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Health Benefits of Physical Activity
During Childhood & Adolescence
Oded Bar-Or
McMASTER UNIVERSITY
ORIGINALLY PUBLISHED AS SERIES
2, NUMBER 4, OF THE PCPFS RESEARCH DIGEST
HIGHLIGHT
"Children are not little adults. Their
responses to activity are quite different from those
of adults. Activity programs should be planned with
these differences in mind."
INTRODUCTION
The beneficial effects to health of
enhanced physical activity (PA) during adult years are
numerous. There is mounting evidence that such benefits
include a reduction in morbidity and mortality from
diseases of several body systems (Bouchard et al., 1994).
Much less evidence is available regarding the effects
of an active lifestyle during childhood and adolescence
on adult health.
The main reason for the paucity of
information on the possible carryover of benefits from
childhood to adulthood is the lack of longitudinal studies
that have followed the same individuals over many years.
Ideally, one would need randomly to assign children
into those who are given enhanced PA programs and those
who remain sedentary over years and then observe the
long-term effects of PA or of inactivity. On ethical
grounds, such studies are hard to justify (it is unethical
to demand that children not engage in PA for an extended
period of time). In addition, they are extremely expensive
and logistically most complicated. A second- best alternative
would be to conduct controlled intervention studies
that last shorter periods and include several groups
of subjects who span a wide age range (from childhood
to middle. Such mixed longitudinal studies are feasible,
but have yet to be launched. Another approach is to
identify adults with and without diseases and question
them about their PA during earlier years. Such retrospective
studies are easier to perform, but their outcome depends
on the ability of people to correctly remember and report
their PA behavior during earlier years. Conclusions
derived from retrospective studies are less valid than
those derived from longitudinal interventions.
The purpose of this article is to
examine briefly the current evidence that enhanced PA
during childhood and adolescence imparts immediate health
benefits, or reduces risk for adult chronic disease.
Emphasis will be given to the following conditions:
obesity, hypertension, abnormal plasma lipoprotein profile,
and osteoporosis. Table 19.1 summarizes the evidence
attesting to such benefits.
SHORT-TERM BENEFITS
Before analyzing the carryover effects
of childhood PA, one should identify the immediate
effects of a training program (or an active lifestyle)
on health-related risk factors. These are measured while
the program is still in progress, or immediately upon
its conclusion. Evidence for such benefits has been
sought from intervention training programs that last
a few weeks or several months at the most. An alternative
approach has been cross-sectional studies that compare
children (or youth) who habitually engage in athletic
pursuit with those who lead a sedentary lifestyle. The
drawback of the latter approach is that differences
in health-related risk between groups might not be a
result of the physical activity per se. They may instead
reflect heredity or events that took place before the
child became physically active.
Body fatness. (See Bar-Or
& Baranowski, 1994, for a review.) Many, although
not all, cross-sectional studies suggest that obese
children and youth are less active than their leaner
peers. There is only scant evidence, though, that inactivity
is a cause of juvenile obesity (Roberts, 1993).
Training studies with nonobese youth have shown little
or no reduction in body adiposity (Wilmore, 1983). However,
enhanced PA with or without a low-calorie diet, did
reduce % body fat or excess body weight in obese children
and youth.
Blood pressure. (See
Alpert & Wilmore, 1994, for a review.) Some cross-sectional
studies show a slightly higher resting blood pressure
among sedentary adolescents compared with their active
peers. Most studies, however, do not show such a difference,
particularly if the groups have the same adiposity level.
Training of healthy, previously inactive children or
adolescents who have a normal blood pressure induces
little (1-6 mmHg) or no drop in blood pressure. However,
in adolescents with hypertension, training over several
months does induce a reduction of both systolic and
diastolic blood pressure. Even though such a reduction
is modest (around 10 mmHg), it may be beneficial for
some individuals with mild hypertension who otherwise
may require medication to control their blood pressure.
The training programs that induced a decline in blood
pressure were comprised mostly of aerobic activities.
In one study (Hagberg et al., 1984), the inclusion of
a five-month weight training regimen following a six-month
aerobic program further reduced the blood pressure of
adolescents with hypertension. Such beneficial effects
of exercise disappear within several months of termination
of the program.
Blood lipids. (See Armstrong
& Simons-Morton, 1994, for a review.) Based on some
cross-sectional studies, children and adolescents who
are physically active, or whose aerobic fitness is high,
have a more favorable blood lipid profile than their
sedentary, or less fit, peers. This difference is particularly
apparent in high-density lipoprotein cholesterol (HDL-C
= the "good" cholesterol), which is higher in the active
groups. Other cross-sectional comparisons, however,
do not reveal such differences. In most of the cross-sectional
studies it is impossible to separate a high activity
level from a high fitness level.
Training studies of several weeks
duration have failed to show any beneficial effect on
the blood lipid profile in healthy children or adolescents.
More beneficial responses have been shown for groups
who have a high coronary risk. These include children
and adolescents with insulin-dependent diabetes mellitus,
obesity, or with at least one parent who has three or
more coronary risk factors.
Skeletal health. (See
Bailey & Martin, 1994, for a review.) The possible
link between skeletal health and PA has received attention
in recent years with the finding that physically active
postmenopausal women, and elderly populations in general,
have a higher bone mineral density (BMD) and less osteoporosis
than less active controls. One of the determinants of
bone health in old age is the peak BMD reached
by young adulthood. Bone mass and BMD subsequently (and
inevitably) decline with the years, until the bones
become fragile.
This topic has an important pediatric
relevance, because the great majority of bone build-
up occurs during adolescence. A question of major public
health relevance is whether enhanced PA during childhood
and adolescence will result in a higher peak BMD. Cross-sectional
comparisons have shown that young athletes in weight-bearing
sports such as gymnastics, soccer and volleyball (but
not in non-weight-bearing sports such as swimming) have
a higher BMD than do nonathletes. Likewise, bones of
the dominant limb in asymmetrical sports,
such as tennis or little-league pitching, have a higher
BMD than the nondominant limb. Conversely, bones of
a limb immobilized for several weeks or months had a
lower BMD than in the contralateral, nonimmobilized
limb.
Retrospective studies, in which adults
were asked about their PA during childhood, suggest
that women who had been physically active during childhood
had a higher BMD in the third and fourth decades of
life than women who had been less active as children.
Longitudinal results of weight-bearing training programs
are equivocal. Most controlled interventions yielded
little or no increase in BMD or bone mass of exercising
adolescents (e.g., Blimkie et al., 1993).
CARRYOVER TO ADULT LIFE
There are several models that may
explain a possible link between an adult persons
health and her or his activity behavior in earlier years.
As suggested by Blair et al. (1989) there are conceivably
three avenues by which an enhanced PA level during childhood
might improve adult health:
- Childhood activity improves child
health which, in turn, is beneficial to adult health.
- An active lifestyle during childhood
has a direct benefit to health in later years.
- An active child becomes an active
adult who, in turn, has a lower risk for disease than
an inactive adult.
Research provides no proof, or disproof,
for any of these links. However, because a sedentary
lifestyle in adults has been proven to entail a high
risk for several chronic diseases (Bouchard et al.,
1994), the most plausible link is that an active lifestyle
during childhood and adolescence would be carried over
through adulthood which, in turn, would reduce risk
for disease. There are, however, no prospective studies
that have tracked activity patterns from childhood to
adulthood. Even though activity patterns and attitudes
toward PA remain quite stable during late adolescence
(but less so around age 1012 years) (Malina, 1990),
there is a low relationship between the two.
HOW MUCH PHYSICAL ACTIVITY?
There are practically no data as to
the optimal dose of PA during childhood and adolescence
that might maintain and/or enhance health. However,
a group of experts from various countries has recently
generated a consensus statement (Sallis & Patrick,
1994), which includes the following guidelines for adolescents:
- All adolescents should be physically
active daily, or nearly every day, as part of play,
games, sports, work, transportation, recreation, physical
education, or planned exercise; in the context of
family, school, and community activities.
- Adolescents should engage in three
or more sessions per week of activities that last
20 minutes or more at a time and that require moderate
to vigorous levels of exertion. There is no formal
consensus statement for preadolescents although Corbin,
Pangrazi, and Welk (1994) have made recommendations
for physical activity levels for this group in a previous
issue of the President's Council on Physical Fitness
and Sports Physical Activity and Fitness Research
Digest.
TABLE 19.1
Possible effects of enhanced physical
activity during childhood and adolescence on risk for
chronic disease.
Observed Variable/Risk
Adiposity/Obesity
Resting Blood Pressure/Hypertension
Blood Lipid Profile
Bone Mineral Density/Osteoporosis
Cross-Sectional Comparisons
Obesity is associated with hypoactivity.
Less active groups have similar or slightly higher BP
compared with active groups.
Young athletes sometimes have a better profile than
sedentary controls (mostly in HDL-cholesterol).
Athletes (weight-bearing activities) have higher BMD
than nonathletes.
Short-Term Effects of Intervention
Programs
General Population: Little or no reduction in
% fat
Obese: reduction in % fat
General Population: Little or no reduction in
blood pressure
Hypertensives: 5 12 mmHg reduction in SBP
and less in DBP
General Population: No improvement in profile
High-risk Population: Improved profile
Immobility induces loss of BMD .
Training over several months induces no increase in
BMD.
General Population: No information
Obese: % fat returns to pretraining levels in
most patients
Carryover to Adult Life
General Population: Noinformation
Hypertensives: BP returns to pretraining values
with in weeks
General Population: No information
High-risk Population: No information
Retrospective data suggest a possible carryover.
CONCLUSION
Based on current information, no long-term
studies exist that support or reject the notion that
physical activity during childhood and adolescence is
beneficial to adult health. There is, however, some
evidence for short-term benefits of enhanced PA during
the early years, particularly among children and youth
who are at a high risk for chronic illness in later
years. Much more research is needed to study this important
issue further. In particular, it is essential to identify
means of keeping young people motivated to maintain
an active lifestyle as they reach young adulthood and
middle age. The contrast between cross-sectional data
and those generated through training studies is intriguing.
The former suggest favorable health characteristics
among active children and youth, compared with sedentary
controls. Training studies, on the other hand, show
little or no beneficial effect of training among healthy
children and youth. This contrast may reflect a preselection
of those who become active, and are healthier to start
with, versus those who choose to pursue a se dent ary
li fest yle. It is po ssib le, th ough , that interventions
more vigorous than those commonly used in research would
yield greater effects. It has been shown, for example,
that army recruits who undergo an intense eight hours
per day training regimen for several months respond
with an increase in bone mineral content (Margulies
et al., 1986) and an improved lipid profile (Rubinstein
et al., 1995). Likewise, it is possible that longer
interventions (e.g., 1 2 years) than those used
in most studies would yield more positive training-induced
results.
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