Physical Activity Guidelines Advisory Committee Report
Part H. Research Recommendations
Physical Activity Guidelines Advisory Committee (PAGAC) members were
requested to consider the research needed to help resolve unanswered or
inadequately answered questions they identified during their review of the
science. Although a substantial amount of research on physical activity and
health has been published since 1995, major gaps still exist in our knowledge
needed to establish cause and effect for various health outcomes and to better
define dose response, especially at the low and high ends of the activity
spectrum. At the end of many of the chapters in Part G: The Science Base, authors have
included a listing or discussion directed at needed research for specific
health outcomes included in their review. This section initially focuses on
some overarching recommendations that are applicable to many of the outcomes
considered during the PAGAC review. Following these general recommendations are
priority recommendations specific to the outcomes considered by each
Overarching Research Recommendations
One issue that became evident during the PAGAC review was the lack of
data on selected subpopulations, especially various race/ethnic groups, persons
of low socioeconomic status (SES), individuals with specific cognitive and
physical disabilities, and obese persons. Some of these groups have been
excluded from participation based on study eligibility/exclusion criteria
(e.g., ability to walk at a moderate pace, read or speak English), or because
study logistics precluded them from easily participating (e.g., travel
distances, study visits during work hours). Since 1995, many studies that
included women as subjects have been published. However, only a few have
provided within-study comparisons of the impact of physical activity on health
outcomes between the sexes.
- Because of the scientific and logistical challenges of including
adequate-sized samples of multiple groups in a study conducted at one
institution, well-designed and executed multi-center studies are needed in
which each research site can have access to subjects who represent various
specific understudied populations. This is critically important in providing
investigators with opportunities to examine interactions between
sociodemographic factors, particularly sex and race/ethnicity or SES, and
physical activity in relation to health and to make inter-group
- Funding agencies should support well-designed studies of individual
understudied populations, especially race/ethnic minorities, persons of low
SES, and individuals with physical and cognitive disabilities, so that major
questions regarding the effects of exercise and effectiveness of physical
activity interventions in each of these populations can be answered. If an
organization funded a number of such studies with at least a core of shared
measures, they would have a well-diversified research portfolio on understudied
populations. Such an approach would more likely answer key questions than would
an approach that requires each investigator to include relatively small numbers
of understudied populations in their studies.
- Journal standards for peer-reviewed articles should require a
reporting of the race/ethnicity (in addition to sex and age) of the sample and
presentation of subgroup analyses by race/ethnicity and/or SES if sample sizes
are sufficient, rather than simply treating these as co-variates and adjusting
In each of the review chapters, the dose-response data currently
available are summarized. Based on these reviews, it is apparent that major
unanswered issues still exist in response to the question, "How much of what
type of activity is enough to improve health?" To have sufficient statistical
power to appropriately evaluate dose response in experimental studies, the
overall sample size needs to be relatively large. In observational studies, it
has been difficult to isolate parameters other than overall activity amount
from data collected using questionnaires. Many experimental studies have used
one level of physical activity as the intervention (usually that included in
recent recommendations) and, consequently, evaluation of any dose-response
effects must rely on post-hoc comparisons. Given that the number of
dose-response questions that can be asked are nearly limitless when considering
various activity characteristics (type, intensity, frequency, duration,
amount/volume), possible health outcomes, and different populations, it is
important that some priority be developed for which dose response questions are
- Some recently published data indicate that physical activity of a
lower intensity and/or smaller amount than is currently recommended may provide
significant health benefits for chronically inactive or unfit adults (who
comprise a large proportion of the American population aged 18 years and over)
and older adults. Both experimental and observational studies are needed to
answer a variety of questions about the nature of benefits provided and
characteristics of dose required at the low end. The range of physical activity
used in the intervention should include a dose below that currently identified
in physical activity recommendations to evaluate its impact and the stability
of this level of physical activity behavior over time.
- There remains a lack of data defining both the shape of the
dose-response curve at the higher amounts and intensities of activity for most
health outcomes and whether an upper limit of benefit exists. Most current
recommendations focus on a minimal or target amount/intensity of activity that
is consistent with much of the population receiving some benefit, but don't
address questions of "optimal" or "maximal" benefit. Studies are need to
clarify the amount of physical activity, defined by metabolic equivalent
(MET)-minutes per week or some other measure, at which additional improvements
in various health outcomes no longer occur or at which increases are negated by
increased adverse medical events.
- To fill the gap in our knowledge about dose response, investigators
should design and conduct studies that evaluate effects of the following
variables at fixed volumes of physical activity: intensity, frequency,
duration, and multiple bouts. Details related to these variables would allow
more precise physical activity guidelines to be developed across the breadth of
activity-related health outcomes.
- Reasonable evidence exists that activity accumulated in short bouts
throughout the day can favorably alter selected biomarkers for cardiovascular
and metabolic diseases and improve cardiorespiratory fitness. However, no
evidence is available that such patterns of activity may be beneficial for
musculoskeletal health. Experimental studies are needed to extend this research
involving activity bouts of different durations, especially multiple bouts
shorter than 10 minutes and a few long bouts per week (e.g., 2 x 75 minutes) on
various health outcomes. Observational studies are needed using assessment
methodologies that will allow accurate quantification of a range of types of
activity in different population groups (e.g., abdominally obese, frail
elderly) and an evaluation of the effect of accumulation of short bouts on
clinical outcomes independent of activity intensity and amount.
The ability of the PAGAC to draw strong conclusions for various outcomes
was limited by the wide variety of questionnaires used to assess physical
activity and numerous different approaches to data analysis and
- Uniform data collection is needed with respect to the type of
physical activity (e.g., leisure-time, occupational) and physical activity
characteristics (e.g., intensity, duration, amount).
- The Compendium of Physical Activity has been very useful in assigning
standardized values of absolute intensity to a wide range of activities, but it
should be updated and expanded to children and youth.
- During the past decade, technology that provides for the objective
assessment of physical activity in relatively large groups of subjects has
increased rapidly, especially through the use of motion sensors and
physiological monitoring. These technologies have the potential to greatly
improve the accuracy and reliability of physical activity assessment in
free-living populations leading to a better understanding of health benefits
and dose response. Development and evaluation of these technologies are needed
for assessing populations with different activity profiles and sociodemographic
- A much better understanding is needed on how the results of physical
activity assessed by new objective measurement methods can be compared to data
collected by commonly used questionnaires.
Physical Activity and Physical Fitness
Physical activity surveillance of the US population has been provided by
the Behavioral Risk Factor Surveillance System (http://www.cdc.gov/brfss/), the Youth Risk
Behavior Surveillance System (http://www.cdc.gov/HealthyYouth/yrbs/index.htm),
and the National Health and Nutrition Examination Survey (http://www.cdc.gov/nchs/nhanes.htm),
but the information provided by these surveys remains quite limited. Also,
longitudinal physical fitness assessment of most population groups in the
United States is non-existent. Lack of these data prevents evidence-informed
decisions regarding the contribution any change in physical activity has on
various health outcomes, such as the rapid increasing incidence of obesity,
metabolic syndrome, and type 2 diabetes (T2D).
- Surveillance of the total activity energy expenditure of
representative samples of the US population needs to be implemented once
appropriate assessment tools have been developed and validated. Such tools
could include either questionnaires or new objective measurement technology, or
a combination of the two.
- Special attention needs to be given to the surveillance of both the
physical activity and physical fitness of the US population at both ends of the
age spectrum toddlers/children and the oldest adults. These groups
constitute a substantial portion of the US population and receive unique
benefits from being physically active, but no national surveillance system for
physical activity or physical fitness data exist for them.
Systematic Reviews and Meta-Analyses
During literature reviews by the PAGAC, it became evident that, for
selected health outcomes in various populations, a large number of studies have
been published since 1995, but neither quantitative systematic reviews nor
meta-analyses have been published. Such reviews would be very helpful in
drawing conclusions about health benefits, modifiers of the effects of physical
activity, and dose response.
- Experts investigating specific health outcomes from physical activity
should assess the nature and volume of recent publications and determine
whether quantitative reviews of the data would contribute to existing
knowledge, help formulate guidelines and policy statements, and help set
Research Recommendations of PAGAC
In the review chapters in Part G:
The Science Base, each subcommittee highlighted areas in
which data are lacking and provided guidance regarding research needs for
specific populations and health outcomes. The following section provides a
consolidation of the key recommendations from these chapters. The varying
format and style of these recommendations reflects the different approaches
that subcommittees took in identifying and articulating research needs in their
- Empirical data are needed that are specific to minority populations
African Americans and Hispanics, in particular.
- Empirical data are needed that are specific to disabled populations,
whether physically or intellectually disabled.
- Additional studies are needed to clarify whether all activities
"count" equally, because limited data now suggest that vigorous-intensity
activities are associated with additional risk reductions, beyond their
contribution to total energy expended, when compared with moderate-intensity
- Additional data are needed to help clarify the shape of the
dose-response curve. An emphasis on two areas of data collection would be
particularly useful: (1) uniform data collection to assess the same domains of
physical activity (e.g., leisure-time, occupational, household, and/or
commuting) across studies, and (2) collection of sufficient details on physical
activity to assess different modes of exercise (e.g., aerobic versus strength
training) as well as energy expenditure and intensity.
- Studies are needed to determine the point (if any) on the
dose-response curve at which no further reduction in all-cause mortality
Studies are needed to answer the following questions:
- What is the time course of acquisition of the cardiovascular health
benefits resulting from increases in habitual physical activity?
- What are the cardiovascular health benefits of varying exercise bout
duration, frequency, and intensity, while controlling for total volume?
- What effect do daily exercise exposures accumulated in short bouts
have on the acquired cardiovascular health benefits of habitual physical
- What are the effects of resistance training on cardiovascular health
and what is the nature of dose-response effects (varying intensity, bout
volume, and frequency of programs)?
- Are there sex differences in cardiovascular health benefits of
habitual physical activity when controlling for activity volume?
- What are the specific harmful effects of physical inactivity on
cardiovascular health and what are the characteristics of the inactivity most
likely to produce harm?
- What are the specific effects of aerobic training, resistance
training, and a combination on selected biomarkers of vascular health, such as
brachial artery flow mediated dilation? What are the dose-response
- What are the main characteristics of an exercise program for
preventing and treating peripheral arterial disease (PAD)? What are the
exercise dose-response patterns, sex differences, exercise modality options,
differential effects on diabetic patients with PAD, on asymptomatic patients
and are biomarkers available to predict exercise responders?
- Available data indicate that regular physical activity is associated
with reduced risk of metabolic syndrome. However, it is not clear whether
physical activity and exercise can be used in treating or reversing metabolic
syndrome, and additional studies will help to clarify this issue.
- Research is needed in diverse populations to determine whether the
effects of physical activity across the range of metabolic health issues,
including metabolic syndrome, T2D, type 1 diabetes (T1D), and gestational
diabetes, differ with race and ethnicity.
- Further examination of the effects of physical activity on metabolic
syndrome and T2D also is warranted to determine whether and how its effects
differ in youth and adults.
- Additional research evaluating dose-response patterns of exercise in
preventing diabetes and cardiovascular outcomes in diabetes would make a
valuable contribution to the metabolic health literature.
- Randomized controlled trials (RCTs) are needed to examine the effects
of exercise on treating T1D in children and adults. Good cardiovascular outcome
data in response to physical activity in T1D is lacking and could potentially
be obtained in adult-onset T1D.
- Clinical studies in post-exercise hypoglycemia are needed to further
study the intermittent high-intensity exercise approach to prevention and to
compare extra carbohydrate versus lower insulin-dosing approaches to treating
- Research is needed on several issues related to gestational diabetes.
For example, RCTs are needed to determine whether physical activity can prevent
gestational diabetes. It also would be useful to have additional dose-response
data on the role of exercise and physical activity in treating gestational
- Additional large scale, multi-site RCTs are needed to more thoroughly
characterize the dose response of physical activity on weight stability, weight
loss, and body composition across a variety of population groups, especially
for those in the normal body mass index range. Only a limited number of RCTs
have addressed these outcomes. Large-scale multi-site RCTs would allow
investigators to more effectively address issues related to susceptibility to
weight gain or resistance to weight or fat loss that may vary by sex,
race/ethnicity, and age. As mentioned in the overarching recommendations, various volumes should
be evaluated within the same study design.
- Determine the most effective strategies for promoting and maintaining
sufficient doses of physical activity to facilitate weight loss and/or weight
stability. It is important to develop effective intervention strategies to
promote and maintain the desired level of physical activity for weight loss
and/or weight stability because adherence to this level of physical activity is
currently less than optimal. Although some strategies have been shown to be
effective for improving adherence to this level of physical activity, the
success of these strategies has been demonstrated in limited samples and
populations. Therefore, additional research in this area is needed.
- Determine how much physical activity is needed to prevent weight
regain following weight loss. Most of the available literature related to this
question is observational or has relied on retrospective analysis of
self-selected and self-reported levels of physical activity. Use of
state-of-the art technology and complete energy balance designs are absent from
the literature. Specifically, it appears that no adequately powered studies of
sufficient duration with randomization have been conducted to examine different
levels of physical activity following weight loss.
- Determine the physical activity effects on total and regional fat
loss from those of weight loss, per se, especially in those people very
susceptible to weight gain in the current social environment and who thus may
be most resistant to weight or fat loss with exercise. Additional RCTs are
needed to distinguish physical activity effects from weight loss effect. In
addition, the large-scale use of imaging techniques is necessary to distinguish
between subcutaneous and visceral fat depots in their responsiveness to
endurance and/or resistance training. The ability of studies to translate
imaging findings into simple anthropometric measures such as the waist or the
abdominal circumference would increase the clinical and personal utility of the
- More research is needed to establish the risks and benefits of
various regimens of physical activity in men and women with a body mass index
of 35 or greater.
- Risk for osteoporotic fractures is strongly influenced both by bone
fragility and by falling. Physical activity is the only therapeutic
intervention that can both increase bone strength and reduce risk for falling.
A large RCT of the effectiveness of physical activity versus anti-resorptive
therapy (e.g., bisphosphonates) on the prevention of fractures is needed.
- Bone mineral density can be measured with a high degree of precision
and remains the best predictor of risk for osteoporotic fracture. However,
studies of animals provide evidence that small increases in bone mineral
density in response to mechanical loading reflect very large increases in bone
strength. Development of better technologies for the non-invasive assessment of
bone strength in humans would provide additional insights into the relative
effectiveness of physical activity to enhance bone strength and reduce fracture
- Dose-response studies are needed to determine the optimal frequency,
intensity and duration of physical activity associated with benefits (minimum
dose) or increased symptoms (maximum dose) among adults with arthritis and
other rheumatic conditions.
- Longitudinal studies of the relationship of lifetime accumulation of
moderate physical activity, particularly walking, and incident arthritis of all
types among the non-elite athlete population are needed. Special attention
should be paid to adequately capture potentially confounding and mediating
Muscle Quantity and Quality
- Studies of the specific modes of physical activity that are most
effective in preventing the age-associated decline in skeletal muscle mass and
function are needed, with a focus on whether age-related changes in other
factors (e.g., nutritional, hormonal) are important mediators of the
- Investigations should identify the underlying mechanisms that limit
the capacity for muscle hypertrophy in response to resistance exercise with
- Design large RCTs to determine whether physical activity can prevent
or delay the onset of functional limitations and/or role limitations in older
adults. Few controlled trials have confirmed the strong evidence from
observational trials that physical activity prevents or delays the onset of
functional and/or role limitations. Given the problem of confounding in
observational studies, large RCTs are needed.
- Determine the dose response of multi-modal activities on improving
functional health and reducing falls. Evidence suggests that
moderate-intensity, multi-modal interventions can help improve functional
health and reduce falls. However, we do not know whether physical activity has
a threshold or dose effect. Studies are needed to determine whether a threshold
below the current recommendations exists and whether higher-intensity
interventions are more or less effective than moderate-intensity
- Determine whether the dose-response effect is relevant to single
component versus multi-modal interventions. We need to know the dose response
for each component of multi-component interventions, not just the dose response
for the total intervention. This would provide information on how to mix
components to achieve maximal benefit for a given amount of time and would help
clarify whether single-mode physical activity interventions would be as
successful at improving functional health as multi-modal interventions. No
trials have addressed this question. Most trials have included multi-modal
interventions in older adults.
- Determine whether physical activity reduces injurious falls (e.g.,
falls that result in fractures) in older adults at risk of falls. Physical
activity reduces falls in older adults at risk of falls; however, little is
known about whether it can reduce injurious falls. An RCT is needed that has
sufficient power to assess whether physical activity can reduce injurious
- Knowledge about the role of physical activity in reducing the risk of
common cancers would benefit from additional evidence gathered from clinical
trials. In the survivorship setting, clinical trials showing a benefit of
physical activity interventions on reducing deaths, recurrences, and reducing
the impact of late or long-term treatment effects also would make a valuable
contribution to our understanding of the needs of this growing population.
- Studies are needed to clarify biological mechanisms linking physical
activity to specific cancers in order to identify associations with less
commonly studied cancers.
- Studies are needed to define the shape of the dose-response curve of
the physical activity-cancer relation in order to determine the effect of
low-intensity activities and accumulated bouts.
- Observational epidemiologic research is needed to identify the dose,
type, and frequency of physical activity on risk of various cancer sites and
subtypes, in addition to identifying the effect of physical activity on risk of
specific cancers within particular population subgroups, including various
races and ethnicities, ages, sexes, and groups at elevated risk of cancer.
- Additional prospective cohort studies and tightly controlled RCTs are
needed, especially for anxiety and sleep disorders. Specifically:
- Additional studies of under-represented groups and of people at
high risk of mental health disorders are needed.
- Selection of potential confounders specific to mental health
risks need to be included in prospective cohort studies.
- Reporting of adherence to and dropout from trials should be
improved, particularly with respect to the impact on the trial's efficacy and
likely population effectiveness.
- Investigators should strive for convergence of subjective and
objective measures of physical activity and should specify the social and
environmental contexts in which physical activity occurs.
- Valid outcome measures need to be selected, refined, and used
- Physical activity exposures and outcomes need to be measured
frequently to permit investigators to model change.
- It would be helpful to conduct additional RCTs comparing the
effects of exercise with other preventive interventions.
- Novel designs that distinguish social moderators and mediators
of outcomes from experimental contamination (i.e., placebo effects) would make
a valuable contribution to the field.
- Studies are needed that manipulate or directly compare standardized
features of physical activity, including type, intensity, and timing, with the
settings in which activity takes place (e.g., group versus solitary, community
versus home, indoor versus outdoor).
- It would be helpful to accelerate the synergy between human brain
imaging studies and neuroscience studies that use animal models of human
disease. This improved synergy could help elucidate biological mechanisms
underlying the benefits of physical activity to mental health. An increased
emphasis on modeling of social-cognitive mediators of mental health outcomes
and studies of gene-environment interactions also would be valuable additions
to the field.
- Determine whether physical activity affects classroom behavior and
academic achievement in children and adolescents.
- Determine whether physical activity affects depression, anxiety, and
cognitive function in children and adolescents.
- Determine the types and amounts of physical activity that are needed
to prevent the development of excessive adiposity during childhood and
- Identify the optimal types and amounts of physical activity needed to
maintain cardiorespiratory and metabolic health during childhood and
- Establish the dose-response pattern for the relation between physical
activity and bone health in children and adolescents.
- Determine how one selects the initial increment (dose) of activity
for individuals who have been inactive that will maximize continued
participation and minimize adverse events. Recommendations have been vague
about the amount of activity a person should initially select.
- Determine how a person should select the size and frequency of
increments to an activity plan for a previously inactive individual that will
maximize continued participation and minimize adverse events. Although a 10%
increase per week has been suggested for youth and young adults, and a 2 to 4
week interval for older adults has been suggested, little research exists to
support such suggestions.
- Determine the incidence and risk factors for adverse events
associated with walking.
- Current literature suggests that risks may be unrelated to either
total volume of walking or intensity (using elevated treadmills). These
findings need to be substantiated in other settings and populations.
- Research is needed on the rate of adverse events in various
populations resulting from participation in various modes of physical activity,
including weight-bearing and resistance training.
- Research is needed to provide evidence-based answers to the following
questions regarding pre-participation medical screening. Does a recommendation
for people to develop an activity plan with a health care provider prevent
adverse events? Does it reduce participation in physical activity? If the
answer to both questions is yes, what is the balance at the population level?
Are such recommendations justified for certain population subgroups? If so,
People With Disabilities
- Prospective cohort studies should be conducted to determine the
frequency, intensity, and duration of physical activity associated with key
health outcomes, including reduction in certain secondary conditions associated
with the specific disability subgroup (e.g., pain in spinal cord injury,
fatigue in multiple sclerosis, reconditioning in intellectual disability).
Studies should be stratified by age, functional level, and severity of
- Multi-center clinical exercise trials should be conducted to achieve
adequate statistical power and to be able to generalize findings to certain
subgroups within the targeted disabilities (e.g., young adults with
paraplegia). A high level of intervention fidelity must be established that
employs the same testing instruments, procedures and training regimen. The
heterogeneity between and within disability groups and the low incidence of
many disabilities make it extremely difficult to obtain an adequate sample size
when recruiting from only one location.
- RCTs are needed to examine the effects of various types of exercise
in addition to the actual training volume (frequency, intensity, duration).
Group exercise such as tai chi or yoga may have additional social benefit,
which may improve outcomes but may also confound the benefit of the specific
dose of exercise. Future studies should control for the social aspect of
exercise so as to obtain accurate data on the exercise regimen itself versus
the social benefits associated with exercising in a group.
- Improved self-report assessment tools should be developed to measure
changes in health in disabled populations. It is difficult to make comparisons
between studies when instruments are not the same or not explained well enough
to make critical comparisons between them. Given the small numbers of many
disabled subgroups, it would be helpful to have a recommended set of
instruments for each targeted outcome with good psychometric properties so that
data from various studies can be compared.
- Development of new and innovative strategies for recruiting disabled
individuals who generally do not volunteer for research studies must become a
high priority. Because most experimental research is conducted with volunteers,
it is difficult to generalize a study's findings to the entire subgroup. People
who volunteer for exercise-related research may be younger and/or have a higher
functional level than the broader population of people with disabilities.
- Determine how self-report measures of social integration and/or
quality of life are associated with objective measures, such as quantifying an
increase in community participation (i.e., increased number of outdoor and/or
social activities, greater amount of time outside the home for social events,
increased employment). The fact that physical activity can improve mental
health and quality of life is an intriguing concept that should be examined in
future research on disabled populations with a more objective and standardized
measurement of these outcomes.
- Develop research designs that categorize subjects by function rather
than disability to increase recruitment and identify key health outcomes that
generalize across disability groups. Given the difficulty in identifying and
recruiting certain populations whose disabilities have low incidence (e.g.,
spina bifida, muscular dystrophy, cerebral palsy), use of the International
Classification of Functioning Disability and Health (ICF) model would allow
researchers to identify specific eligibility criteria by impairments (e.g.,
lower extremity paralysis) and/or activity limitations (e.g., unable to walk)
rather than by disability.
Women During Pregnancy and the Postpartum
- Additional RCTs are needed evaluating activity regimens with
different dose patterns on the course of labor and delivery.
- RCTs are needed to determine whether physical activity will help
prevent gestational diabetes.
- More research is needed on dose response looking at the role of
exercise/physical activity in treating gestational diabetes.
- Studies to compare effects of physical activity during pregnancy and
the postpartum period in diverse race-ethnic groups are needed.
- Research is needed to examine the effect of physical activity in
reducing risk of T2D in women with a history of gestational diabetes.
Racial and Ethnic Diversity
- An increased number of Federally-funded studies should be powered to
include sufficient representation of at least one ethnic/minority or lower SES
population, with sufficient sample size to permit subgroup analyses by
race/ethnicity or SES. Strict exemption criteria should be rigorously
- Cultural proficiency of recruitment and retention approaches and
adequacy of resources directed toward recruitment and retention should be
scrutinized by grant review committee members with special expertise in this
area, similar to the separate assessments of adequacy of study methods and
analytical approaches by review committee statisticians.
- Federal program officers should manage and balance their portfolios
to ensure that racial/ethnic differences in physical activity-related exposures
and outcomes are under active investigation, and should use requests for
applications (RFAs) and other mechanisms to direct funding toward disparities
examination and elimination.
- Journals should require reporting of race/ethnicity, sex, and SES of
samples in the abstract as well as the body of the text.
- Subgroup analyses should be requested when sample size is sufficient,
and further data desegregation should be encouraged to examine interactions
between sociodemographic characteristics, e.g., sex-ethnicity,
- Abstraction databases should include search criteria that permit
ascertainment of inclusiveness (i.e., subgroup analyses by race/ethnicity or
- Specific research questions deserve particular emphasis, such as the
precise role in weight maintenance of racial anthropomorphic variations in
resting or activity-related energy metabolism (as opposed to or in concert with
age- or sex-related differences) in body composition.
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