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body composition Body composition is the component of health-related 
fitness that relates to the relative amount of adipose tissue, muscle, bone, 
and other vital components (e.g., organs, connective tissues, fluid compart-

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Copyright © National Academy of Sciences. All rights reserved.
Educating the Student Body: Taking Physical Activity and Physical Education to School
Relationship to Growth, Development, and Health
 
123
ments) that make up body weight. Most feasible methods for assessing 
body composition are based on models that divide the body into fat and fat-
free (all nonfat constituents) components (Going et al., 2012). Although fat 
mass and adipose tissue are not equivalent components, fat mass is easier 
to estimate than adipose tissue, and it is correlated with performance and 
disease risk. In settings in which estimation of body fat is difficult, weight-
for-height ratios often are used as surrogates for body composition. Indeed, 
definitions of pediatric overweight and obesity have been based on BMI, 
calculated as weight in kilograms divided by height squared. Child and 
adolescent obesity defined by BMI remains at all-time highs. Population 
surveys indicate that approximately 33 percent of all boys and girls are 
overweight, and nearly one in five are obese (Ogden and Flegal, 2011). 
The tendency for excess fatness to persist from childhood and adolescence 
into adulthood (Daniels et al., 2005), coupled with the strong association 
between obesity and chronic disease (Weiss and Caprio, 2005; Barlow, 
2007), has caused great concern for future obesity levels and the health of 
youth and adults alike (IOM, 2005, 2012b). 
The increase in prevalence of obesity is undoubtedly due to a mismatch 
between energy intake and expenditure. Population surveys have shown 
that few children and youth meet recommended levels of daily physical 
activity (see Chapter 2). Prospective studies have shown a significant and 
inverse relationship between habitual physical activity and weight gain 
(Berkey et al., 2003), and in some studies physical activity is a better pre-
dictor of weight gain than estimates of calorie or fat intake (Berkey et al., 
2000; Janssen et al., 2005). These relationships are better established in 
adults than in children and youth, although even in preschool children, low 
levels of physical activity, estimated from doubly labeled water, were found 
to be indicative of higher body fat content (Davies et al., 1995). While 
studies of exercise without caloric restriction generally show only small 
effects on body weight, significant albeit moderate reductions of body fat 
are generally reported (Eisenmann, 2003). Moreover, even in the absence 
of significant weight loss, exercise has beneficial effects on risk factors for 
cardio metabolic disease (Ross and Bradshaw, 2009; Gutin and Owens, 
2011).
Body mass index Changes in weight for height with growth and 
maturation for U.S. boys and girls are described in CDC growth curves 
(Kuczmarski et al., 2000). Current growth curves were derived from U.S. 
population surveys conducted before the increase in weight for height 
that defines today’s pediatric obesity epidemic. In boys and girls, BMI 
declines during early childhood, reaching its nadir at about ages 5-6, and 
then increases through adolescence. A gender difference emerges during 
puberty, with males gaining greater fat-free mass than females. Both the 

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Copyright © National Academy of Sciences. All rights reserved.
Educating the Student Body: Taking Physical Activity and Physical Education to School
124