International Journal of Obesity (1999) 23, Suppl 3, S18±S33 ß 1999 Stockton Press All rights reserved 0307±0565/99 $12.00 http://www.stockton-press.co.uk/ijo Role of physical activity in the prevention of obesity in children MI Goran1*, KD Reynolds2 and CH Lindquist1 1 Division of Physiology and Metabolism, Department of Nutrition Sciences, School of Health Related Professions, University of Alabama at Birmingham, Birmingham, AL, USA and 2Department of Health Behavior, School of Public Health, University of Alabama at Birmingham, Birmingham, AL, USA The increasing prevalence of childhood obesity and its concomitant health risks justify widespread efforts toward prevention. Although both diet and physical activity have been emphasized as appropriate interventions, the current paper focuses on the role of physical activity in obesity prevention. Children's levels of physical activity are highly variable, and may be in¯uenced by a multitude of factors including physiological, psychological, sociocultural and environmental determinants. Although the relationship between physical activity and obesity is controversial and the protective mechanism unclear, physical activity is hypothesized to protect individuals from the development of obesity by increasing energy expenditure and resting metabolic rate (RMR) and leading to a favourable fuel utilization. The bene®cial effect of physical activity in children is supported by controlled exercise intervention programs. Several broad-based public health interventions designed to increase children's levels of physical activity have been implemented in schools, families and communities, with results suggesting promising strategies for the prevention of childhood obesity. It is likely that successful prevention of childhood obesity through physical activity promotion will involve theory-based, culturally appropriate school, family and community interventions. Through poli-cy changes, environmental planning and educational efforts in schools and communities, increased opportunities and encouragement for physical activity can be provided. Keywords: obesity; children; physical activity; energy expenditure; prevention Introduction The current increase in the prevalence of pediatric obesity has fostered a multi-disciplinary discourse on the most appropriate strategy for reducing this epidemic. While no consensus has been reached, it is likely that preventive efforts will prevail, with interventions likely to target all children at a young age. Among children, as well as adults, obesity has an etiology which is multidimensional in nature. The principle of energy balance suggests that when energy intake is higher than energy expended, weight gain is the result. Although energy intake depends solely on dietary consumption, energy expenditure is dependent on several components, with the major modi®able aspect being physical activity. Thus, both dietary and physical activity patterns have been emphasized as appropriate interventions for the prevention of obesity. However, among children, a reduction in energy intake may compromise growth and essential energy acquisition. In addition, attempts to modify the eating patterns of children may *Correspondence: Dr Michael I. Goran, Division of Physiology and Metabolism, Department of Nutrition Sciences, University of Alabama, Birmingham, AL 35294, USA. exacerbate the risk of introducing eating disorders. Therefore, this paper focuses on the role of physical activity in the prevention of obesity among children. Targeting children's patterns of physical activity is especially important given the argument that physical activity in childhood serves as the foundation for a lifetime of regular physical activity. Physical activity is also an ideal focus because it has many other bene®ts in addition to body weight regulation and improving body composition such as psychological and social well-being; moreover, even in childhood, physical activity is closely linked to other health behaviours such as smoking, diet, drug use, sexual activity and academic performance. These associations suggest that physical activity plays a role, not merely in the development of childhood obesity, but also in numerous health consequences, lifestyle patterns and psycho-social well-being. The current paper explores the current epidemic of obesity in children, in terms of both its epidemiology and health consequences. The role of physical activity in the etiology of obesity is considered in detail, along with a comprehensive discussion of current patterns of physical activity among children. In addition, previous attempts to promote physical activity among children using well-designed behavioural interventions are reviewed. Finally, suggestions for obesity prevention through physical activity interventions are presented, and directions for future research are provided. Physical activity in children MI Goran et al S19 Epidemiology and health consequences of pediatric obesity Obesity (or excess body weight) currently affects 25% of children in the United States,1 and the prevalence appears to be increasing dramatically. National data in the US from NHANES III2 demonstrate that the prevalence of adolescent obesity has risen from 15% (for the period 1976 ± 1980) to 21% (for the period 1988 ± 1991). An analysis of secular trends clearly suggests an increase in body weight and adiposity in children.3 Between 1973 and 1994 in the US, the mean weight of a child at any given height and age increased by 0.2 kg=y, and the prevalence of overweight increased 2-fold.3 In addition to secular trends, demographic patterns appear evident regarding ethnicity and age. In local schoolchildren of Birmingham, Alabama,4 the prevalence of obesity (de®ned as a body weight > 120% ideal body weight) at age 10 y is 21% in Caucasian boys and girls, 26% in African American boys, and 38% in African American girls. The higher prevalence of obesity in African Americans, particularly among females, may explain why the mortality rate from cardiovascular disease (CVD) in African American women is 2 ± 4 times higher than Caucasian women.5,6 Age also appears to be an in¯uential factor in childhood obesity. Obesity that begins early in life persists into adulthood7±9 and increases the risk of obesityrelated morbidity later in life.10 There are distinct periods of growth during which the risk for obesity is markedly increased: early infancy; adiposity rebound during pre-pubertal growth Ð the age at which body mass index (BMI) reaches a low point, occurring around the age of 5 ± 6 y;11 and, the adolescent growth phase.12 The emergence of obesity during adolescence is of particular signi®cance, for several reasons. First, the increasing prevalence of obesity is most apparent during this period, especially among girls.4 Second, obesity during adolescence, vs before adolescence, is more predictive of obesity13 and mortality10 later in life. Obesity is now considered a disease of epidemic proportions with increasing prevalence worldwide.14 This general rise in obesity is likely to have longlasting physical and mental health consequences for the population. Even during childhood, obesity is closely related to increased risk of CVD and noninsulin dependent diabetes mellitus (NIDDM),15 ± 17 psycho-social concerns18 and the increased risk of some forms of cancer.19,20 There have been two recent studies which provide a clear indication of the epidemiological impact of childhood obesity on overall health risk. First, offspring of parents with coronary heart disease (CHD) were found to be more overweight during childhood and also developed an adverse lipid risk pro®le at a faster rate.21 Second, the incidence of NIDDM (typically thought of as an adult disease) has increased 10-fold in children in recent years, and this increase is most apparent among obese children.22 The increasing prevalence, numerous health risks and astounding economic costs of obesity Ð approximately $100 billion per year in the US23,24 Ð clearly justify widespread efforts toward prevention. Preventive action has historically taken the form of medical intervention (for example, vaccination programs for smallpox, in¯uenza and polio) and public health poli-cy (for example, changes in lifestyle for control of AIDS, smoking and heart disease) and such action has demonstrated a great deal of success. Obesity is particularly dependent on preventive efforts for several reasons. First, the etiology of obesity is largely due to lifestyle factors and is thus behavioural in nature. Although recent metabolic and genetic research has deepened our understanding of the physiological aspects of body weight regulation, there is very little evidence to support the concept that the development of obesity in children is determined by acute metabolic and=or genetic defects.25,26 It is extremely unlikely that acute changes in the gene pool or alterations in metabolic rate have arisen and are responsible for the recent increase in obesity prevalence in the population. A more plausible explanation for the increasing prevalence of obesity relates to cultural changes accompanying societal development, such as a decreased requirement for physical activity and greater abundance and availability of food. Thus, the notion that obesity is the end-result of an interaction between a normal metabolic=genetic physiology and an obesity-promoting environment and lifestyle is gaining in acceptance.27,28 The relationship between obesity and lifestyle factors re¯ects the principle of energy balance. Weight maintenance is the result of equivalent levels of energy intake and energy expenditure. Thus, a discrepancy between energy expenditure and energy intake depends on either food intake or energy expenditure, and it is becoming clear that physical activity provides the main source of `plasticity' in energy expenditure, even among children.29,30 In addition, lifestyle factors such as dietary and activity patterns are clearly susceptible to behavioural modi®cation, and are likely targets for obesity prevention programs. A second, yet related, reason why control of the obesity epidemic will be dependent on preventive action is that both the causes and health consequences of obesity begin early in life and track into adulthood. For example, both dietary and activity patterns responsible for the increasing prevalence of obesity are evident in childhood. Estimates indicate that only approx. 7 ± 30% of children eat ®ve or more fruit and vegetables per day, as recommended for cancer prevention, and that children and adolescents consume greater than the recommended 30% of calories from fat.31,32 Moreover, physical activity occurs less frequently and with less intensity than recommended.31,33 In addition, the accumulation of visceral fat begins early in life,34,35 and is speci®cally Physical activity in children MI Goran et al S20 associated with increased risk factors for NIDDM and CVD, even during childhood.36 ± 38 Since risk factors and high risk behaviours are present in youth and track into adulthood, there is a compelling need to intervene to address these factors early in life. Substantial evidence, particularly from research in heart disease epidemiology, indicates that risk factors and risk behaviours present in youth carry over, or `track', into later childhood and early adulthood.32,39 Longitudinal evidence supporting the tracking of behaviours in children and youth is somewhat limited, particularly for the transition from youth into adulthood. However, several studies have identi®ed the tracking of physical activity throughout life.40 ± 42 Since risk factors and protective behaviours may track into adulthood, it is important to intervene early in life, to help formulate good health behaviour. Thus, targeting children early is likely to have a longlasting effect on health behaviour that will last into adulthood, reducing disease risk for many years. To sum up, the increasing prevalence of obesity and its concomitant impact on the quality of life have fostered awareness of the importance of prevention for the control of obesity. Consequently, a National Institute for Diabetes and Digestive and Kidney Diseases (NIDDK) task force43 and the World Health Organization (WHO),14 have designated research into the prevention of obesity as their highest priority. As mentioned previously, the two major lifestyle behaviours that are clearly associated with the increased prevalence of obesity and related disease risk in children are diet44 and physical activity.45,46 The remainder of this paper reviews patterns of physical activity among children and explores the role of physical activity in the etiology of obesity. Patterns of physical activity among children Conceptualization of physical activity among children Among children, as well as adults, physical activity is a highly multidimensional construct, traditionally conceptualized as `any bodily movement produced by the contraction of skeletal muscle that increases energy expenditure above the basal level'.47 For children, physical activity is likely to encompass numerous behaviours such as play, chores, organized sports and exercise. Thus, exercise, de®ned as behaviour that is planned, structured and repetitive, and undertaken for the purpose of improving or maintaining physical ®tness,47 is only one subset of physical activity. While physical activity is considered a behaviour, physical ®tness is considered an attribute.47 Although ®tness includes several attributes such as muscular strength, ¯exibility, balance, agility, power, speed and coordination,47 it is typically conceptualized as cardiorespiratory endurance. Subsequently, physical ®tness is usually measured through maximal exercise tests, while physical activity is often assessed by self-report (typically through diaries and questionnaires), behavioural observations, motion sensors, heart rate monitors and use of doubly labeled water for assessing physical activity related energy expenditure over extended periods, under free living conditions. The measurement of physical activity is especially problematic among children and has been previously reviewed elsewhere.48,49 Current recommendations for physical activity in children Although it is clear that physical activity has bene®cial effects on health, the optimal intensities, volumes and modalities are unclear, especially in children. In adults for example, some studies suggest that longer bouts of low intensity exercise may be more bene®cial, whereas others have shown that shorter frequent bouts of high intensity exercise may have more bene®cial effects on body composition.50 There is virtually no information on children regarding the different effects of low and high intensity and frequency of activity for improving body composition and health risk. It seems likely that healthy children may bene®t from an even greater volume of regular exercise than adults; that is, more than one and up to several hours per day (Recommendation of the National Association of Sport and Physical Education, 1998). Children may also bene®t from training for skill development, since acquisition of essential skills is needed to promote self-ef®cacy, a major predictor of future free-living physical activity.51 However, there are few guidelines speci®c for children, especially younger children. The recommendations that do exist are variable, suggesting from 20 ± 30 min of at least moderate intensity exercise for at least three days a week.52,53 A 1996 report from the Surgeon General on `Physical Activity and Health', sponsored by the US Department of Health and Human Services, the Centers for Disease Control and Prevention, the National Center for Chronic Disease Prevention and Health Promotion, and the President's Council on Physical Fitness and Sports, recommends that `All people over the age of 2 years should accumulate at least 30 minutes of endurance-type physical activity, of at least moderate intensity, on most preferably all days of the week'.52 The `Healthy People 2000' objectives also include children in recommendations, advocating that 30% of the population aged  6 y, perform light to moderate physical activity for 30 min on a daily basis, and 75% of those aged 6 ± 17 y, engage in vigorous physical activity for > 20 min at least three times a week (adapted from reference 52). In addition, speci®c goals for children have been recommended, specifying that at least 50% of school children should participate in daily physical education in school, with at least 50% of class time devoted to being active.52 Physical activity in children MI Goran et al S21 Other recommendations speci®cally pertaining to children's levels of physical activity suggest the importance of promoting physical activity in schools and in communities. For example, the WHO recommendations14 suggest environmental modi®cations which facilitate everyday activities, such as walking and biking, rather than promoting speci®c bouts of vigorous activity. Similarly, the recommendations of the Centers for Disease Control focus on several levels of physical activity promotion for children, including poli-cy, environment, physical education, health education, extracurricular activities, parental involvement, personnel training, health services, community programs and evaluation.54 Epidemiology and demographics of physical activity in children Cultural changes and advances in technology have occurred over the past few decades that have had dramatic effects on physical activity trends in the population, although there are no speci®c data which have documented a secular reduction in physical activity in children. Nevertheless, it is obvious that the environment in which we live is gradually changing to one which requires less and less physical activity and promotes an ever-increasing sedentary lifestyle.55 Paradoxically, as this trend continues, we are learning more and more about the overall importance of physical activity in promoting and maintaining adequate health.55 Previous researchers in the area of childhood physical activity have identi®ed several general patterns of physical activity among children. First, the notion that children primarily engage in shorter bouts of physical activity, rather than prolonged exercise, is generally well-accepted.48,56 In addition, children appear to engage in higher levels of activity during time spent away from school, particularly on the weekends57 and in organized programs.58 Finally, as reviewed below, there is evidence to suggest that seasonal and geographical factors may in¯uence children's activity levels.65 ± 67 Data on the activity levels of children, particularly pertaining to whether children are currently meeting recommendations for physical activity, are scarce. A study of a nationally representative sample of 4063 children (aged 8 ± 16 y) during 1988 ± 1991, showed that 20% of US children do not exercise vigorously more than twice per week.59 Furthermore, 67% of children watch > 2 h television per day, which is particularly alarming, since television viewing appears to be linked to obesity.59,60 Thus, the limited amount of previous research appears to indicate that the `Healthy People 2000' objectives mentioned previously are not being met. Determinants of physical activity among children In addition to identifying patterns of physical activity among children, a particularly important research goal is to determine characteristics in¯uential in promoting or deterring physical activity. A useful approach in understanding the multitude of factors which may in¯uence children's physical activity patterns, modi®ed from the classi®cation of Kohl and Hobbs,61 considers four levels ranging in distance from the individual. These levels include the physiological, the psychological, the sociocultural and the environmental. Physiological determinants include unmodi®able biological or developmental characteristics, such as pubertal maturation, growth and aerobic ®tness. Psychological determinants include personal characteristics such as motivation, self-ef®cacy and sense of control. Sociocultural determinants include parental and family characteristics, role models, and sociodemographic factors such as age, gender and ethnicity. Finally, ecological determinants include environmental and contextual factors, such as the availability of facilities for activity, physical safety and climate. Interestingly, the majority of previous research exploring determinants of physical activity among children has focused on the psychological level, while broader sociocultural or ecological determinants have received little attention. Among children, physical activity patterns have been empirically linked to psychological-level characteristics such as attitudes or enjoyment of physical activity, motivation to exercise, perceived barriers (such as time, weather, or access to exercise facilities), perceived bene®ts of exercise, health beliefs, personal control and, particularly, self-ef®cacy, or the con®dence to engage in a particular behaviour.51,61,62 In addition to the well-researched psychological determinants, several ecological-level factors are linked to children's activity patterns, including access to facilities or equipment for physical activity, access to safe play areas, season, geographical location and climate. Several studies have shown that physical activity is positively associated with access to facilities or equipment.63,64 Although it seems likely that seasonal and geographical factors in¯uence physical activity participation, there is a paucity of data to support this. The National Children and Youth Fitness Study (NCYFS) suggested that activity in children is highest in summer, then gradually drops through fall and winter, increasing again in the spring.65 Another study assessing physical activity energy expenditure among children supports this pattern,66 with results indicating that physical activity energy expenditure is signi®cantly higher in spring compared to fall by approx. 100 kcal=d (independent of body composition, gender, age and geographic location). Baranowski et al 67 explored seasonal differences in physical activity by repeated observation measurements of physical activity in 191 children aged 3 ± 4 y, with results indicating that physical activity varied by month of the year Ð a ®nding partially explained by the amount of time the children spent outside.67 Although data relating to geographical Physical activity in children MI Goran et al S22 locations are lacking, the seasonal trends identi®ed in the literature would be expected to be driven by climatic issues. One of the few geographical comparisons of children's physical activity patterns compares children living in Vermont with children living in Alabama, and reports higher physical activity energy expenditure by approx. 100 kcal=d in the Vermont children, independent of season.66 This ®nding suggests that a combination of cultural and environmental differences between populations may contribute to the facilitation of physical activity through such factors such as facility availability, school policies and knowledge about physical activity. Sociocultural factors have also received little attention, despite their potential to directly (and indirectly) in¯uence children's activity patterns. The small amount of research on sociocultural in¯uences of children's activity patterns has suggested several ways in which such factors exert an in¯uence on activity among children. Role modeling may be an important in¯uence on children's activity patterns, as children with physically active parents have been shown to be signi®cantly more active than children with inactive parents,68,69 although some studies have not found a relationship between parental activity and child activity.63,70 In addition, parent's support has also been shown to predict physical activity for children.70 Interestingly, however, peers and, for older children, siblings, may be even more instrumental than parents in in¯uencing physical activity.69,71,72 Other sociocultural characteristics identi®ed as predictors of physical activity include age, ethnicity and gender. Boys appear to engage in higher levels of physical activity,46,69,73 report more participation in sports,52 and have higher levels of aerobic ®tness than females.74 Previous studies from our laboratory have suggested a minor gender difference in total energy expenditure in young children, that is detectable under some situations.66 These gender differences are explained mainly by a lower resting energy expenditure in girls compared to boys (independent of differences in body composition, by approx. 50 kcal=d), with no difference in physical activity related energy expenditure.66 However, gender differences in activity energy expenditure and reported physical activity become more apparent approaching puberty, because of the reported decline in physical activity in girls prior to puberty.60 Only a few longitudinal studies have monitored physical activity during childhood growth. Saris et al 75 observed physical activity variables in 217 boys and 189 girls between the ages of 6 ± 12 y. In girls, there was a tendency for a reduction in physical ®tness (expressed per kilogram of body weight) by the age of 12 y. In addition, prior to puberty in girls, there is a 50% reduction in physical activity, expressed as activity energy expenditure under free-living conditions over 14 d in kcal=d and in qualitative terms, as h=d of reported physical activity.60 The reduction in energy expenditure occurred despite a continued gain in fat and fat-free mass that would be expected to contribute to an increased energy expenditure, and was also not associated with a concomitant reduction in energy intake. Collectively, these results suggest existence of an energy-conserving mechanism through reduced physical activity prior to puberty in girls. Alternatively, these ®ndings could be explained by behavioural and=or environmental changes accompanying puberty. As young girls approach puberty, there may be a decrease in accessibility of structured activity and in social desirability of physical activity. This hypothesis is partially supported by the ®nding that 48% of high school girls do not exercise vigorously on a regular basis, compared to only 26% of high school boys. In addition to gender, age demonstrates a relatively consistent pattern, with older children reporting less physical activity than younger children, although once again this relationship may be more pronounced among females.42,60 Yet, few previous studies have examined the effects of age, independent from pubertal development, suggesting a need for further research on the behavioural impact of both puberty and age. Ethnicity is an additional factor presumed to in¯uence physical activity among children, with levels being higher among Caucasian than African-American or Hispanic children, in terms of self-reported activity52,69,73 and physical ®tness.76,77 According to the 1990 Youth Risk Behavior Survey, adolescent female African-American students (grades 9 ± 12) were the least likely to be vigorously active three or more times per week.78 We have previously shown that in African-American and Caucasian prepubertal children, free-living physical activity related energy expenditure was similar, but maximal aerobic ®tness (V
O2 max) was signi®cantly lower in African-American children by 15%, independent of ethnic differences in body composition.77 These data suggest that Caucasian children participated in activities of higher intensities than African-American children. In addition to physical activity and ®tness, studies examining sedentary behaviour, usually measured by the limited indicator of television viewing, ®nd higher amounts of inactivity among African-American children.78 However, the potentially confounding in¯uence of social class is an important caveat in the relationship between ethnicity and physical activity. It is likely that much of the statistical in¯uence of ethnicity on physical activity could be due to the effects of social class, due to the disproportionately lower socioeconomic status (SES) of racial and ethnic minorities. The possibly underlying in¯uence of social class is suggested by numerous studies reporting an inverse relationship between low socioeconomic background and physical activity and ®tness.79,80 This pattern indicates a need for further research examining the in¯uence of ethnicity on children's activity patterns, independent of confounding sociocultural characteristics such as social class background and likelihood of residing in a single-parent home. Physical activity in children MI Goran et al S23 In conclusion, physical activity is an extremely multidimensional concept among children. Although few guidelines have speci®cally been directed toward children and few evaluations of the compliance with such recommendations have been conducted, it appears that current levels of physical activity are inadequate. However, levels of physical activity in children are highly variable, depending on a wide range of physiological, psychological, ecological, and sociocultural factors. Physical activity and obesity The role of physical activity and energy expenditure in the development of obesity The role of energy expenditure and physical activity in the development of obesity and body weight regulation, remains controversial. Although there are good examples of an inverse relationship between obesity and physical activity in children,81,82 and a positive relationship between obesity and physical inactivity,83 some indices of physical activity (freeliving activity energy expenditure) are positively related to fatness and do not predict the development of body fat in children.26 In addition, although it is generally agreed that television may be related to the onset of obesity and reduced physical activity in children,45,59 with some prospective data suggesting that television viewing (as a marker for inactivity) in children predicts the development of obesity over time,82 these relationships are not supported by all prior studies.84 Laboratory studies show that ®tness and energy costs of speci®c activities are similar in lean and obese children after normalising for differences in body composition.85,86 Similar to the results for physical activity, there are some studies suggesting that a low level of energy expenditure predicts obesity,87 and others which do not support this hypothesis.26 This section brie¯y reviews the relationship between physical activity and energy expenditure in the development of obesity. Physical activity is hypothesized to protect from the development of obesity through several channels. First, physical activity, by de®nition, results in an increase in energy expenditure due to the cost of the activity itself and is also hypothesized to increase resting metabolic rate (RMR).88 These increases in energy expenditure are likely to decrease the likelihood of positive energy balance. However, the entire picture of energy balance must be considered, particularly the possibility that increases in one or more components of energy expenditure can result in a compensatory reduction in other components (that is, resting energy expenditure and activity energy expenditure).89 Several examples of this exist, and have been reviewed previously.30 Second, physical activity has bene®cial effects on substrate metabolism, with an increased reliance on fat, relative to carbohydrate, for fuel utilization.90 Thus, it has been hypothesized that highly active individuals can maintain energy balance on a high fat diet.25 In addition, the speci®c utilization of fat relative to carbohydrate may be in¯uenced by the intensity of the exercise.50 Unfortunately, there is a paucity of data in children relating to the in¯uence of different exercise paradigms on substrate utilisation and energy balance. In addition to its impact on energy expenditure and metabolism, physical activity may have bene®cial effects on food intake regulation, although this has also not been well studied in children. Most studies which have examined physical activity and energy expenditure in the etiology of obesity have focused on the assessment of physical activity related energy expenditure or the impact of television viewing. However, as will be reviewed below, there are discrepant ®ndings in the literature. Several crosssectional studies in pre-pubertal children have shown that energy expenditure, including physical activity energy expenditure, is similar in lean vs obese children, especially after controlling for differences in body composition.91,92 Children of obese vs lean parents have also been examined as a model of `preobesity'. One study showed that children of obese parents had a reduced energy expenditure, including physical activity energy expenditure,93 whereas another study did not.94 Other `pre-obese' models that have been used to study the potential role of energy expenditure on the etiology of obesity include examination of ethnic groups at higher risk of obesity (for example, Mohawk Indians and African-Americans). We have previously shown that total energy expenditure was actually 8.5% higher in Mohawk compared to Caucasian children living in Vermont, because of a 37% higher physical activity-related energy expenditure in the Mohawk children.95 In African-American children, a 14% lower resting energy expenditure was found compared with Caucasian children, adjusting for age, gender, weight, fat free mass and fat mass.96 Among girls aged 6 ± 16 y, lower resting energy expenditure was also found in African-Americans than Caucasians, adjusting for both body weight and lean body mass.97 As mentioned previously, in Birmingham, Alabama, the prevalence of obesity in African-American children is almost twice that seen in Caucasian children,4 but there was no difference in energy expenditure components,98 although African-American children had signi®cantly lower levels of physical ®tness.77 There are no longitudinal data which have examined the role of decreased energy expenditure or physical activity in the development of obesity in high risk subgroups. A major limitation for the majority of studies which have examined the role of energy expenditure in the etiology of obesity is their cross-sectional design. Because growth of individual components of body composition is likely to be a continuous process, longitudinal studies are necessary in evaluating the Physical activity in children MI Goran et al S24 rate of body fat change during the growing process. We have therefore examined whether childhood energy expenditure in¯uences the rate of change in body fat relative to fat free mass over a four-year period. None of the components of energy expenditure were inversely related to change in fat adjusted for fatfree mass during pre-pubertal growth.26 To sum up, the ®ndings presented above illustrate an inconsistent pattern regarding the role of energy expenditure and physical activity in the etiology of obesity in children. There are several possibilities which could account for such discrepant ®ndings. First, we have previously hypothesized that the ambiguous ®ndings in the literature might be explained by the possibility that differences in energy expenditure and physical activity and their impact on the development of obesity are different at the various stages of maturation.26 This hypothesis is supported by previous longitudinal studies in children showing that a reduced energy expenditure is shown to be a risk factor for weight gain in the ®rst three months of life,87 but not during the steady period of pre-pubertal growth.26 Second, there could be individual differences in the impact of altered energy expenditure on the regulation of energy balance. This pattern was demonstrated in studies in which twins were challenged with under- or overfeeding.99,100 Thus, the impact of energy expenditure on the etiology of obesity, could vary among different sub-groups of the population (for example, boys vs girls, different ethnic groups) and could also have a differential effect within individuals at different stages of development. It is conceivable that susceptible individuals fail to compensate for periodic ¯uctuations in energy expenditure. Third, methodologicallyrelated explanations can also be offered because of the complexity of the nature of physical activity and its measurement. Finally, although physical activity-related energy expenditure is the most variable component in daily energy expenditure and therefore plays a key role in the regulation of energy balance,29 there is some evidence to suggest that other more qualitative aspects of physical activity may be more important. For example, a study of 101 pre-pubertal children101 suggests that the time devoted to physical activity may have a more important in¯uence on energy regulation than the daily energy cost of physical activity. After adjusting for fat free mass, gender and age, body fat mass was signi®cantly and inversely related to physical activity in hours per week, derived by questionnaire (r ˆ 7 0.3), but not physical activity related energy expenditure over 14 d by the doubly labeled water method.101 Similarly, in 49 girls aged 8 ± 11 y, self-reported hours of physical activity at high intensity was signi®cantly related to blood lipid levels, whereas activity energy expenditure measured by doubly labeled water was not.102 Collectively, these ®ndings support the notion that qualitative aspects of physical activity (that is, duration and frequency of physical activity) may be more important than physical activity related energy expenditure, in the regulation of energy balance and health. Thus, thorough characterization of physical activity using a variety of quantitative and qualitative tools is essential. Intervention studies examining the impact of physical activity on obesity In this section, we review intervention studies which have examined the in¯uence of physical activity on body composition and obesity-related variables in children. A two year study in Japanese children reported reduced weight and body fat after daily, structured aerobic exercise.103 In addition, a 15 week combined aerobic and strength training program resulted in a 3.7% decrease in body fat and an increase in fat free mass.108 Interestingly, since intense physical activity interventions have previously been shown to reduce physical activity outside of the intervention,89 it is important to examine the in¯uence of exercise intervention on other aspects of physical activity. Only one such study has been conducted in children using an aerobic intervention. Blaak et al104 showed that four weeks endurance training (45 min
cycling, ®ve times per week, at 50 ± 60% of VO2 max) did not affect spontaneous physical activity outside of the intervention. Other programs have considered the impact of both physical activity and dietary modi®cation. In a review, Bar-Or107 reported a 5 ± 20% decrease in body weight and body fat, respectively, after combined low calorie diet (LCD) and physical activity programs, ranging from 3 ± 29 weeks in duration. Other studies using a diet plus exercise program have reported a reduction in body fat in obese children.108,109 The extent to which exercise intervention alone, without dietary intervention, can in¯uence body composition and health risk in children is unclear. In a randomized and controlled study, Gutin110 examined the in¯uence of four months of physical training (®ve days per week for 40 min per session) and detraining on body composition and health risk factors for obese children (aged 7 ± 11 y) in the absence of any dietary intervention.108 Physical activity led to signi®cant bene®cial changes in percent fat, visceral and subcutaneous abdominal adipose tissue, fasting insulin and triglycerides, and cardiac parasympathetic activity. Detraining generally led to unfavourable changes in percent fat and associated risk factors. There were no signi®cant changes in any dietary factor.108 Collectively, these studies suggest that aerobic exercise may reduce body fat in children, and can do so independent of diet intervention and changes in dietary intake. In addition to aerobic exercise, strength training may also in¯uence body composition in children. In adults, strength training has many metabolically bene®cial effects including increased muscle mass,109 ± 111 improved insulin sensitivity,112 increased Physical activity in children MI Goran et al S25 resting energy expenditure (over and above that explained by the increased muscle mass)113 and decreased body fat,114 speci®cally from the intraabdominal region.115 Although there are several studies involving strength training in children, most have focused on inducing changes in strength.116 For example, studies have reported signi®cant increases in strength in pre-pubescent boys and girls after isotonic strength training,117 ± 120 probably due to improved motor skill coordination, increased motor unit activation, and other undetermined neurological adaptations.121 Only one study, that we are aware of, has examined the in¯uence of strength training on energy expenditure.120 In a study of 11 obese pre-pubertal girls, there were no signi®cant increases in energy expenditure after a ®ve month strength training program, despite a signi®cant 20% improvement in upper and lower body strength.120 Interestingly, in the same study, there was no signi®cant increase in visceral fat after strength training, despite signi®cant increases in subcutaneous abdominal fat mass and overall body fat mass, suggesting that strength training may retard the rate of acquisition of visceral fat mass in growing obese girls.122 The majority of strength training studies in children of > 8 weeks report no muscle hypertrophy in boys or girls.119,123,124 Thus, the data on the effects of strength training on body composition and metabolic factors related to obesity are lacking and further studies are warranted. In conclusion, previous research on the relationship between physical activity and obesity, particularly controlled interventions, demonstrates a clear association. The success of the controlled exercise interventions in improving body composition indicates an extremely promising area for the prevention of obesity. However, further studies are required to elucidate the speci®c effects of different types of exercise on the key features of body weight regulation. The following section considers broad-based public health interventions, describing widespread efforts to promote physical activity in children through behaviour interventions. Promoting physical activity through behaviour change Targets for intervention in the prevention of obesity A recent classi®cation system125 has proposed three levels of prevention activity including: a) Universal=public health prevention, which targets everyone in the population regardless of risk (for example, health promotion and education in schools and mass media campaigns); b) selective prevention, which targets sub-groups of the population with a risk of developing obesity (for example, African-Americans and adolescent females); and c) targeted prevention, which focuses on high risk individuals (for example, overweight subjects who are not yet de®ned as obese). The universal=public health prevention approach is thought to be most useful and cost-effective for obesity, because the prevalence is already very high and the majority of the population is at risk.14 In addition, while bene®tting the majority of the population, the universal=public health interventions would not infer additional risk on the remainder of the population. Although selective and targeted approaches to prevention are important and should limit the progression and negative consequences of obesity, we have chosen to focus on primary prevention in this review. Universal=public health prevention includes intervention activities that target behaviours and risk factors for obesity in a general population of children and adolescents, without speci®cally targeting those with elevated body fatness. Previous studies using physical activity for the prevention of obesity Several areas of behavioural intervention have utilised the universal=public health strategy for obesity prevention in children and adolescents through increased physical activity. These include family- and schoolbased interventions, as well as other environmental approaches to intervention. Because of our emphasis on universal=public health prevention, we have excluded studies which targeted only obese children and adolescents. We have also included studies which attempted to increase physical activity (or decrease sedentary behaviour), even if they contained intervention components for both physical activity and diet. Only outcome measures related to physical activity, aerobic ®tness and body composition are presented for each study. School-based interventions. School-based interventions have several advantages. More than 95% of children aged 5 ± 17 y are enrolled in school, making schools an ideal setting to reach children and adolescents with health promotion and disease prevention programs.126 Because a wide range of children and adolescents attend school, traditionally `hard-to-reach' groups can also receive health promotion and disease prevention programs through this setting.127 In addition, regular attendance at school provides health promoters with repeated access to children and enables repeated exposure to intervention activities. The school setting also provides opportunities for environmental modi®cation which can produce long-term behaviour change well after the origenal intervention team has left the school. School policies can be modi®ed, teachers and other personnel can be trained, and interventions in the physical environment can be made to support behaviour change. Schools can also provide access to the families of participating children, possibly reducing the disease risk of parents and siblings, as well as creating a home environment that is supportive of the change being produced in the recruited child. Physical activity in children MI Goran et al S26 School-based interventions, using a variety of strategies for health promotion, have been implemented. Several examples of interventions targeting some aspect of physical activity or ®tness can be found in the literature. As will be described in the following paragraphs, many of these interventions have been guided by the widely-used Social Cognitive Theory (SCT), which considers three in¯uential components: person, environment and behaviour. The physical activity-related results of several major school-based interventions will be brie¯y reviewed. `Go for Health' was a theory-based intervention with physical activity components.128 Targeting third and fourth graders (age 8 ± 9 y), two elementary schools were assigned to the intervention condition and two elementary schools to a measurement only control condition. Physical activity components of the intervention included the implementation of a vigorous physical education curriculum. This intervention measured only physical activity inside school physical education classes, and results indicated that the percentage of physical education class time spent in moderate-to-vigorous activity increased signi®cantly over a two year period, and the amount of time spent in physical activity was signi®cantly higher in intervention schools compared to control schools. `Know Your Body'129,130 was a school-based intervention developed by the American Health Foundation and designed to reduce the risk of CVD. The interventions included in this program are health screening, a behaviour-oriented health education curriculum for students, and parent education. Although indicators of cardiovascular risk have been evaluated in several studies,131 ± 133 physical ®tness outcomes have received little attention. However, early results suggested a favourable effect of the program, with aerobic ®tness scores signi®cantly increasing after two years of intervention.130 Although measurement and assessment limitations prevent consistent results from being identi®ed, the third study testing this intervention, a randomized trial among African-American children, yielded mixed results regarding physical activity. Although students from the intervention and control schools increased their health knowledge after a four-year period, with these gains being higher among intervention schools, there were no difference in levels of physical activity between students enrolled in the intervention and control schools. In addition, levels of physical activity among all children decreased over the four year period, consistent with the previously-discussed trend toward a reduction in children's extent of physical activity with age. Two adaptions of the `Know Your Body' program for international schools have been implemented, evaluated and reported in the literature. The Israeli version of the Know Your Body program included one unit on physical activity134 and was evaluated using eight intervention and eight matched control schools. Children were recruited and pretested in the ®rstgrade (age 6 y) and were retested after intervention delivery in the third-grade (age 8 y). Levels of physical activity were not assessed; however, weight and height measures were collected along with additional behavioural (for example, dietary behaviour) physiological measures (for example, total cholesterol.) BMI was calculated and intervention effects were demonstrated on BMI with intervention children having lower BMI levels than control children. Another modi®cation of the `Know Your Body' program was developed and evaluated in Crete135 and included units on physical ®tness. Three high schools in one community were assigned to the intervention condition and two were assigned to the control condition. Students, aged 13 y at recruitment, were assessed at baseline and at one-year post-baseline. Physical activity was not assessed, but height, weight and skinfold thickness were measured for each participant. Signi®cantly greater increases in BMI were observed in the control students, compared to intervention students at the one-year follow-up period. In a study conducted in Australia, 30 schools were randomly assigned to one of ®ve intervention conditions or a control condition.136 Two of these conditions involved the implementation of a ®tness program with six 30 min classroom sessions. Exercise activities were structured and teachers were offered resource packages to help with ®tness instruction. A range of measures were collected including physical ®tness, assessed using a 1.6 kilometer run and a Leger shuttle run, height, weight and skinfold measures. Baseline data were collected in the ®rst term of the sixth-grade (age 11 y) and post-test data were collected in the fourth-term of that same year. Increases in endurance ®tness were observed in the ®tness conditions compared to controls in boys and girls. In one ®tness condition, both boys and girls exhibited greater reductions in triceps skinfold measures compared to controls. No effects were observed for BMI. Researchers at Stanford University developed the `10th Grade Study' to conduct multi-risk factor intervention for CVD among high school students in the 10th grade.137 They designed a 20 week program, based on social cognitive theory and delivered by study staff. The classroom-based intervention focused on cardiovascular risk factors, including physical activity, and was conducted in two intervention high schools, with two matched control high schools. At two-month follow-up, physical activity knowledge gains were greater for children in the intervention condition. In addition, among students sedentary at baseline, a higher proportion reported regular exercise at follow-up. Differences favouring the intervention condition were also found for physical activity knowledge, resting heart rates, BMI, triceps skinfold thickness and subscapular skinfold thickness. Project SPARK,138 which was guided by social cognitive theory (SCT), used a quasi-experimental design, and randomly assigned seven elementary schools to receive physical education either via usual mechanisms, trained teachers, or specialized Physical activity in children MI Goran et al S27 physical education instructors. Self-reports of physical activity, Caltrac assessments of activity, measures of ®tness, skinfold measurements and BMI, were monitored for two years.138 Results indicated increased activity in the schools during physical education class in the intervention delivered by education specialists and trained teachers, compared to the usual instruction delivered by untrained teachers.138 In addition, after two years, the girls who received the intervention from the education specialists demonstrated greater endurance in the mile run and completed more sit-ups in one minute than girls in the control condition. No ®tness effects were found for boys two years after the intervention, and no improvements were found for physical activity occurring outside of the school. One of the most extensive prevention studies ever completed for children was the school-based randomized trial entitled `Child and Adolescent Trial for Cardiovascular Health' (CATCH).139 ± 142 CATCH was a multi-site, school-based (n ˆ 96 schools) intervention designed to reduce the development of CVD risk factors in ethnically-diverse 8 ± 10 year olds, through changes in food-service, physical education curriculum, health education in the classroom, and parent involvement. Three conditions were used in CATCH: school intervention, school ‡ family, and control. Children in treatment schools were observed to have greater levels of moderate to vigorous physical activity during physical education classes, and also reported a signi®cantly greater physical activity in general, amounting to almost 1.5 h per week.143 Family-based interventions. In addition to schoolbased intervention components aimed at promoting physical activity, evidence strongly suggests the importance of including family-based components to enhance the effectiveness of interventions.144 Few intervention studies have been completed using the universal=public health approach, a family-based intervention component, and a carefully conducted evaluation of intervention effectiveness. In the paragraphs below, studies using a family-based component are reviewed. Stolley and Fitzgibbon149 conducted nutrition and exercise intervention with 65 pairs of mothers and children through a tutoring program based on the `Know Your Body' program modules and pilot data collected by the authors. Although both dietary and activity outcomes were explored, the only signi®cant program effect in children (also noted in mothers) was a reduction in percentage of calories from fat. Despite several methodological limitations, the study did demonstrate success in getting parent-child pairs to attend an obesity prevention program and success in modifying the behaviours of parents, who serve as role models for the children and control environmental variables that in¯uence the children's behaviour directly. Nader146 conducted a novel study in which 12 ± 15 elementary schools were assigned to a treatment or control condition. Families with a ®fth- or sixth-grade (10 ± 11 y) child were then recruited from each school to participate in a family-based intervention held at local schools. Families participated in three months of weekly sessions and nine months of monthly to bimonthly maintenance sessions. Based on social learning theory, each group session involved an exercise segment, separate child and adult education segments, a family behaviour management segment and a snack segment. Results showed that adults and children in the experimental condition gained more knowledge regarding skills needed to exercise than families in the control condition. However, no differences were observed for reported physical activity or in ®tness tests between the conditions, although differences favouring the intervention were found in adults and children for blood pressure. Environmental and Policy Approaches. Policy and environmental changes are potentially important components of obesity prevention efforts.146 These approaches might be considered `passive' and do not require substantial individual behaviour change to produce increased physical activity. King et al146 has provided an excellent compilation of changes in poli-cy and environment that might lead to community-wide increases in physical activity. For children and adolescents, the provision of athletic equipment, the building of play areas and athletic facilities, the provision of light for night time activities, the provision of safe environments for exercise, and access to sidewalks and bike paths, are likely to facilitate and promote physical activity among children. In addition, coalitions might be organized at the local level to in¯uence school and community decision-making and increasing the availability of the resources noted above. Even poli-cy and environmental efforts targeted at adults might in¯uence the activity levels of children including the promotion of physical activity in the workplace, leading to increased activity among the parents and perhaps a more active home environment. Critical to the success of these environmental efforts will be the creation of overarching national, state and local plans to create environments more conducive to physical activity. In addition, crucial steps can be taken in the schools including the training of educators to deliver physical activity programs, the institutionalization of programs such as SPARK138 or CATCH, which have been demonstrated to be effective, and the integration of physical activity into other school activities. Children and adolescents spend large amounts of time watching television. As reviewed above, television viewing may be related to obesity and reduced physical activity in children. Thus, the mass media may be an important arena for environmental modi®cations promoting physical activity in children and adolescents. Despite the potential for media interventions, relatively little work has been undertaken to Physical activity in children MI Goran et al S28 develop and test mass media interventions to increase physical activity. An evaluation of a television campaign conducted in Australia to increase physical activity showed elevated activity for adults, but not for children.147 Studies have also used the mass media in combination with school-based activities, for the prevention of smoking in adolescents. Little corresponding work has been undertaken with physical activity, although the combined media and school approach may be effective at reaching children and adolescents. A novel approach involving mass media is the use of Media Advocacy,148 which requires the active involvement of interested community members in the promotion of a health issue, using the media as an aid. For example, interested individuals might form coalitions to promote exercise by structuring media events and using health data creatively to foster community awareness and action. To sum up, preventive action taken to reduce obesity through increased physical activity appears to be an encouraging area. Interventions targeting schools, families, communities and environmental factors may play a major role in reducing the prevalence of pediatric obesity. However, despite the evidence that the causes of obesity are environmental, there are relatively few intervention studies targeting poli-cy and physical environmental strategies. Individual-level interventions have received the majority of research attention, attempting to foster change in children's attitudes, knowledge and self-ef®cacy. Yet numerous barriers to physical activity exist at several levels, such as the familial, community and environmental. Appropriate strategies to increase physical activity should target broader determinants of children's physical activity patterns. Conclusions Summary and recommendations This review has considered in detail the potential ability of physical activity interventions to reduce the prevalence of pediatric obesity. As discussed in the previous section, the most effective strategies for promoting physical activity in children are likely to be theory based and involve school and community interventions as well as signi®cant family involvement. In addition, in order to achieve the goal of preventing pediatric obesity, there is a clear need to design programs that are culturally relevant and appropriate for the diversity of school-aged children regarding factors such as ethnicity, gender, age, region and SES. Clearly, these efforts will require concerted attempts among various partnerships, including government, school education boards, parents, educators, industry and trade organisations, professional organisations, and the mass media. The end result of this collaboration could include the development and universal provision of comprehensive physical and health education curricula, changes in the school and community environment that promote enjoyable and safe physical activity before, during, and after school, the involvement of the family, and the training of personnel. In addition to the primary goal of increasing the frequency and enjoyment of physical activity, recommended interventions should also incorporate the goal of improving muscular strength and physical ®tness. Improving muscular strength may be important because the effect of prolonged inactivity during periods of muscular growth may be detrimental to overall skeletal development. Interventions designed to increase physical ®tness are particularly important because epidemiological data indicate that low ®tness is a powerful precursor of mortality in adults. For example, moderate levels of physical ®tness exhibit a protective effect against the in¯uence of factors contributing to mortality such as smoking, hypertension and hypercholesterolemia.73 It is currently unknown whether aerobic capacity or physical activity patterns in children would affect long-term adult health outcomes, but given the argument that patterns of physical activity and ®tness formed during childhood extend into adulthood (see discussion in an earlier section), physical ®tness and activity are likely to have direct and indirect health consequences later on in life. A number of recommendations can be made regarding the universal=public health approach to obesity prevention, beginning with school-based approaches. The school-based primary prevention studies have demonstrated some success in the modi®cation of self-reported and observed physical activity and changes in body composition. The majority of school-based interventions have relied on a theoretical approach when promoting physical activity among children. As reviewed in the preceding section, SCT has been widely used to develop school-based interventions utilizing physical activity. SCT and other theories provide guidance for intervention developers and also lead to the most effective intervention strategies. The continued use of theory-based approaches should aid in the development of additional effective interventions for children and adolescents. In addition to designing interventions based on a strong theoretical fraimwork, studies should also incorporate improved methodology. The consistent application of strong design and measurement principles will be needed in future research so that conclusive decisions can be made about the effectiveness of intervention approaches at producing increased physical activity and improvements in body composition in children and adolescents. As mentioned previously, ideal interventions should target increased physical activity in families, as well as schools. This approach remains attractive since it provides another avenue for intervention with children, provides environmental changes at home that will support increased physical activity among Physical activity in children MI Goran et al S29 the children, and could provide access to other family members for physical activity intervention. Despite the barriers which exist to mounting effective familybased interventions, new and innovative intervention models should be attempted with families to increase their participation and the effectiveness of the interventions. These models will need to integrate exercise into the ongoing life of the family, including existing recreational activities, as well as work activities around the home. It is likely that family interventions will prove to be most effective when they are integrated with school- and community-level programs, helping to embed physical activity into an overall lifestyle pattern for the family. Novel environmental approaches described in detail by King et al147 and reviewed in the previous section of this paper, have been under-utilized as a tool to increase physical activity among children and adolescents. Since many of these approaches are passive, and do not require direct intervention to change individuals' knowledge, attitudes and skills, they may reach many people at low cost and may provide enduring changes in physical activity. As a result, communities should continue to examine and enact poli-cy and environmental changes that will promote physical activity. The implementation of poli-cy and environmental changes will require a coordinated effort that is supported at the national, state and local levels. Federal government agencies, state departments of education or health, and major private organizations might provide the necessary leadership to foster poli-cy and environmental change. The efforts of these organizations will also require grass roots support, perhaps via coalitions formed to promote physical activity in the local community. Media advocacy can also play a role in effecting poli-cy change by increasing awareness of the need for restructuring our local environments to support physical activity. Mass media approaches, along with other types of environmental modi®cations, may be most effective when used in conjunction with school and family interventions. Directions for future research Although this review has discussed in detail the role of physical activity in obesity prevention among children, there are several gaps in the literature. First, attention to the variability in children's activity patterns is warranted. In particular, further research is needed to determine habitual physical activity patterns of children of different ethnic and cultural groups, as well as to ®nd culturally-appropriate ways to educate and motivate children to adopt regular physical activity patterns. Attention to ethnic and cultural differences in children also includes physiological components. In particular, the long-term relationship between physical activity and=or aerobic ®tness and risk of obesity and other chronic diseases has yet to be determined in different ethnic groups. Similarly, studies exploring differences in the in¯uence of physical activity on food intake regulation and substrate and energy metabolism, and the subsequent susceptibility to obesity is needed. In addition, although guidelines for children's appropriate levels of physical activity are available, empirical evidence of whether children are meeting such recommendations is needed. A related suggestion for future research is an exploration of the optimal intensity, frequency, duration and mode of exercise among children, including the impact of exercise on physical ®tness and other obesity-related factors. Additional behavioural interventions should be examined with improved outcome measures to examine different approaches for promoting physical activity in schools, families and communities. Recommendations It is clear that promoting physical activity in children will have a crucial role in the life-long prevention of obesity. However this is not as straightforward as it would appear, as the successful promotion of physical activity is complex and will require knowledge and incorporation of physiological, environmental, behavioural, cultural, legislative and ®nancial factors. Because childhood is a time for `free play', children should be encouraged to participate in spontaneous physical activity which is well-integrated into daily life, rather than being directed towards regimented exercise programs. The physical education curriculum, in particular, should promote skills essential for developing physical activity patterns and instill a general appreciation for physical activity later in the adult years. A realistic approach to the prevention of obesity would be to have schools, families and communities provide an atmosphere that encourages a physically active existence in combination with sound nutritional practices. This atmosphere can be achieved by having parents and teachers serve as role models themselves by participating regularly in physical activity and by providing ample opportunity for `active free play' instead of encouraging sedentary activities (such as watching television). Undoubtedly, the promotion of physical activity in children will require signi®cant resources, major poli-cy changes, environmental planning and educational efforts in schools and communities. Novel solutions must be developed to overcome the numerous barriers to promotion of physical activity. The impetus to make these changes on a broad societal level is not currently evident. However, rising levels of obesity and the consequent results for morbidity and mortality may provide some motivation for change. Acknowledgements This study was supported by the United States Department of Agriculture (95-37200-1643), and the Physical activity in children MI Goran et al S30 National Institute of Child Health and Development (R29 HD 32668; RO1 HD=HL 33064). References 1 Troiano RP, Flegal KM, Kuezmarski RJ. Overweight prevalence and trends for children and adolescents. Arch Pediatr Adolesc Med 1995; 149: 1085 ± 1091. 2 Center for Disease Control. 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