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Cardiovascular heart disease (CHD) remains one of the greatest
contributors to morbidity and mortality in industrialized nations
and as a result is responsible for a substantial amount of money
spent by health care systems to both treat CHD and its symptoms.
It is now well accepted that CHD has its genesis in childhood despite
the fact that clinical symptoms of the disease do not become apparent
until later in life (Berenson et al., 1998;
Kannel and Dawber, 1972;
Lauer et al., 1975).
Since 1998, The American Academy of Pediatrics, 1998
as stated that elevated cholesterol levels in children and adolescence
increases the risk for CHD later in life although the exact risk
remains unknown. In response to the rising incidence of CHD in adults,
the American Heart Association and other governing bodies have continued
to emphasize the importance of exercise in childhood as a means
of preventing CHD later in life (Kavey et al., 2003).
Several landmark studies have repeatedly identified a sedentary
lifestyle as a major risk factor for the development of CHD in adults
(Blair et al., 1995;
Morris et al., 1980;
Paffenbarger and Hyde, 1980).
Unfortunately, unlike studies involving adults, the role regular
exercise has on CHD risk factors in children and adolescents remains
unclear. The review aims to describe what is known about the effects
of exercise training in children and adolescents on the following
blood lipids and lipoproteins: total cholesterol (TC), high density
lipoprotein cholesterol (HDL-C), low density lipoprotein cholesterol
(LDL-C), and triglycerides (TG). The following paragraphs describe
in detail studies that described mode, frequency, duration and intensity
of the exercise. Although Gilliam et al., 1978b
did not meet these criteria, they are included because they represent
one of the first training studies to have both exclusively included
girls in the subject pool and examined exercise training effects
on blood lipids and lipoproteins.
In addition, the review is limited to those studies that only examined
exercise-training effects in apparently healthy children and adolescents.
Table 1 clearly demonstrates
that the effect of exercise training on blood lipids and lipoproteins
is equivocal. Gilliam and Burke (1978)
reported a six-week study involving 14 females ages 8-10 years.
The subjects participated in various aerobic activities for 35 minutes
per session. The results showed a significant (p<0.05) increase
in HDL- C levels with no change in TC levels. No other variables
were reported. The main flaw in this study was a lack of a control
group. Additionally, intensity was described as "strenuous"
but was not quantified, the length of the study was short (six weeks)
and the frequencies of the exercise sessions were not reported.
In 1980, Gilliam and Freedson (1980)
conducted a second training study on 11 girls ages 7-9 years. This
time they included a control group and the length of the study was
extended to 12 weeks. After a four-day per week training program
at a "moderately high" intensity, no alterations in TC
or TG were observed. No other variables were reported. Lack of an
appropriate tight control on training intensity clouds the interpretation
of this study.
Linder et al., 1983
examined the effect of an eight-week walk/jog program at a heart
rate (HR) intensity of 80 % of peak HR on 29 boys, ages 11-17 years.
No effect was observed for TC, TG, HDL-C, or LDL-C. The inherent
problem in this study was the inclusion of boys who are at differing
maturational stages.
Savage et al. 's (1986)
walk/jog/run program with 8-9 year old boys resulted in no alterations
in TC or LDL-C or HDL-C levels after the 11-week study. However,
they did note an overall improvement in the TC/HDL ratio.
Ignigo and Mahon (1995)
examined the effects a 10-week exercise-training program had on
TC, TG, HDL-C and LDL-C in boys and girls ages 9-10 years. Eighteen
children participated in an exercise training program and 10 children
served as controls. The exercise program included 60 minutes of
aerobic activity, three times per week at an exercise intensity
that elicited heart rates of 160-180 b·min-1 (80-90 %
of peak HR). TG was the only variable that was favorably altered
after the 10-week exercise intervention. Again, the control for
exercise intensity is not clear. Although the authors mentioned
the use of heart rate monitors, they also mentioned that heart rates
were monitored by pulse counting and thus it is not clear how many
subjects were using heart rate monitors at any one time. Additionally,
the inclusion of both boys and girls in a relatively small sample
size may result in an affect that is independent of the exercise
intervention.
Blessing et al's. (1995)
16 week training study is one of the longest to date, however intensity
was not clearly described nor accurately controlled for. Their subjects
were 25 males and females who ranged in age from 13-18 years. The
16-week training program involved 40 minutes of various aerobic
activities at an intensity that was to approach 90% of previously
determined peak work capacity. Intensity was measured by the subjects
obtaining a radial pulse. The results showed a positive alteration
in TC, HDL-C, LDL-C, TC/HDL-C levels after the 16 weeks of exercise
training. The inherent problem with this study is the inclusion
of both males and females in the same study. Additionally, the age
range of 13-18 years, is too broad due to the differing maturational
stages of this group.
Rowland et al., 1996
conducted a 13-week study that included 34 boys and girls ages 10-13
years. First, there was not a control group. Instead, the subjects
acted as their own controls to try and minimize the genetic effects
of trainability between subjects. However, this study design does
not control for the effects growth and maturation may have on the
measured variables. Second, although heart rate monitors were used
to measure exercise intensity, only seven subjects used the monitors
during each exercise session and as a result, exercise intensity
was only collected on each subject for one out of the three exercise
sessions each week. A final source of error is again subject heterogeneity.
As previously mentioned, the inclusion of adolescent boys and girls
in the subject pool makes interpretation of blood lipid and lipoprotein
changes difficult.
Stergioulas et al., 1998
examined the effect exercise training had on HDL-C levels in 18
boys' ages 10-14 years. The subjects were chosen from a group of
1000 Greek subjects who participated in a survey that was conducted
in 1993. HDL-C levels increased significantly (p < 0.05) after
the eight-week training program. There were several inherent problems
with this study. First, it is difficult to ascertain how exercise
intensity was measured. They indicated that exercise was set at
75 % of physical working capacity that was an exercise heart rate
of 170 b·min-1. However, it is not clear whether a peak
exercise test was completed prior to the exercise intervention or
whether peak heart rate information was gathered from the Greek
survey results of 1993. If exercise heart rate was estimated, than
it is questionable that a heart rate of 170 b·min-1 would
be accurate for boys with an age range of 10-14 years. Second, the
authors do not describe whether or not heart rate was monitored
during the exercise sessions. A final source of error is subject
heterogeneity. Although only boys participated in the study, their
maturity level was not assessed. Assessment of maturity level is
pertinent because there were most likely significant differences
in the boys who ages ranged from 10-14 years and, as mentioned above,
testosterone has been shown to adversely affect the blood lipid
and lipoprotein profile of males.
Stergioulas et al. (2006)
conducted a second study with 10-14 year old boys. In this study
all subjects completed peak exercise tests for the determination
peak HR. The subjects completed 4 training sessions per week at
80 % of their peak HR for 8 weeks. Significant, positive alterations
were observed for all variables at the end of the 8 weeks. However,
it again needs to be pointed out that the probable maturity differences
among the subjects makes the data difficult to accurately interpret.
Stoedefalke et al., 2000
has the longest well controlled exercise training study to examine
the effects of exercise training on post menarchial 13-14 year old
girls. The 20-week study included 20 experimental subjects and 18
control subjects. All subjects underwent peak exercise tests to
determine maximal HR values. Subjects exercised three times per
week for 20 minutes on either a treadmill or cycle ergometer. Exercise
intensity was kept at 75-80% of maximal HR as verified by HR monitors.
No significant change in TC, HDL-D, LDL-C or TG was observed in
either group.
Welsman et al., 1997
examined the effect two separate modes of aerobic training had on
TC levels in 35 girls' ages 9-10 years. The exercise intervention
lasted eight weeks and exercise intensity was set at approximately
80 % of peak HR. All subjects underwent peak exercise tests to determine
peak HR values. No change in TC or HDL-C was observed in either
group. Subjects exercising on the cycle ergometers wore heart rate
monitors so that exercise intensity could be accurately measured.
Subjects who participated in the aerobic dance program underwent
a pilot study to determine which routines would consistently elicit
heart rates above 150 b·min-1. The principal weakness
of this study was that the study only lasted eight weeks. Additionally,
if the subjects in the aerobic dance group experienced a decline
in submaximal HR than the dance routines may not have been rigorous
enough to elicit HR levels of 150 b·min-1 in the latter
weeks of the study.
Tolfrey et al., 1998
conducted a very well controlled study with 48 prepubertal boys
and girls of which 28 of the subjects completed an exercise training
intervention. They controlled for exercise intensity by using HR
monitors and through constant encouragement, they were able to have
all subjects maintain an exercise intensity of 79% of peak HR. The
subjects pedaled on cycle ergometers three times per week for 12
weeks. The results showed that there was no difference over time
for TG and TC between the two groups. However, the exercise group
experienced an increase in
HDL-C and a decrease in LDL-C levels. Changes in the blood lipid
profile were independent of alterations in peak VO2.
In fact, the control group started out with a higher peak VO2
and maintained the greater peak VO2 until the end of
the study suggesting that it is the exercise training which directly
effects blood lipid profiles and not peak VO2. This was
the first study that had adequately controlled for exercise intensity
and, although it is probably unrealistic to expect children to continue
to exercise at a constant intensity, doing the same mode of exercise
outside of an experimental setting, the study does advance our knowledge
of the effects a highly structured exercise training program has
on blood lipids and lipoproteins in prepubertal children. The major
design flaw is the inclusion of both boys and girls in the study.
Additionally, as mentioned above, few studies have lasted longer
than 12 weeks and it would have been beneficial to observe whether
a longer training period resulted in more dramatic differences.
Tolfrey et al., 2004
conducted a second training study with 34, 10-11 year old boys and
girls. All subjects exercised three times per week at 80 % of peak
HR. Again all subjects wore HR monitors for the 12-week exercise-training
program. Unlike other studies, the study was unique in that exercise
duration was individualized to match energy expenditure targets.
Two groups were established. A LOW group that expended 100 kcal·kg-1
and the MOD group that expended 140 kcal·kg-1. The exercise-training
program elicited no change in TC, HDL-C or LDL-C irrespective of
exercise duration and energy expenditure. The authors suggest that
the exercise volume may have been insufficient to elicit a change.
Williford et al. (1996)
is the only study to examine exercise-training effects in black,
male adolescents. Twelve boys completed a 15 week, 5 day per week
exercise training program. The exercise sessions took place for
30 minutes during a regularly scheduled physical education class.
The subjects jogged at 70-90 % of their pre-determined peak heart
rates. It is not clear how HR was monitored. Unique to this study
was the inclusion of a weight-training program that took place two
times per week. The 15-week exercise-training program resulted in
significant increases in HDL-C and significant decreases in LDL-C.
No change in TC occurred. The authors point out that further research
is needed regarding the effects of ethnicity and the effects of
exercise training on blood lipids and lipoproteins.
| CONCLUSIONS
AND RECOMMENDATIONS FOR FUTURE RESEARCH |
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Part
of the difficulty in determining whether exercise training
has a positive effect on the blood lipid profile of children
is that well controlled studies remain scarce. Additionally,
methodological problems present in the majority of the exercise
training studies limits the ability to make a conclusive,
evidence based statement regarding the effect exercise training
has on blood lipid levels in normolipidemic children.
- small
numbers of subjects in each study;
- low
or no representation of girls;
- inclusion
of both boys and girls in the subject pool;
- inclusion
of boys and girls at different maturational stages in the
subject pool;
- exercise
training regimes that do not adequately control for exercise
intensity;
- exercise
training regimes that do not last longer than 8 weeks;
- exercise
training studies that do not have an adequate volume to
elicit a change.
In
addition it should be noted that studies conducted prior to
1990 need to be viewed with caution due to the lack of interlaboratory
lipid and lipoproteins measurement standardization.
The main flaw in all of the above studies is that the exercise
intervention length is probably not long enough. Superko,
1991,
in his review of the adult literature, concluded that exercise
training programs lasting longer than four months, are needed
to see a positive alteration in blood lipids and lipoprotein
levels in most adults.
Therefore, although the research appears to be equivocal with
regard to the effects of an exercise-training program on blood
lipids and lipoprotein levels in children and adolescents,
it may be that the exercise interventions were not long enough.
Additionally there is a clear lack of representation of adolescent
females. Therefore it is not possible to make an evidence
based conclusion of the exercise training effects on blood
lipids and lipoproteins in this population.
From a broader perspective, it is important to determine whether
regular exercise training in children and adolescents results
in regular exercise participation through the lifespan. Ideally,
a longitudinal study which examines the effects of exercise
training from the primary school years through adulthood would
be best. Major goals of such a study would include the following:
- an
examination of the fitness variable changes measured periodically
from puberty, through adolescence into adulthood.
- observation
of selected behavior changes as they relate to exercise.
- the
effect family exercise patterns have on exercise patterns
in children.
- the
effect the PE curriculum has on promoting exercise through
the lifespan.
The
second area of research should focus on the effects exercise
has on children who already exhibit a risk for CHD. Children
to be included would be those with a strong family history
of CHD, who are hyperlipidemic, hypertensive or obese. Currently
there are data which support the notion that regular exercise
reduces the severity of both hyperlipidemia and hypertension
in children. Well controlled exercise training studies in
children who had adverse blood lipid and lipoprotein profiles
showed positive alterations in their profiles (Rimmer et al.,
1997;
Stergioulas et al., 1998;
Tolfrey et al., 1998).
Similarly, Alpert and Wilmore's (1994)
review of the literature found a decline in blood pressure
in hypertensive children after exercise although few of the
subjects reached normal levels.
Clearly the effects of exercise training on the blood lipid
and lipoprotein levels of normolipidemic children and adolescents
are equivocal. Of the 14 studies reviewed, six observed a
positive alteration in the blood lipid and lipoprotein profile
(Blessing et al., 1995;
Savage et al., 1986;
Stergioulas et al., 1998;
Stergioulas et al., 2006;
Tolfrey et al., 1998; Williford et al., 1996),
five of the studies observed no alteration in the blood lipid
and lipoprotein profile (Gilliam and Burke 1978;
Gilliam and Freedson 1980;
Linder et al., 1983;
Rowland et al., 1996;
Welsman et al., 1997; Stoedefalke et al., 2000)
and one study observed a negative effect on HDL-C but an overall
improvement in the lipid and lipoprotein profile due to the
decrease in the TC/HDL ratio (Savage et al., 1986).
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