Culture and Society Industry, Government
Community
School and Peers Family and Home
FIGURE 3-1 Simplified ecological systems theory model.
intake exceeds energy expenditure (Figure 3-2). Both aspects of energy imbalance (i.e., food and beverage intake and physical activity) interact with and are affected by multiple factors within each of the four ecological layers. The two innermost layers describe factors operating within the indi- vidual (including genetic factors, ethnic identity and culturally determined attitudes and beliefs, psychosocial factors, and current health status) and those operating within the physical and social locations and situations that define daily behavioral settings (Booth et al., 2001). The key behavioral settings for children and youth are the home, school, and community. As noted in the framework developed by the Partnership to Promote Healthy Eating and Active Living, behavioral settings are affected either directly or indirectly by a variety of other factors that potentially constitute primary and secondary leverage points for effecting changes (Booth et al., 2001).
These leverage points include the major sectors that affect the food system, opportunities for physical activity or sedentary behavior, and information and education regarding dietary behaviors and physical activity. The outer- most layer on the framework in Figure 3-2 reflects the critical concept of an overlay of social norms and values, that is, the social fabric that cuts across all the layers and processes below. Social norms and values both determine and respond to collective social and institutional processes within the con-
DEVELOPING AN ACTION PLAN 85 Social Norms and Values
Primary and Secondary Leverage Points
• Food and Agriculture • Education • Media
• Government • Public Health • Health Care
• Land Use and Transportation
• Leisure • Recreation Behavioral Settings
• Home • School • Community
Energy Intake
Energy Expenditure
Energy Imbalance Obese Children and Youth Genetic, Psychosocial, and
Other Personal Factors Food and
Beverage Intake
Physical Activity
FIGURE 3-2 Framework for understanding obesity in children and youth.
NOTE: In this diagram energy intake is depicted as excessive when compared to energy expenditure, leading to a positive energy balance (or energy imbalance) resulting in obesity.
text of the larger U.S. culture. This framework, which emphasizes the need for obesity prevention efforts to leverage the interests and actions of a number of stakeholders working within and across multiple settings and sectors, guided the review of evidence and the development of recommen- dations in this report.
86 PREVENTING CHILDHOOD OBESITY
OBESITY PREVENTION GOALS
Clear specification of obesity prevention goals is essential in shaping an action plan and evaluating its success. Pertinent issues for setting obesity prevention goals for populations include concepts of optimum population BMI and healthy weight levels, potential effects on food intake and patterns of physical activity and inactivity (the primary modifiable determinants of obesity), as well as attitudes and social norms related to food and eating, physical activity and inactivity, body size, and dietary restrictions (WHO, 2000; Kumanyika et al., 2002). For children and youth, these consider- ations must be framed not only within the context of healthy physical, psychological, and cognitive development but in recognition that the in- creased prevalence of childhood obesity has broadened the emphasis of dietary guidance to address the overconsumption of energy-dense foods and beverages and physical activity patterns (ADA, 2003, 2004).
For individual children and youth, obesity prevention goals focus on maintaining energy balance (calories consumed versus calories expended).
As discussed in greater detail later in the chapter, this involves engaging in healthful dietary behaviors and regular physical activity. Healthful dietary behaviors include choosing a balanced diet, eating moderate portion sizes, and heeding the body’s own satiety cues that indicate physiological fullness.
It is currently recommended that children and adolescents accumulate a minimum of 60 minutes of moderate to vigorous physical activity each day (see section on physical activity).
Children’s food and beverage intake and their physical activity and sedentary behavior patterns can be influenced by a variety of environmental factors, including the availability and affordability of healthful foods, ad- vertising messages, and opportunities to participate in physical activity within communities (Richter et al., 2000). Although individuals and fami- lies are embedded within broader social, economic, and political environ- ments that influence their behaviors and may either promote or constrain the maintenance of health (IOM, 2001), such environments may also serve as contexts for change. These are the settings in which relationships are formed (e.g., home environment and support networks), and they represent a collection of formal and informal community institutions that monitor the behavior and safety of residents (Leventhal and Brooks-Gunn, 2001).
As will be noted throughout this report, changing the social, physical, and economic environments that contribute to the incidence and prevalence of childhood obesity—especially in populations in which the problem is longstanding and highly prevalent—may take many years to achieve. There- fore, the committee acknowledges that numerous intermediate goals, in- volving step-by-step improvements in diet patterns and physical activity levels of children and youth, are necessary for assessing progress. The ulti-
DEVELOPING AN ACTION PLAN 87
mate aim of obesity prevention in children and youth, however, is to create, through directed social change, an environmental-behavioral synergy that promotes positive outcomes both at the population and individual levels.
Box 3-1 summarizes these long-term and intermediate goals, which will be discussed in greater detail throughout the report.
BOX 3-1
Goals of Obesity Prevention in Children and Youth
The goal of obesity prevention in children and youth is to create—through directed social change—an environmental-behavioral synergy that pro- motes:
• For the population of children and youth
♦ Reduction in the incidence of childhood and adolescent obesity
♦ Reduction in the prevalence of childhood and adolescent obesity
♦ Reduction of mean population BMI levels
♦ Improvement in the proportion of children meeting the Dietary Guidelines for Americans
♦ Improvement in the proportion of children meeting physical activity guide- lines
♦ Achieving physical, psychological, and cognitive growth and develop- mental goals
• For individual children and youth
♦ A healthy weight trajectory, as defined by the CDC BMI charts
♦ A healthful diet (quality and quantity)
♦ Appropriate amounts and types of physical activity
♦ Achieving physical, psychosocial, and cognitive growth and developmental goals
Because it may take a number of years to achieve and sustain these goals, intermediate goals are needed to assess progress toward reduction of obe- sity through policy and system changes. Examples include:
• Increased number of children who safely walk and bike to school
• Improved access to and affordability of fruits and vegetables for low-income populations
• Increased availability and use of community recreational facilities
• Increased play and physical activity opportunities
• Increased number of new industry products and advertising messages that promote energy balance at a healthy weight
• Increased availability and affordability of healthful foods and beverages at supermarkets, grocery stores, and farmers markets located within walking distance of the communities they serve
• Changes in institutional and environmental policies that promote energy balance
88 PREVENTING CHILDHOOD OBESITY
Optimum BMI and Healthy Weight
The concept of optimum BMI can be applied to populations. For coun- tries such as the United States, where undernutrition is not as common as in developing countries,3 a BMI-distribution median of around 21 kg/m2 may be optimal (WHO, 2000). Population weight goals for obesity prevention in adults can also be stated in terms of decreasing the proportion that exceed the threshold of 30 kg/m2, although this goal includes both prevent- ing new cases of obesity and reducing weight among those already over the threshold.
The same principles are appropriate for assessing the population of children in the United States in pursuit of the committee’s primary objec- tive: to stop, and eventually reverse, current trends toward higher BMI levels. Also, as discussed in Chapter 2, there are particular concerns about the population of obese children becoming heavier. Achieving this objective would have the effects of reducing the mean BMI as well as decreasing the proportion of children and youth in the population that exceeds the thresh- old definition of obesity.
Available research does not currently allow the committee to define an optimum BMI for children and youth. It suggests, however, that future research toward this aim should be focused on defining the associations between BMI and objective measures of concurrent and future growth and between BMI and physiological and psychological morbidity, mortality, and health (Robinson, 1993; Robinson and Killen, 2001).
Analogous to the current practice for adults, the committee recom- mends the use of BMI for assessing individual and population changes in children and youth over time and in response to interventions. Population weight goals for childhood obesity prevention should be stated in terms of changes in the mean BMI and in the shape of the entire BMI distribution.
Alternatively, goals can be stated in terms of decreasing the proportion of children or youth who exceed particular thresholds—e.g., 75th, 85th, 90th, 95th, or 97th percentiles of BMI for age and gender on the CDC BMI charts. In the absence of an appropriate evidence base, however, threshold goals are necessarily somewhat arbitrary and sacrifice substantial informa- tion about the rest of the distribution as well as substantial statistical power to detect differences between groups and over time (Robinson and Killen, 2001).
3Hunger and food insecurity persist in the United States. In 2002, 35 million individuals including 13.1 million children lived in food insecure households (an estimated 11 percent of all U.S. households); 3.5 percent (3.8 million) of U.S. households were food insecure with hunger (Nord et al., 2003). Additionally, rates of micronutrient deficiencies remain unaccept- ably high in certain subgroups of the U.S. population (Wright et al., 1998; Ballew et al., 2001;
Ganji et al., 2003; Hampl et al., 2004).
DEVELOPING AN ACTION PLAN 89 The current CDC guidelines for healthy weight in children and youth are in the range of the 5th to 85th percentiles of the age- and gender-specific BMI charts. Therefore, a child whose weight tracks in that range—that is, he or she does not cross to lower than the 5th or higher than the 85th percentiles—would be considered to be in the healthy weight range accord- ing to these definitions.
The CDC BMI charts are mathematically smoothed curves of the pooled growth parameters of children and adolescents sampled in cross-sectional national health surveys conducted from 1963 to 1994. An analogy would be to consider the curves as compiled from a series of “snapshots” of large national samples made at different times over three decades. But because the sample sizes at each age level get much smaller at the extremes of the distributions, the growth curves may be more prone to errors at the upper and lower ends.
Because of the increases in body weight that occurred in the 1980s and 1990s—after the second National Health and Nutrition Examination Sur- vey (NHANES II) conducted in 1976-1980—a decision was made not to include the NHANES III (1988-1994) body-weight data in the revised 2000 BMI charts for children aged 6 years or older. The NHANES III data would have shifted the affected curves (weight-for-age and BMI-for-age) upward, which was considered to be biologically and medically undesirable. How- ever, the fact that the CDC BMI charts were developed from data for a prior time period in which children were leaner, on average, leads to an occasionally confusing situation—for example, where more than 5 percent of the population is above the 95th percentile—but this is readily clarified in the context of the charts’ historical source.
The CDC BMI charts are derived from cross-sectional samples of chil- dren (data for different age groups are based on different children). That is, they do not directly represent the longitudinal growth trajectory for the same set of children who have been measured as they age.4 Therefore, it is not known whether an individual child’s height, weight, or BMI should be expected to follow along the same percentile curve over time in order to maintain health or whether there are health implications of variations throughout childhood (e.g., crossing percentiles by going from the 20th percentile at age 1 to the 60th percentile at age 5 to the 40th percentile at age 12). Mei and colleagues (2004) found that shifts in growth rates were
4The latter approach has been used to develop longitudinal growth charts that are used in several other countries (Tanner and Davies, 1985; Cameron, 2002). These types of charts are generally developed from smaller, and potentially less representative, samples.
90 PREVENTING CHILDHOOD OBESITY
common during birth to 6 months and less common in children aged 2 to 5 years. More research is needed to determine whether there is an increased prevalence of “crossing” percentiles in different populations or during dif- ferent age intervals and whether there are associations between crossing percentiles and health-related outcomes.
The problem is how to proceed despite this lack of certainty. The committee concluded that because the CDC BMI charts are based on large national samples of the U.S. population of children and youth, they are the best available tools for assessing growth in clinical and public health set- tings. Although there are many unknowns about how to apply this infor- mation to individual children, and clinicians face difficulties in making generalizations regarding normal growth trajectories, experience suggests that children who demonstrate rapid changes—that is, frequently crossing up or down percentiles—may require special health-care attention. Health- and medical-care professionals should be consulted regarding growth- related questions for individual children as they can assess a child’s own growth trajectory in context (see Chapter 6).
ENERGY BALANCE
Obesity prevention involves maintaining energy balance at a healthy weight while protecting overall health, growth and development, and nutri- tional status. Energy balance refers to the state in which energy intake is equivalent to energy expenditure, resulting in no net weight gain or weight loss. In adults, who have stopped growing, this relationship between energy intake and output must be equal and reach a zero net energy balance to prevent body storage of extra calories5 from food as fat and result in weight gain, which represents a positive energy balance. Strictly speaking, growing children, even those at a healthy body weight, must be in a slightly positive energy balance to satisfy the additional energy needs of tissue deposition for normal growth. However, for the purpose of simplicity in this report, the committee uses the term “energy balance” in children to indicate an equality between energy intake and energy expenditure that supports normal growth without promoting excess weight gain.
In children, energy expenditure constitutes the calories used for basal metabolism, processing of food, maintenance and repair of the body, and daily physical activity—in addition to the calories required for normal growth and development. Inappropriate weight gain (excess fat storage) results when energy expenditure is consistently exceeded by energy intake over time.
5In this report the term “calories” is used synonymously with “kilocalories.”
DEVELOPING AN ACTION PLAN 91 Energy intake is the calories ingested in the form of food and beverages.
Children require a dietary pattern consisting of a variety of foods that provide all the necessary nutrients to support normal growth and develop- ment, as well as regular physical activity. Thus, a balanced diet refers to the consumption of appropriate amounts of a wide variety of nutrient-dense foods that provide adequate amounts and proportions of macronutrients (protein, fat, and carbohydrates) as well as sufficient essential micronutri- ents (vitamins, minerals) and dietary fiber, in addition to providing ad- equate energy to meet the needs of maintenance, growth, and development.
Although “energy intake = energy expenditure” looks like a fairly basic equation, in reality it is extraordinarily complex when considering the mul- titude of genetic, biological, psychological, sociocultural, and environmen- tal factors that affect both sides of the energy balance equation and the interrelationships among these factors (Figure 3-2). For example, the amount, type, and intensity of physical activity influence body composition and physical fitness, which in turn influence the energy cost of physical activity (Hill et al., 2004).
There are several concepts regarding energy balance and weight gain in children and youth that the committee determined were important to clarify:
• Genetics is a factor in excess weight but it is not the explanation for the recent epidemic of obesity (Koplan and Dietz, 1999). Although inher- ited tendencies toward weight gain may be a partial explanation for excess weight in children, as discussed below, there have been no measurable changes in the genetic composition of the population during the recent decades that could explain the significant increases in obesity.
• Growth spurts do occur at several points throughout childhood and adolescence, but it cannot be assumed that a child will lose his or her excess weight at those times. Many experienced clinicians assess an indi- vidual child’s relative weight status by examining the consistency of that child’s weight or BMI percentiles over time. Thus, for example, after the age of about 4 years, normally growing children who are in the 20th or 50th or 65th percentile for weight would be expected to remain around these same percentiles for weight, during the remainder of their childhood.
However, what can be considered normal variation to that pattern is not yet known, and is an important research question.
• Physiological reasons for a child’s excess weight should be carefully explored by health-care professionals. However, the identifiable medical conditions that cause childhood obesity are rare and are not the principal underlying causes of the current obesity epidemic in the population.
• The perceptions of what healthy children should “look like” differ among generations, cultures, and individuals. However, it is important that obesity not become the norm in society for children and youth as it poses
92 PREVENTING CHILDHOOD OBESITY
serious health risks during childhood that can continue throughout adult life.
In the simplest terms, energy balance represents calories consumed versus calories expended, although as noted above, many individual vari- ables can affect that balance. The discretionary variables under an individual’s control on a daily basis are dietary energy intake and the energy expended during physical activity.6 Daily energy intake is deter- mined by the calorie content of the specific food and beverages consumed.
Energy expenditure above resting metabolism is largely dependent on the nature and intensity of the activity and is often measured in calories per minute of activity (e.g., walking at a moderate or brisk pace of 3 to 4.5 miles per hour on a level surface expends between 3.5 and 7 calories per minute as measured in adults [CDC, 2004]). Knowing this, it is possible to determine the amount of physical activity that would be required to “burn off” the energy contained in a given food (Box 3-2). The relatively high amount of physical activity required to balance the calories in many pre- ferred foods highlights the challenges of maintaining energy balance under conditions of a sedentary lifestyle and when surrounded by abundant food in large portions at relatively low cost. Much remains to be learned regard- ing the interactive effects of diet and physical activity—for example, the
6Resting metabolism also contributes to daily energy expenditure but it is not subject to modification by the individual in the short term. Resting metabolic rate changes as a function of body mass and composition which generally takes weeks or months to change under an applied regimen.
BOX 3-2
Balancing Food Intake and Physical Activity
• One small chocolate chip cookie (50 calories) is equivalent to walking brisk- ly for 10 minutes.
• The difference between a large chocolate chip cookie and a small chocolate chip cookie is estimated to be about 200 calories or about 40 minutes of raking leaves.
• One hour of walking at a moderate pace (20 minutes/mile) uses about the same amount of energy that is in one jelly-filled doughnut (300 calories).
• A fast food meal containing a double patty cheeseburger, extra-large fries, and a 24 ounce soft drink is equal to running 21/2 hours at a 10 minute/mile pace (1500 calories).
SOURCE: DHHS, 2001b.