Introduction

Considerable attention has been placed on the increasing prevalence of obesity in our society. One population-based study

(Mokdad et al., 1999) indicated that the prevalence increased from 12 to 18.9% from 1991 to 1998. Another study (Troiano

& Flegal, 1998), using a different set of national data, indicated that the prevalence of obesity increased from 14.5% to 22.5%

over the past 2 decades. The trends are similar for men and women and are consistent across different ages and

socioeconomic classes. While all of these data are cross-sectional, the magnitude and repeatability of the changes suggest

that these trends reflect real changes in the population prevalence of obesity. 

There are major public health consequences associated with these trends. Obesity has been linked to a variety of serious

diseases and metabolic disorders including Type II diabetes, hypertension, dyslipidemia, coronary heart disease, gall bladder

disease, respiratory diseases, various cancers and osteoarthritis (Pi-Sunyer, 1999). The direct economic costs of treating

obesity in the United States are estimated at over 70 billion dollars (Colditz, 1999). The fact that obesity increases with age

combined with the rapidly expanding elderly population suggests that the problems are likely to even get worse in the near

future. These trends are troubling and have caused obesity to be recognized as one of the leading public health problems

facing our society. The World Health Association released a report on preventing and managing the global epidemic of

obesity (World Health Association, 1998). At the same time, the National Institutes of Health released a report on the

identification, evaluation and treatment of overweight and obesity (National Institutes of Health, 1998). In 1999, the

American College of Sports Medicine convened a meeting to describe the role of physical activity in the prevention and

treatment of obesity and its co-morbidities. These recent developments reveal the current emphasis being placed on

addressing the obesity epidemic. 

Because there is little reason to suspect changes in genetics over this short time frame, the most common explanation for the

recent trends in obesity is that a variety of environmental and behavioral influences have combined to make it harder for

individuals to maintain weight balance. Hill & Melanson (1999) recently

presented a conceptual model describing some of these influences. The model

suggests that the availability of high fat, energy dense foods, coupled with large

portion sizes and low costs of fast foods have combined to drive up energy intake.

Conversely, declines in work related physical activity, reduced activity of daily

living and increases in sedentary behavior tend to drive down daily energy

expenditure values. While some of these changes may be subtle, they appear to be

exerting large influences on the population. 

Obesity is clearly a multifactorial problem but there is some evidence that

physical inactivity may play a more pivotal role in the overall process. An

ecological review of secular trends (Jebb & Moore, 1999) recently suggested that

the prevalence of obesity is more strongly related to decreases in energy

expenditure than to increases in energy intake. DiPietro (1999) reached a similar

conclusion in a review of studies on the prevention of weight gain. She indicated

that habitual physical activity plays a critical role in attenuating the age-related

weight gain that is commonplace in our society. She further suggested that an

increasing amount of activity might be needed to maintain a constant body weight

with age.

Series 2, No. 6 July, 1996

President’s Council on Physical Fitness and Sports 

Series 3, No. 12 Dec. 2000

Published quarterly by the 

President’s Council on

Physical Fitness and Sports

Washington, D.C.

★

Guest Authors:

Gregory J.Welk

Iowa State University, Ames, IA1

Steven N. Blair

The Cooper Institute, Dallas, TX

★

Co-Edited By:

Drs. Chuck Corbin and Bob Pangrazi

Arizona State University

Physical Activity Protects against the Health Risks of Obesity
2

between physical fitness and body composition and their

combined effect on health2. First we will review the

epidemiological evidence for a protective effect of

physical fitness on the health risks associated with

obesity. Probable mechanisms for this protective effect

will then be described. A summary of these observations

will focus on the implications of these findings for the

treatment and prevention of obesity. 

Epidemiological Evidence for the Protective

Effect of Physical Activity

The science of epidemiology is predicated on identifying

and explaining causes of diseases in the population. The

general approach is to compare the prevalence or rates of

diseases in populations exposed to certain potential risk

factors against populations not exposed to these risk

factors. This is typically evaluated with a statistic known

as a relative risk ratio (death rate in those exposed to a

risk factor/death rate in those not exposed to that factor).

Different exposure variables are studied to identify the

factors that increase risks for specific diseases or

outcomes. In these studies, it is important to discount

other competing explanations through progressive

advances in design and more complex analyses.

Examples from the Aerobics Center Longitudinal Study

will be used to describe the progression of

epidemiological research used to examine the protective

effects of physical activity/physical fitness on the health

risks associated with obesity. 

The Aerobics Center Longitudinal Study

The Aerobics Center Longitudinal Study (ACLS) is an ongoing observational study of individuals who have come

to the Cooper Clinic in Dallas, Texas for preventive

medical examinations. A major strength of the database is

that objective measures of cardiorespiratory fitness from

maximal exercise treadmill tests are included in the

comprehensive medical exam. This has allowed the

relationships between physical fitness and a variety of

health outcomes to be examined in a large cohort over an

extended period of time.

Preliminary evidence for a protective effect of fitness

came from a study focused primarily on the benefits of

cardiorespiratory fitness for reducing risks of all cause

mortality (Blair et al., 1989). As a way to document the

persistence of these effects, comparisons were made for

individuals in three different categories of body mass

index - BMI (< 20, 20-25 and > 25). In these analyses,

individuals in the bottom 20% of the age and sex group

distribution for treadmill time were classified as unfit and

While physical inactivity appears to be a strong

contributing factor to the etiology of obesity, studies on

the effectiveness of physical activity in promoting weight

loss have been less than encouraging. Because physical

activity increases energy expenditure, it is reasonable to

assume that the combination of diet plus exercise would

be more effective than dieting alone for weight loss. A

recent review of 13 controlled studies found that this is

rarely the case (Wing, 1999). Only two of the 13 studies

reported statistically significant differences in weight loss

from the diet plus exercise regimen compared to diet

alone. The conclusion from this review was that

"...exercise does not significantly increase initial weight

loss over and above that obtained with diet only." When

the effect of exercise on weight loss was studied without

corresponding dietary modifications, exercise was found

to promote weight loss in only one of five randomized

controlled trials (Ross & Janssen, 1999).

These results present a confusing picture. Physical activity

is clearly viewed as being essential to the prevention of

weight gain but fairly ineffective (at least in clinical trials)

at promoting weight loss. On an individual level, the

failure of exercise to contribute to weight loss in a

predictable manner could be a major deterrent to

continued involvement in an exercise program.

Overweight individuals who begin an exercise program

would likely remain motivated only if they see some

changes in body composition resulting from their efforts. 

On the positive side, recent evidence suggests that

physical activity confers health benefits that are largely or

entirely independent of changes in body composition.

These findings suggest that overweight and obese

individuals can obtain the same benefits of physical

activity as lean individuals. While the overall health

benefits of physical activity have become well accepted,

the general assumption (even among many within the

scientific community) has been that the benefits are

contingent or dependent on corresponding changes in

body composition. While physical activity can lead to

changes in body composition, the amount of change

depends on an individual’s genetics, individual variability

in metabolism and possibly other lifestyle behaviors. The

cellular and metabolic adaptations occurring as a result of

physical activity appear to be independent of these

changes. Therefore, an overweight or obese person can

have good cardiovascular health as long as he/she remains

active and possesses a reasonable level of fitness. This is

clearly an important message that should be conveyed to

individuals who have struggled with weight control all

their lives. 

The purpose of this article is to review the relationships
7

Physical Activity and 

Fitness Quote

“If a larger percentage of population became

physically active, the public health burden 

associated with obesity would be greatly reduced.

This conclusion should not be interpreted to dismiss

the health risks associated with obesity, but rather to

emphasize the moderating influence of physical

activity and physical fitness on these risks.”

Gregory J. Welk 

Iowa State University, Ames, IA 50011

Steven N. Blair

The Cooper Institute, Dallas, TX 75230

Please Post

President’s Council on Physical Fitness & Sports

200 Independence Avenue, S.W., Washington, DC 20201

(202) 690-9000 • FAX (202) 690-5211
4

hypertension, current smoking status, and low fitness).

Low fitness was found to be an independent predictor of

mortality in all body mass index groups even after

adjusting for all of the other mortality predictors. This

suggests that being unfit is a risk that adds to the risks

already associated with other risk factors. The magnitude

of the relative risks for low fitness in this study was as

high or higher than those for the other established comorbidities. 

Summary of Epidemiological Evidence 

The studies reviewed above demonstrate a strong

protective effect for physical activity and physical fitness

on the health risks associated with obesity. Consistent

differences in health risks were evident between fit and

unfit individuals among all body composition categories.

Persistence in the proposed protective effect is also

evident in the fact that the results have been found to be

similar for a number of different indicators of body

composition, including BMI, body fatness, fat free mass

and waist circumference. These effects were also found

to be independent of potential confounding variables as

well as the presence of other risk factors. While the

relationships reviewed here were demonstrated with

fitness as an outcome, similar relationships have been

reported in studies using physical activity as the primary

exposure variable (Paffenbarger & Hyde, 1984).

Additional work is needed but the current findings provide

compelling evidence to support the view that the health

risks of obesity can be largely managed through physical

activity.

The reason this evidence has been relatively slow to

surface is that few epidemiological studies have examined

physical activity and body composition as related, but

independent, exposures. With respect to health, studies

linking physical activity and fitness with health typically

control for some aspect of body composition. In contrast,

studies linking body composition with health have rarely

considered the moderating influence of physical activity

and fitness. The erroneous assumption has been that

overweight or obese individuals would, by default, be

unfit. In a comprehensive review of the current literature

on this topic, Blair and Brodney (1999) identified 24

articles that evaluated health outcomes across levels of

both physical activity/fitness and some indicator of body

composition. Sufficient data were available to summarize

outcomes of all-cause mortality, coronary heart disease,

hypertension, type 2 diabetes mellitus and cancer. The

studies consistently showed that active or fit men and

women were protected against the health risks of

overweight or obesity. The authors concluded that

"...active obese individuals actually have lower morbidity

and mortality than normal weight individuals who are

sedentary." (p. S646).

Probable Mechanisms for a Protective

Effect

Physical activity provides protection against the health

risks of obesity primarily by reducing and/or reversing the

development of a progressive disease process known as

insulin-resistance syndrome or Syndrome X. This

syndrome is closely associated with obesity and is

characterized by the clustering of insulin resistance and

hyperinsulinemia, dyslipidemia, essential hypertension,

glucose intolerance, and an increased risk of non-insulin

dependent diabetes mellitus and cardiovascular disease

(Landsberg, 1996; Opara & Levine, 1997; Timar, Sestier,

& Levy, 2000). Before describing the moderating

influence of physical activity, the general mechanisms

underlying insulin resistance syndrome will first be

described. 

While research is still accumulating on the specific

mechanisms, the general disease process is related to the

presence of abdominal body fat - particularly visceral fat

(Despres, 1993; Chisholm, Campbell, & Kraegen, 1997).

Abdominal body fat is characterized by an increased

responsiveness to lipoprotein lipase. Because of its high

lipolytic activity, abdominal adipocytes readily release

free fatty acids (FFA) into the circulation. These FFA are

carried directly to the liver through the portal circulation

where they are converted into very-low density

lipoproteins (VLDL) and ultimately LDL cholesterol. The

high levels of FFA may also lead to enhanced lipid

oxidation and reduced glucose oxidation. These changes

can result in high levels of blood glucose and lead to a

reduced sensitivity to insulin. The increased levels of

FFA in the portal circulation also act directly to inhibit

insulin clearance by the liver resulting in

hyperinsulinemia. High levels of insulin in the presence of

elevated glucose tend to promote reductions in insulin

sensitivity and hasten the development of non-insulin

dependent diabetes mellitus. The increased insulin levels

also are associated with increases in blood pressure as

insulin is thought to increase sodium retention in the

kidney. Thus, high levels of abdominal obesity are directly

related to dyslipidemia (elevated levels of LDL and

triglycerides), hypertension, hyperinsulinemia, reduced

glucose sensitivity and ultimately non-insulin dependent

diabetes mellitus. 

Physical activity induces beneficial metabolic changes
5

that limit the progression of this underlying disease

process. A major benefit of physical activity is that it leads

to specific reductions in levels of abdominal obesity (Ross

& Janssen, 1999). The enhanced lipolytic activity of these

fat cells allows fatty acids to be released and metabolized

more readily than other fat depots. While overall weight

loss from exercise is limited, studies have indicated that

changes in abdominal body fat levels can occur without

corresponding weight loss or total fat loss. Studies have

also demonstrated that physical activity has a direct effect

on improving overall metabolic profiles (e.g.

normalization of insulin levels and improved glucose

homeostasis). In a review of these studies (Kelley &

Goodpaster, 1999), the improvements in insulin action

were found to occur without concomitant changes in

weight and/or body composition. While there appears to

be some chronic adaptations that occur as a result of

regular exercise performed over a period of time, there are

also observations that indicate that some benefits are

related to the acute effects following individual bouts of

activity. The fact that physical activity exerts some of its

positive influence independent of changes in body

composition is consistent with the findings from the

epidemiological studies reviewed earlier.

Summary and Conclusions

The general assumption in our society is that overweight

or obese individuals are probably physically inactive and

unfit. Conversely, most people would assume that a thin

individual is probably physically active, physically fit and

healthy. These characterizations are gross

oversimplifications and don’t take into account the

complex metabolic relationships that influence body

composition, fitness and health. It is clearly possible for

overweight individuals to maintain high levels of fitness.

Through participation in regular physical activity, it is also

possible for overweight individuals to have good health

and low risks for chronic disease. 

While the public health concerns about the increasing

prevalence of obesity are well founded, they may be

misplaced. The epidemiological studies reviewed here

suggest that the health risks of obesity are largely

controlled if a person is physically active and physically

fit. The protection appears to come, at least in part, from

positive metabolic changes that occur as a result of regular

participation in physical activity. Because most of these

changes have been found to occur independent of changes

in body composition it may prove more successful to

promote physical activity for its own sake - without

emphasizing or expecting corresponding changes in body

composition. If a larger percentage of the population

became physically active, the public health burden

associated with obesity would be greatly reduced. This

conclusion should not be interpreted to dismiss the health

risks associated with obesity, but rather to emphasize the

moderating influence of physical activity and physical

fitness on these risks. The implications of this distinction

for the clinical treatment of obesity and for health

promotion in general are described below. 

Implications for Treatment

With regard to treatment, physicians routinely counsel

patients about the health risks of obesity and typically

prescribe various weight loss treatments to address the

issue. Guidelines published by the National Institutes of

Health (1998) recommend that physicians test for the

presence of other coexisting diseases (co-morbidities)

such as diabetes and hypertension since the presence of

additional risk factors has been shown to compound

overall health risks. The results reviewed here suggest that

low fitness (and/or lack of physical activity) should be

considered as equally important when evaluating health

status of overweight or obese individuals. As previously

stated by Wei et al., (1999), it is as important for a

clinician to assess an obese patient’s fitness status as it is

to measure fasting plasma glucose an cholesterol, evaluate

blood pressure and inquire about smoking habits. 

Implications for Health Promotion

Weight loss is frequently cited as one of the most common

reasons for beginning an exercise program. In light of the

negative results from studies on physical activity and

weight loss, continued efforts in this direction may be

counterproductive. Many obese individuals have become

frustrated with exercise since their efforts often do not

lead to corresponding changes in weight or body

composition. Rather than encouraging these individuals to

lose weight it is more appropriate and probably more

effective to encourage them to just become physically

active. If regular physical activity is performed, physical

fitness will improve to the extent possible and lead to

positive metabolic outcomes. These changes will provide

significant benefits for health regardless of any changes in

body weight. The focus on behavior rather than an

outcome (e.g. weight loss) also offers motivational

advantages for obese individuals since it increases the

likelihood that they can be successful and feel positive

about their efforts. Specific guidelines and strategies for

using physical activity in this way have previously been

published (Leermakers, Dunn, & Blair, 2000).
6

The President’s Council on 

Physical Fitness and Sports Research Digest 

is now available on-line at http://www.indiana.edu/~preschal

Lee Haney, Chairman

Elizabeth Arendt, M.D., St. Paul, MN

Billy Blanks, Sherman Oaks, CA

Jeff Blatnick, Halfmoon, NY

Ralph Boston, Knoxville, TN

Don Casey, East Rutherford, NJ

Timothy Finchem, Ponte Vedra Beach, FL

Rockne Freitas, Ed.D., Honolulu, HI

Zina Garrison, Houston, TX

Lauren Gregg, Charlottesville, VA

Jimmie Heuga, Avon, CO

Jim Kelly, Buffalo, NY

Judith Pinero Kieffer, Los Angeles, CA

Deborah Slaner Larkin, Pelham, NY

Nikki McCray, Washington, D.C.

Albert Mead III, Atlanta, GA

Jack Mills, Columbia, SC

Ellen Hart Peña, Denver, CO

Ken Preminger, Atherton, CA

Amber Travsky, Laramie, WY

Executive Director—Sandra Perlmutter

The President’s Council on Physical Fitness and Sports

The President’s Council on Physical Fitness and Sports (PCPFS) was established in 1956 through an Executive Order by

President Dwight D. Eisenhower as part of a national campaign to help shape up America’s younger generation. Today, the

PCPFS serves as an advisory council to the President and Secretary of the Department of Health & Human Services on

matters involving physical activity, fitness and sports to enhance and improve the health of Americans of all ages.

The PCPFS enlists the active support and assistance of individual citizens, civic groups, private enterprise, and voluntary

organizations to promote and improve the physical activity and fitness of all Americans and to inform the public of the

important link which exists between regular activity and good health.

Twenty (20) individuals from the sports, fitness and health fields are appointed by the President to serve as members of the

Council. They are:

Please note that the appropriate language for the citation of this resource is:

The President’s Council on Physical Fitness and Sports Research Digest.

200 Independence Avenue, S.W., Washington, DC 20201 • (202) 690-9000 • FAX (202) 690-5211

It is important to point out that only modest amounts of

physical activity are needed to attain some protection

against the health effects of obesity. The cutpoints used to

define the high risk (low fitness) category in the ACLS is

the lowest 20% of the age-specific fitness distributions for

both men and women. When expressed in absolute levels

of fitness, the cutpoints for young adults (20-39 years) are

approximately 8.1 and 10.5 METS (or a VO2 max of 28 –

35 ml/kg/min), for women and men, respectively. Similar

cutpoints for middle aged adults (40-59 years) are

approximately 7 and 9.5 METS (or a VO2 max of 25 – 33

ml/kg/min) . These fitness levels can be achieved with

modest amounts of physical activity. A recent study

examining activity levels of ACLS participants (Stofan,

DiPietro, Davis, Kohl, & Blair, 1998) indicated that an

average leisure time energy expenditure of 525 to 1650

kcal per week for males and 420 to 1260 kcal/week for

females were associated with moderate to high levels of

fitness. These levels of activity can easily be attained by

following the current public health guidelines for physical

activity, which recommend 30 minutes of moderate

activity on most days of the week (Pate et al., 1995). 
3

those in the top 40% were classified as high fit. The

results suggested that fit individuals with higher BMI

values had lower mortality rates than unfit individuals

with low BMI’s. Because this study only examined

moderate levels of overweight (BMI > 25), additional

confirmation was required to test the strength of the

findings. 

A subsequent study (Barlow, Kohl, Gibbons, & Blair,

1995) was conducted to examine similar relationships

among a larger sample (nearly 26,000) of men across

three higher BMI ranges (< 27, 27-30 and > 30). In all

three BMI categories, moderately fit individuals and

highly fit individuals were found to have lower relative

risk for all-cause mortality than individuals in the unfit

category. A follow-up study (Lee, Jackson, & Blair, 1998)

repeated the analyses after excluding men with previous

myocardial infarction, stroke or cancer at baseline and

adjusting for exam year, smoking habit and alcohol intake.

The definition of unfit participants was the same as in

previous studies but in this study, all individuals above the

20th percentile were classified as "fit". Comparisons also

were made using the fit and lean individuals (< 25 BMI)

as the reference group rather than making comparisons

separately for each BMI category. Unfit individuals in

each BMI category had greater risk of dying as compared

with the fit and lean group (relative risks ranged from 1.68

to 2.24). An interesting observation from this study was

that there were no differences in relative risk for fit

individuals in the three different BMI categories. Fit

overweight men had no greater risk than fit lean men

(See Figure 1).

Figure 1.

A limitation of the previous studies was a reliance on BMI

as the indicator of body composition. To address this

issue, another study (Lee, Blair, & Jackson, 1999)

examined the relationship of cardiorespiratory fitness to

mortality with more specific measures of body

composition. The study evaluated the risks for all-cause

mortality among men who had received a body

composition assessment (either sum of 7 skinfolds or

underwater weighing) and a cardiorespiratory fitness

assessment. Individuals were classified as lean if they had

body fat levels less than 16.7%, normal if they had body

fat levels from 16.7 - 25% and obese if they had body fat

levels greater than 25%. Relative risks for unfit

individuals ranged from 1.62 to 2.07 compared with lean

and fit men (See Figure 2). The investigators also

examined the relationships on a subsample of participants

who also had waist circumference measurements. These

participants were divided into 3 waist girth strata (< 87

cm, 87 to 99 and > 99). The relative risks for these

comparisons were even higher – ranging from 2.05 to

4.88. The greatest risk was found for the lean unfit group

but this may have been due to the smaller sample from

this particular category. The main message from the study

was that risks are higher for unfit men at all three waist

circumference categories. 

Figure 2.

The most comprehensive analysis on the topic compared

the relative importance of various cardiovascular risk

factors on cardiovascular disease and all-cause mortality

across different body composition categories (Wei et al.,

1999). In this study, the investigators categorized men

from the ACLS database into three different BMI levels (<

25, 25-29.9 and > 30) and then sub-classified by the

presence or absence of six different mortality predictors

(cardiovascular disease, type II diabetes, cholesterol,
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1

The concepts behind this originated while Dr. Welk was the Director of the Childhood and Adolescent

Health Division, Cooper Institute.

2 Much of the content for the present article was based on the recent ACSM Roundtable meeting and the subsequent

publication of various review articles (Blair & Bouchard, 1999).