age teenagers report not meeting recommended levels of physical activity, and 10 percent classify themselves as inactive. Approximately half are enrolled in a physical education class, but only one-third attend such classes daily. More than two-fifths of younger children, aged 9 to 13, report not participating in organized physical activity outside of school; slightly less than one-quarter indicate that they do not participate in any free-time physical activity during nonschool hours. These results indicate a widespread but largely preventable public health problem whose causes and possible solutions present a challenge to understand.
In the next chapter, longitudinal data are analyzed to determine what is known about changes in physical activity levels over time and the direction of these changes. Trends in other areas, including land use patterns and travel behavior—the focus of this study—are examined for their possible contribution to these changes.