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pelinks4u EDITORIAL
How
Physical Educators Can Help to Combat Obesity
 Author:
Bernard Gutin, PhD
Academic
affiliations:
Adjunct Professor
of Nutrition, University of North Carolina-Chapel
Hill;
Emeritus Professor
of Pediatrics and Physiology, Medical College
of Georgia;
Emeritus Professor
of Applied Physiology, Teachers College, Columbia
University.
Contact
information: bernardgutin@yahoo.com
Because the prevalence of child obesity has
been increasing to epidemic levels over the
last few decades, many researchers have investigated
approaches designed to prevent this public
health problem. Practitioners like physical
education (PE) teachers or nutritionists look
to such research projects for guidance concerning
how they can help youths to develop healthy
bodies. Unfortunately, many research strategies
have failed to provide evidence that compared
to control groups, the interventions were
successful in preventing obesity. Thus, the
trials did not provide clear guidance to practitioners.
In this essay, I’ll analyze some factors
that distinguish successful from unsuccessful
research interventions. This analysis may
help research scientists design more effective
interventions and also help practitioners
to be more effective in their efforts. My
main message is that research projects and
professional efforts are more likely to be
successful if they emphasize adequate doses
of vigorous physical activity (PA) rather
than restricting energy intake (Gutin,
2008). Thus, physical educators have
a central role to play in preventing obesity.
Successful preventive interventions can be
distinguished from unsuccessful ones by analysis
of three factors: 1) the biologic outcomes
used to evaluate the success of the intervention;
2) the "dose" of PA emphasized in
the intervention; and 3) the aspects of diet
stressed in the intervention.
Evaluating Successful Obesity Interventions
In reviewing the biologic outcomes used to
evaluate the success of intervention trials,
many of the unsuccessful trials have used
changes in Body Mass Index (weight/height2),
or BMI, as the main biologic outcome. This
is an attractive procedure because BMI is
easily measured, and is highly correlated
with a fatness index such as percent body
fat. Thus BMI is quite useful for epidemiologic
studies of obesity in large numbers of subjects.
However, BMI is not a good index to use to
assess the changes in body composition that
result from interventions that include varying
amounts of PA because it is possible for PA
interventions to increase fat-free mass (FFM)
at the same time that they reduce fat mass,
with the result that BMI does not change.
In fact, we recently found that youths who
engaged in an after-school exercise intervention
for 3 years, reduced their fatness compared
to the control subjects, but increased so
much in FFM that their BMIs increased more
than the BMIs of the control subjects (Gutin
et al, 2008).
In this instance, if BMI had been used as
the outcome variable it would have appeared
that the intervention had the paradoxical
effect of making the kids fatter. Because
the adverse cardiometabolic health effects
of obesity are due to excessive fatness, rather
than high levels of FFM (Gutin et al, 2007),
such a result might miss detecting favorable
health outcomes. It is also important to note,
that already during the childhood years, poor
lipid profiles are associated with abdominal
visceral adiposity, independent of total body
fatness (Owens et al,
1998; Owens et al, 2000; Gutin et al, 2007),
and that PA interventions have a favorable
effect on visceral adiposity in obese and
non-obese youths (Owens
et al, 1999; Gutin et al, 2002; Barbeau et
al, 2007).
In order for research projects to provide
a fair test of the effectiveness of an intervention,
they should use the most sophisticated techniques
available for measurement of total body or
visceral fat, such as dual-energy x-ray absorptiometry
or magnetic resonance imaging. Of course,
PE teachers or nutritionists working in practical
settings don’t have access to such expensive
equipment. That is why I emphasize that these
techniques should be used in research projects,
not in schools. Instead, teachers who want
to keep track of how their students are progressing
might use surrogate indices of total body
or abdominal fatness, such as skinfold thicknesses
or waist girth; these indices focus the attention
of the teachers and students on fatness rather
than weight.
How Much Physical Activity is Enough?
With respect to PA dose, there are three main
components to consider: 1) the weekly volume
of PA, 2) the duration of the intervention,
and 3) the intensity of the PA. Weekly volumes
of ~150 minutes/week of moderate to vigorous
PA (e.g., ~50 minutes/session, ~3 sessions/week)
have produced favorable changes in body fatness
and cardiometabolic health of youths who were
sedentary and obese prior to the intervention
(eg, Gutin et al, 1999;
Gutin et al, 2002) .
However, in youths who were not obese at
the outset of the project, doses of this amount
have not been successful in preventing the
increase of fat and associated health disorders
(eg, Tolfrey et al,
1998; Tolfrey et al, 2004). This suggests
that the effective dose needed for obesity
prevention is greater than the dose needed
for obesity treatment. This idea is supported
by the results of recent studies showing that
moderate to vigorous PA doses of ~300 minutes/week
(~1 hour/day) have been successful in improving
the body composition of a wide variety of
youths (Barbeau et al,
2007, Strong et al, 2005).
With respect to the duration of the intervention,
periods of 2-4 months have been successful
with obese youths (Gutin
et al, 1999), whereas longer periods
of 6-10 months and longer have a better record
of success with non-obese youths (Barbeau
et al, 2007; Gutin et al, 2008). With
respect to intensity of PA, it seems that
youths who maintain high intensity levels,
as might be seen in activities such as soccer,
basketball and dance, are more likely to show
favorable changes in fatness and fitness than
youths who engage in moderate intensities
such as walking to school (Barbeau
et al, 1999; Gutin et al, 2002).
Much work still needs to be done by exercise
scientists to clarify the dose-response relations
in different populations of youths. For PE
teachers, the available data suggest that
more emphasis should be given to vigorous
PA than to moderate PA. However, teachers
must keep in mind that a given dose of PA
places more of an overload on the body of
a sedentary and obese child than it does on
the body of a less sedentary child. An effective
way to quantify the relative physiologic impact
of a given PA dose is to use heart rate monitors
to provide feedback to students and teachers
concerning how hard students are actually
working.
Nutrition and Obesity
The role of diet in obesity prevention is
quite controversial. For obesity treatment,
many trials have produced favorable effects
on weight by helping obese youths to go on
diets, i.e., reduce their energy intake. Moreover,
most adults are aware that they gain weight
when they eat more (as over a holiday period)
and they lose weight when they go on a reduced-calorie
diet. These experiences lead to the common
belief that children who become fatter than
their age-mates must ingest greater amounts
of energy than their peers. Thus, many intervention
trials have been designed to encourage youths
to reduce their energy intake. But this may
be a serious error. In fact, recent research
shows that leaner youths (i.e., those with
lower percent body fat values) actually ingest
more energy than fatter youths (Stallman-Jorgensen
et al, 2007). Here again we see the
importance of distinguishing weight from fat.
At first this research finding seems paradoxical
- fatter kids eat less than lean kids? But
the paradox disappears when we consider two
related facts: the leaner kids are also the
ones who do the most vigorous PA, and vigorous
PA stimulates the development of FFM, which
is associated with a higher resting metabolic
rate (Johnstone et al,
2005). This picture of lean youths
playing vigorous sports and ingesting large
amounts of energy is one that many PE teachers
will remember from their own childhoods.
Does this mean that diet is unimportant?
Not at all. In fact, by having a high amount
of energy throughput, the active youths have
a greater chance of obtaining the nutrients
needed for optimal growth. For example, optimal
growth of bone mass requires ingestion of
sufficient amounts of dairy products, calcium
and vitamin D. Thus, we should emphasize ingestion
of sufficient amounts of energy and nutrients,
while reducing ingestion of foods with empty
calories such as sugared drinks.
Parenthetically, another factor that has
been consistently linked with obesity is TV
viewing, which is sometimes used as a surrogate
for sedentariness. However, it seems that
the deleterious effects of TV watching is
less due to its effect on energy expenditure
and more to its influence on diet. That is,
watching TV exposes youths to advertisements
for nutrient-poor foods and keeps the youths
within easy reach of such snacks.
By no means are these emerging research findings
conclusive or non-controversial; much remains
to be learned about these matters. Nonetheless,
exercise scientists have a responsibility
to bring these recent research findings to
the attention of the PE profession and PE
teachers have a major responsibility to help
their students develop healthy bodies.
Summary
In light of the recent research findings,
the following recommendations for physical
education teaching seem reasonable:
- Pay more attention to body composition.
At a minimum, teachers and administrators
should resist classifying kids as obese
on the basis of an elevated BMI without
considering the amount and type of PA they
do. Some states have adopted the practice
of sending home individual BMI values as
part of the report cards. Although measuring
BMI in whole schools or states may be a
good way to call attention to the obesity
epidemic, it may be misleading in the case
of individual children.
If possible,
teachers should incorporate skinfold or
waist circumference measurements into
their assessment programs. Students and
their parents should be aware that skinfold
fatness or fat stored around the midsection
is especially harmful to children’s
health. In contrast, we want healthy children
to develop reasonably high levels of muscle
and bone mass. This is why it’s
useful to measure skinfold fatness and
waist girth rather than weight to keep
track of children’s health.) And
this information can serve as a foundation
for informing students about the importance
of body composition.
-
Physical educators have an important
role to play in helping youths to achieve
the recommended dose of at least 60 minutes/day
of moderate to vigorous PA. This amounts
to ~300 minutes/week, which is much more
than can be achieved within the confines
of PE class alone. After-school, weekend
and vacation hours provide discretionary
time that can be used for such activity.
In addition to assuring that students
obtain reasonably large amounts of PA
in class, physical educators are also
responsible for developing skills and
an appreciation for PA that can encourage
youths to participate in PA during out-of-school
time.
-
Give greater emphasis to vigorous PA
for optimal development of fitness, body
composition and cardiometabolic health.
Although moderate PA such as walking to
school can use up calories, research suggests
that vigorous PA has a more favorable
effect on development of healthy muscles
and bones.
- Youths who are vigorously active should
not be encouraged to limit their energy
intake. It is likely that a high energy
and nutrient intake is consistent with the
biologic demands of healthy growth. Instead,
the emphasis should be on ingestion of nutrient-rich
foods rather than nutrient-poor foods like
sugared drinks and snacks.
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