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Educating the Student Body: Taking Physical Activity and Physical Education to School
116
Educating the Student Body
in the abdominal cavity is more metabolically active (reflected by free fatty
acid flux) than adipose tissue in other areas (Arner and Eckel, 1998), and
higher amounts of visceral adipose tissue are associated with greater risk of
metabolic
complications, such as type 2 diabetes and cardio vascular disease
(Daniels et al., 1999; He et al., 2007; Dencker et al., 2012). In contrast,
subcutaneous fat, particularly in the gluteofemoral region, is generally asso-
ciated with a lower risk of cardiometabolic disease. Age- and sex-associated
variations in fat distribution contribute to age- and sex- associated differ-
ences in cardiometabolic disease prevalence. Girls have more sub cutaneous
fat than boys at all ages, although relative fat distribution is similar. After
a rapid rise in subcutaneous fat in
the first few months of life, both sexes
experience a reduction through age 6 or 7 (Malina and Roche, 1983;
Malina and Bouchard, 1988; Malina, 1996). Girls then show a linear
increase in subcutaneous fat, whereas boys show a small increase between
ages 7 and 12 or 13 and then an overall reduction during puberty. The
thickness of subcutaneous fat on the trunk is approximately one-half that
of subcutaneous fat on the extremities in both boys and girls during child-
hood. The ratio increases with age in males during adolescence but changes
only slightly in girls. In males the increasing ratio of trunk to extremity
subcutaneous fat is a consequence of slowly increasing
trunk subcutaneous
fat and a decrease in subcutaneous fat on the extremities. In girls, trunk and
extremity subcutaneous fat increase at a similar rate; thus the ratio is stable
(Malina and Bouchard, 1988). As a consequence, the sex difference in the
distribution of body fat develops during adolescence. It is important to
note that changes in subcutaneous fat pattern do not necessarily represent
changes in abdominal visceral adipose tissue.
Tracking of subcutaneous fat has been investigated
based on skinfold
thicknesses and radiographs of fat widths in males and females across a
broad age range (Katzmarzyk et al., 1999; Campbell et al., 2012). Results
indicate that subcutaneous fat is labile during early childhood. After
age 7 to 8, correlations between subcutaneous fat in later childhood and
adolescence and adult subcutaneous fat are significant and moderate.
Longitudinal data on tracking of visceral adipose tissue are not available,
but percent body fat does appear to track. Thus children and especially
adolescents with higher levels of body fat have a higher risk of being overfat
at subsequent examinations and in adulthood, although variation is con-
siderable, with some individuals moving away from high fatness categories,
while some lean children move into higher fatness categories.
In
cross-sectional studies, active children and adolescents tend to have
lower skinfold thicknesses and less overall body fat than their less active
peers (Loftin et al., 1998; Rowlands et al., 2000; Stevens et al., 2004;
Lohman et al., 2006), although the correlations are modest, reflecting
variation in body composition at different levels of physical activity, as
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
117
well as the difficulty of measuring physical activity. Longitudinal studies
indicate small differences in fatness between active
and inactive boys and
girls. Although some school-based studies of the effects of physical activ-
ity on body composition have reported changes in BMI or skinfolds in the
desired direction (Gortmaker et al., 1999; McMurray et al., 2002), most
have not shown significant effects. High levels of physical activity are
most likely needed to modify skinfold thicknesses and percent body fat.
In adults, visceral adipose tissue declines with weight loss with exercise. In
contrast, in a study of obese children aged 7-11, a 4-month physical activity
program resulted in minimal change in abdominal visceral adipose tissue
but a significant loss in abdominal subcutaneous adipose tissue (Gutin
and Owens, 1999). In adults, decreases in fatness with exercise are due
to a reduction in fat cell size, not number (You et al., 2006); whether this
is true in children is not certain but appears likely. Given that adipocyte
hypertrophy may trigger adipocyte hyperplasia (Ballor et al., 1998),
energy
expenditure through regular physical activity may be important in prevent-
ing excess adipose tissue cellularity. Regular physical activity also affects
adipose tissue metabolism so that trained individuals have an increased
ability to mobilize and oxidize fat, which is associated with increased levels
of lipolysis, an increased respiratory quotient, and a lower risk of obesity
(Depres and Lamarche, 2000).
Cardiorespiratory System
The ability to perform sustained activity under predominantly aerobic
conditions depends on the capacity of the cardiovascular and pulmonary
systems to deliver oxygenated blood to tissues and
on the ability of tissues
(primarily skeletal muscle) to extract oxygen and oxidize substrate. By
age 2 the systems are fully functional, although young children lack the
cardio respiratory capacity of older children and adults because of their
small size (Malina et al., 2004). Children’s aerobic capacity and conse-
quently their ability to exercise for longer periods of time increase as they
grow. Maximal aerobic power (liters per minute) increases fairly linearly in
boys until about age 16, whereas it increases in girls until about age 13 and
then plateaus during adolescence (Malina et al., 2004; Eisenmann et al.,
2011). Differences between boys and girls are small (~10 percent) during
childhood and greater after the adolescent growth spurt, when girls have
only about 70 percent of the mean value of boys. Changes with age and sex
differences are explained largely by differences
in the size of the relevant
tissues. Dimensions of the heart and lungs enlarge with age in a manner
consistent with the increase in body mass and stature (Malina et al., 2004).
The increase in the size of the heart is associated with increases in stroke
volume (blood pumped per beat) and cardiac output (product of stroke vol-
