Introduction

Prevention is widely recognized as an indispensable strategy to turn the tide of the global epidemic of obesity. The increasing number of people affected and the difficulty, cost, and low yield of therapeutic approaches point out to prevention as the premier focus of our efforts to reduce and eliminate obesity. Furthermore, because once established obesity is a protracted and difficult-to-treat condition, it also makes sense to focus prevention efforts on the younger generations, when health and nutrition education can shape good dietary practices and avoid excess weight gain. But, in spite of the recognized relevance of prevention, there are very limited scientific data pointing to specific successful approaches, and rigorous, controlled clinical trials are relatively few. This paper presents a brief overview of recent efforts in the US to develop successful prevention interventions for childhood obesity.

Obesity prevention in the US

During the first half of the last century, child health concerns in the US were primarily focused on reducing infectious diseases, improving nutrition in low-income communities and insuring intake of essential nutrients during early life. During and immediately after the Second World War, food and diet policies were mainly concerned with insuring adequate food intake in all the population (the first US Recommended Dietary Allowances—RDA—were issued in 1941). In the 60s, President Johnson's War on Poverty continued the focus on preventing undernutrition, introducing federal food assistance programs targeting poor families and schoolchildren. Still, the problem of obesity was not completely ignored, as evidenced by a 1966 report of the Department of Health,¹ which concluded that obesity was already a serious public health problem in the US. A few leading nutrition scientists, most notably Jean Mayer, also raised the issue of childhood obesity, and began to explore prevention and treatment options. In the early 60s, Jean Mayer introduced an 'obesity control' program in elementary and high schools in the city of Newton, a suburb of Boston.² This study, like several others that followed, targeted children who were already obese, and attempted to modify risk factors in the school environment while encouraging behavioral changes that would result in reduced energy intake and increased physical activity. These fundamental premises of childhood obesity prevention have not changed substantially since then.

In the following decades, research on the causes and mechanisms of obesity rapidly increased, and the availability of longitudinal national surveys revealed the rapid increase in obesity prevalence, both in children and adults.^{3, 4, 5, 6} Significant progress was made in quantifying the secular trends of obesity, its association with specific comorbidities, and the role played by ethnicity and socioeconomic status.^{7, 8} Clearer consensus definitions and standard criteria for the diagnosis of obesity were developed, and approaches to management and monitoring were proposed.^{9, 10} Still, compared with other chronic conditions, research on obesity prevention still lags behind, likely because of the extremely complex set of factors determining excess weight gain, and the multiple social, cultural, and economic elements linked to human food intake behavior. Furthermore, research efforts have been split between targeting the increasing number of already obese individuals and developing truly primary prevention interventions for the whole population. In the case of children, this latter focus requires additional efforts to avoid any adverse effects on those children who are not obese or those who have a higher risk for undernutrition.

Design of prevention studies

While a number of health and nutrition interventions targeting children reported weight or BMI outcomes, only a few were specifically designed to reduce BMI or body adiposity. Many have focused on subgroups of children at higher risk, based on their initial weight for age or BMI, or on other risk indicators, such as having one obese parent or a sedentary lifestyle. Studies targeting the population as a whole and aiming at primary prevention are relatively fewer.

Most primary prevention studies included one or more of the following components: (1) dietary intervention, aimed at reducing total energy intake; (2) nutrition/health education, intended to improve knowledge and behaviors related to food choices and to favor healthier lifestyles; (3) physical activity programs, aimed at increasing the time spent in moderate or vigorous activity, and/or reducing time spent in sedentary activities; (4) behavior modification and support, sometimes involving the family and/or members of the community. Table 1 summarizes prevention studies for childhood obesity in North America, as recently listed in a review by Campbell (2002) for the Cochrane Collaborative. As described in the table, there is a wide variety of selection criteria, sample size, attrition rates, and duration of the intervention. Outcome indicators are more consistent, and usually include body weight or BMI, and, in a fewer cases, indices of body fatness such as skinfolds.

Table 1 - Trials for the prevention of childhood obesity in the USA (Campbell, 2002).

Full table

The majority of published studies would not fit the category of randomized clinical trials. Some used historical controls, or compared a single group before and after the intervention.¹¹ This type of design is especially problematic in the pediatric age range, since significant secular changes in body weight, mental development, and learning may occur during the time span of the intervention. Sample size and unit of analysis are also important design elements for trials targeting schoolchildren. Interventions that are delivered at school are greatly dependent on the school environment (physical resources, food service personnel, attitude and experience of teacher and school director, etc), and thus cannot be generalized unless these factors are addressed in the design. This is possible only if sufficient number of schools are included and used as the unit of randomization. Instead, a number of studies have included only two schools: one interventions and the other control.^{12, 13} Finally, when some measurements are to be done in only a subset of children (eg, blood work), these children should be selected at random and not by volunteering, but this is frequently not the case.¹²

Priority targets for prevention interventions

Physical activity/inactivity

It is generally recognized that human energy balance can be more readily modified by changes in dietary energy intake than in energy output (voluntary physical activity). However, most healthy children are naturally active, and physical education is (or should be) a basic component of children's early education, thus providing a target of opportunity for school-based programs. Thus, physical activity is usually a core component of prevention interventions. Several studies (not necessarily focused on obesity prevention) have demonstrated the feasibility of increasing physical activity in the school environment.^{14, 15} More recently, researchers have focused on the amount of time children spend in sedentary activities (TV viewing, working or playing in the computer), suggesting that a consistent reduction of daily sedentary time may be as or more important than short periods of vigorous activity for maintenance of long-term energy balance.¹⁶ Studies exploring one typical sedentary activity, TV viewing, seem to support this notion. The association between time spent watching TV and obesity was first pointed out by Gortmaker and Dietz in the 1980s,¹⁷ and confirmed in subsequent reports.¹⁸ Moreover, preliminary evidence indicates that reducing TV viewing may be an effective component of primary prevention programs for elementary and middle-school children. Two interventions, one in California¹³ and the other in Massachusetts,¹⁹ reported a relative decrease in BMI in children who received 6 months to 2 y of an intervention that included restriction in TV viewing.

Dietary intake

As mentioned above, energy intake is recognized as a major determinant of energy balance. But in the case of children, there are constraints for reducing energy intake as a means to reduce body weight, even for overweight children. First, there is always concern on affecting the normal growth rate by caloric restriction; second, at least in the US, federally funded food assistance programs, such as the School Lunch Program, have specific requirements on the amount of calories that must be served in school meals, and any school-based interventions must comply with these guidelines. Thus, dietary interventions must usually rely on indirect measures, such as reducing the energy density of foods. Common strategies for reducing energy density include using low-calorie products, offering more fresh foods and vegetables, and reducing portion size and number of energy-dense, nutrient-poor items, such as deserts. Several studies have applied these strategies in school cafeterias and have reported success in reducing children's ad libitum intake of total and fat-derived calories.^{20, 21}

Behavioral change

This is an essential component of both prevention and management programs for childhood obesity. The majority of interventions have followed a model based on social learning theory, aiming at modifying individual, behavioral, and environmental factors associated with obesity risk. The intervention would seek to enhance children's knowledge about diet, physical activity, and health, and promote their self-efficacy for healthful behaviors. Many studies have documented variable levels of success in this area, particularly with elementary and middle school children.^{21, 22} However, these behavioral changes not always translate into measurable differences in BMI or body adiposity.

A factor usually not considered in prevention research but clearly related to sustainability of diet-based interventions is food cost. In a recent review, Drewnowski and Specter²³ reviewed the evidence showing that an 'unhealthy' diet (high in saturated fat, low in fresh fruits and vegetables) is significantly less expensive than a healthy diet. Addressing this issue will be critical to insure the acceptability of dietary recommendations in the low-income segment of the population, which is the one suffering higher rates of obesity.

Our experience with the Pathways study,^{21, 24, 25} a randomized controlled trial for the primary prevention of childhood obesity, exemplifies some of the key opportunities and challenges in this area. Unlike other studies, Pathways selected body fat as the outcome variable, measured by a combination of bioelectrical impedance and skinfolds, validated against deuterium oxide dilution.²⁶ The study included 41 schools serving eight American Indian communities in several regions of the US. The focus on American Indian children was justified by the high risk of obesity in these communities.^{27, 28} The study experimental design defined the school as the unit of analysis and used a stratified randomization scheme in order to insure a balanced BMI distribution in control and intervention groups. The components and aims of the Pathways intervention are listed in Table 2. The intervention, which lasted three school years, resulted in a significant reduction in total energy intake, percent of calories derived from fat, and knowledge and behaviors related to food intake and healthy lifestyle. However, it did not reduce the percent body fat or BMI in intervention schools compared to controls. This may be because the intervention did not last long enough, or due to weight gain during the 3 months of summer recess, or due to a compensatory increase in the dietary energy intake at home. Nevertheless, the study demonstrated the feasibility of introducing important changes in the school environment, and several of the intervention components were permanently adopted by schools, within and outside the study.

Table 2 - Components of the pathways intervention.

Full table

Conclusions

As noted above, the number of carefully controlled prevention studies targeting children is still relatively small. School-based interventions have had some clear successes in reducing dietary energy intake, fat intake, and limiting sedentary activities, primarily TV viewing. Perhaps it is not surprising that more targeted, relatively short-term interventions have been able to document successful changes in BMI than open, longer-term ones. However, the eventual effectiveness and sustainability of short-term interventions will have to be assessed in larger studies. There is also increasing concern on the 'obesogenic' environment.²⁹ Some of the characteristics of this environment, in which maintenance of energy balance is made particularly difficult, include: wide availability of high-energy density, inexpensive foods; increasing opportunities to consume it throughout the day; reduced energy demands of daily activities (automobile, elevators, escalators); increasingly sedentary leisure time (TV, video, movies); and limited opportunities for recreational physical activity (lack of safe open spaces, time demand from work). Clearly, children and their families are subjected to this environment, and therefore any effort to modify behaviors associated with excess weight gain will have to address those societal and cultural factors, or at least consider their potential impact on school- or household-based prevention programs.

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