The current epidemic of obesity is caused largely by an environment
that promotes excessive food intake and discourages physical activity.
Although humans have evolved excellent physiological mechanisms to
defend against body weight loss, they have only weak physiological
mechanisms to defend against body weight gain when food is abundant.
Control of portion size, consumption of a diet low in fat and energy
density, and regular physical activity are behaviors that protect
against obesity, but it is becoming difficult to adopt and maintain
these behaviors in the current environment. Because obesity is
difficult to treat, public health efforts need to be directed toward
prevention.
J. O. Hill is director of the Colorado Clinical Nutrition
Research Unit, University of Colorado Health Sciences Center, Denver,
CO 80262, USA.
J. C. Peters is with the Procter and Gamble Company, Cincinnati,
OH 45224, USA.
*
To whom correspondence should be addressed. E-mail:
james.hill@uchsc.edu
Obesity has reached epidemic
proportions in the United States and is threatening to become a global
epidemic (1). According to the classification scheme
established by the World Health Organization (1), 54% of
U.S. adults are overweight [a body mass index (BMI) 25 kg/m2] and 22% are obese (BMI 30 kg/m2) (2). The prevalence of overweight has
risen dramatically over the past two decades, and if this trend
persists, the entire U.S. adult population could be overweight within a
few generations (3). This alarming increase is also present
among the nation's youth; 25% of U.S. children are overweight or
obese (4). Obesity represents a serious threat to health
because it increases the risk of developing many chronic diseases, such
as diabetes and cardiovascular disease (5).
What is causing the dramatic rise in overweight among the
population? Although research advances have highlighted the importance of molecular genetic factors in determining individual susceptibility to obesity, the landmark discoveries of leptin, uncoupling proteins and
neuropeptides involved in body weight regulation, cannot explain the
obesity epidemic. Our genes have not changed substantially during the
past two decades. The culprit is an environment which promotes
behaviors that cause obesity. To stop and ultimately reverse the
obesity epidemic, we must "cure" this environment.
What behaviors contribute to obesity, and how does the environment
foster these behaviors? On the simplest level, obesity can arise only
when energy intake exceeds energy expenditure. Our current environment
is characterized by an essentially unlimited supply of convenient,
relatively inexpensive, highly palatable, energy-dense foods, coupled
with a lifestyle requiring only low levels of physical activity for
subsistence. Such an environment promotes high energy intake and low
energy expenditure. Under these circumstances, obesity occurs more
frequently because, while the body has excellent physiological defenses
against the depletion of body energy stores, it has weak defenses
against the accumulation of excess energy stores when food is abundant.
An individual's body weight and body composition are determined
by interactions between the environment and genetics (6). The environment's contribution to obesity must be thought of in terms
of how it increases the frequency of behaviors that increase the risk
of positive energy balance. With positive energy balance, body mass
increases in order to restore energy balance. In this sense, obesity
can be viewed not as a result of defective physiology, but as the
natural response to the environment. Within any given environment, an
individual's becoming obese is not a certainty, but an event that
occurs with a certain probability. Some individuals can avoid obesity
in unsupportive environments by maintaining a pattern of healthy
behaviors. Genetic makeup also plays a role in that it determines the
strength of an individual's physiological defense against gaining and
maintaining an obese body fat level. Genetic factors are critically
important for determining how different individuals respond within a
given environment. This is best illustrated by the differences in body
weight among individuals living in a common environment.
In a society in which food availability is not limited, weight
maintenance, whether at normal or elevated body fat level, is
accomplished primarily by the regulation of food intake (7). Small differences in metabolic efficiency between individuals, historically the focus of much research, are insufficient to explain the prevalence of obesity. For any given genotype, resting energy expenditure has only a limited capacity to adjust to changes in food
intake in order to maintain energy balance. Even under extraordinary circumstances such as fasting or forced overfeeding, energy expenditure is changed by only 5 to 10% (8). Changes of this magnitude are insufficient to blunt the effects of large alterations in food
intake on body weight and composition.
What Environmental Factors Promote Overeating?
Food availability and portion size. One way in which
the current environment promotes obesity is by providing more frequent opportunities for the consumption of large quantities of food. A
variety of highly palatable, inexpensive foods is available nearly
everywhere. Compounding this is a growing trend in the United States
toward larger portions. This is especially evident in so-called fast
food restaurants, where "super sizing" of menu items is
commonplace. Our culture's apparent obsession with "getting the best
value" may underlie the increased offering and selection of larger
portions and the attendant risk of obesity.
High-fat diets. The effects of diet composition on the
development of obesity can be clearly seen in animal models. Obesity is
rare in experimental animals maintained on a low-fat diet, even when
they are housed in small cages that limit physical activity. In
contrast, providing sedentary animals with ad libitum high-fat diets
(35% of energy from fat) reliably produces increases in energy
intake, increases in efficiency of body fat gain, and obesity (9). For example, the average percent body fat of mice was found to increase in direct proportion to the percentage of energy as
fat in the diet (10). Furthermore, although the
overall prevalence of obesity increased as dietary fat increased, so
did the variation in response between different animals fed the same
level of dietary fat. At the highest levels of dietary fat, some
animals became markedly obese, most gained significant amounts of body
fat, and a few did not gain appreciable fat compared with control
animals fed very low-fat diets.
Studies in humans also support a role for dietary fat in the
development of obesity. In numerous studies, total energy intake was
higher when subjects consumed diets relatively high in fat than when
they ate lower fat diets (11). Although the primary impact
of high-fat diets on obesity may be through effects on food intake,
body fat storage also occurs at a greater rate when excess energy comes
from fat than when it comes from carbohydrate or protein
(8).
Behavioral factors can modulate the effect of dietary fat on the
development of obesity. A recent report (12) described the
results of a 6-month trial in which subjects came to a research "supermarket" to obtain either full-fat or reduced-fat foods. Subjects who selected full-fat foods had, on average, a higher energy
intake and gained weight during the trial, whereas subjects selecting
from reduced-fat foods did not increase energy intake or change body
weight. Dietary restraint (a measure of the extent to which conscious
control is exerted on food intake) provided protection against
the obesity-promoting effect of the high-fat, high-energy density
diet. Restrained eaters, as compared with unrestrained eaters, avoided
increases in energy intake and weight gain on the full-fat diet and
lost weight on the low-fat diet. This suggests that careful control of
food intake can prevent weight gain on a high-fat diet. The advantage
of the low-fat diet was that weight gain was avoided without the
need for high levels of dietary restraint.
The apparent effect of fat per se on energy intake in these studies is
difficult to separate from an effect of energy density. Subjects tended
to eat a constant weight of food on both high- and low-fat diets.
Because fat provides more energy per gram than other nutrients,
high-fat foods have a higher energy density than low-fat foods. It is
possible that the energy density of the diet rather than the dietary
fat was responsible for the increased energy intake. This hypothesis is
supported by studies which show that energy intake varies with energy
density when dietary fat content is maintained at a constant level
(13).
Practically speaking, the energy density of many foods, especially
those with a caloric density above 3 to 4 kcal/g, varies directly with
the percentage of fat in the foods. However, an increasing number of
fat- and calorie-modified foods that are low in fat have an energy
density that is as high as or higher (because of low fiber and water
content) than that of either the foods they replaced or more
traditional low-fat foods such as whole grains, fruits, and vegetables.
Including these novel high-energy foods in the diet may not limit total
energy intake as would occur when composing a diet from among
more-traditional low-fat foods. Recognizing the importance of energy
density is a step forward in understanding how food composition affects
total energy intake, but reductions in dietary fat may still be the
most effective means of reducing the likelihood of excessive energy
consumption. Although not all energy-dense foods are high in fat, few
high-fat foods are low in energy density.
The debate over dietary fat and obesity. Not all
investigators agree that dietary fat promotes the development of
obesity (14). Proponents of a counter viewpoint argue that
obesity prevalence has increased whereas the percentage of energy
intake from dietary fat has declined, and that reductions in dietary
fat produce only modest reductions in body weight. While it is true
that dietary surveys indicate that fat as a percentage of energy intake
has declined from 37 to 34% over the past decade, fat intake in grams per day has remained essentially constant at 80 g/day over the same
period (15). Further, some investigators have suggested that
in the wake of public education to reduce fat intake, dietary fat
intake may be underreported, so that the dietary fat estimates from
these recent dietary surveys may be low.
The major impact of reducing dietary fat is likely not on
reversing obesity but on preventing weight gain in the first place. Given the strength and redundancy of physiological mechanisms defending
against weight loss, it is remarkable that reductions in dietary fat
(without specifying reductions in total energy intake) have been
uniformly associated with some, albeit modest, loss of body weight
(16).
Laboratory studies in rodents and humans have consistently demonstrated
the obesity-producing effects of high levels of dietary fat, but there
are as yet no large prospective trials testing the hypothesis that
reducing dietary fat and energy density can prevent obesity. This
should be an important priority for future research.
What Environmental Factors Promote Physical Inactivity?
Low levels of physical activity are associated with an
increased risk of obesity (17), and our current environment
tends to discourage physical activity. Advances in technology and
transportation have reduced the need for physical activity in daily
life. The appeal of television, electronic games, and computers has
increased the time spent in sedentary pursuits among children and
adults alike. A large portion of our population already lives a
sedentary life, and it is difficult to imagine that this trend will not continue in the future. A low level of physical activity is associated with a low daily energy requirement and will cause obesity unless food
intake is limited accordingly.
Facilitating this trend is the fact that most children in the United
States do not engage in daily physical activity at school. Cutbacks in
mandatory physical education programs have contributed to overall
declines in children's physical activity levels. Even when these
programs are available, they are often taught by untrained individuals,
involve little actual physical activity, and do not focus on the fun
aspects of physical activity (17).
Interaction of Food Intake and Physical Activity
What do we really know about the interaction of food intake
and physical activity patterns? Will consumption of a low-fat, low-energy
density diet prevent obesity in sedentary individuals, and will regular
physical activity protect even those people eating a high fat,
high-energy density diet? The answers to these questions seem obvious,
but there is little direct empirical evidence to prove the
effectiveness of these strategies. Clinical data, however, provide
strong evidence that physical activity mitigates the effects of
fat-rich diets on energy balance. In a series of studies, subjects were
given ad libitum access to low-, medium-, and high-fat diets under
sedentary or physically active conditions (18). At each level of dietary fat, the sedentary condition produced more positive energy balance than the active condition, highlighting the protective effect of physical activity. For each level of activity, higher dietary
fat increased the degree of positive energy balance, highlighting the
obesity-promoting effect of fat (energy density). Overall, energy
balance under any condition was determined not by diet or activity
alone, but by the interaction between the two.
Figure 1 shows the hypothetical
risk of developing obesity in subjects eating a high-energy density
diet (for example, a typical Western diet). These individuals can
maintain a relatively low risk of obesity by engaging in high
levels of physical activity, by high dietary restraint, or by a
combination of moderate activity with some dietary restraint. For
sedentary individuals, avoidance of positive energy balance and obesity
will likely require a high level of dietary restraint or consumption of
a diet low in energy density. Figure 1 illustrates what would happen
with a high-energy density diet. As the energy density of the diet
declines, the height of all of the bars in the figure should be
reduced.
Fig. 1.
Hypothetical risk of obesity in individuals
consuming a diet high in energy density. This risk can be modified by
physical activity and by conscious limitation of total energy intake
(dietary restraint). As the energy density of the diet decreases, the
risk of obesity (the height of all of the bars) would be expected to decline.
[View Larger Version of this Image (56K GIF file)]
What Can We Do to Cure the Environment?
To combat the epidemic of obesity, we must first cure the
environment. There are at least three major ways to promote behaviors that protect against obesity. One step would be a consumer education effort to reduce portion sizes, which may help to limit opportunities for "passive overeating." The food and restaurant industries should be encouraged to take responsible actions by reducing portion sizes,
especially of high-energy density foods.
A second step would be to increase the availability of foods that are
low in fat and low in energy density. Simply telling people to eat such
diets will bring only limited success, given the current food supply.
Foods that are naturally low in fat and energy density, such as fruits,
vegetables, and whole grains, should be made easily available and
affordable in both restaurants and grocery stores. The development of
more low-energy density foods that taste as good as the high-energy
density versions may also facilitate consumption of low-energy density
diets.
We also need to foster a preference for less energy-dense foods
in young children. We must learn more about how children's eating
patterns develop and how they can be modified. Studies suggest that
very young children are good regulators of energy intake
(19), and that they can "unlearn" this regulation over time. Consistently asking a child to "clean your plate," for
example, may accustom them to ignore or override physiological
satiety cues.
Successful public health efforts in the area of diet may require
collaboration among the agricultural and food industries (to provide
appropriate foods), educators (to promote healthy choices), government
(to provide incentives), and researchers (to investigate the mechanisms
underlying the regulation of food intake).
A third major step would be to make the environment more
conducive to physical activity. Increasing the physical activity level
of the general population is an enormous challenge. Public health
efforts could involve development of appropriate incentives. People who
engage in regular physical activity generally have fewer health
problems and fewer days absent from work than sedentary people. Active
individuals could be rewarded with reduced insurance costs or
additional vacation time, for example.
Schools should encourage children to engage in daily physical activity.
Trained instructors should teach basic physical education skills and
expose children to fun physical activities that set the stage for
lifelong habits. There are critical research needs in this area. We
need to understand more about the factors within the environment that
affect physical activity patterns, how these patterns develop in
children, and how they "track" into adulthood. We also need to
investigate how much physical activity is required to prevent the
development of obesity under different environmental circumstances and
at different ages. We need to find creative ways to counter attractive
sedentary pursuits. Children are more likely to participate in physical
activity if the activity is fun and if parents also participate.
It is unlikely that we will reverse a sedentary lifestyle solely by
promoting increased leisure time activity. We must also increase
physical activity in the routines of daily life--for example, by
occasionally using physical activity rather than meals as the focal
point of family gatherings.
What Are the Barriers to Altering the Environment to Prevent
Obesity?
Health professionals, the general public, and policy-makers have
not recognized obesity as a serious threat to health. Despite overwhelming evidence about the consequences of obesity and clear indications that obesity has reached epidemic proportions, it remains
low on the list of important public health problems.
A second barrier is the perception that we do not know how to
prevent obesity. Evidence from the National Weight Control Registry (NWCR), which monitors individuals who have successfully maintained a
weight reduction for at least 1 year, suggests that we can prevent weight gain by promoting the behavioral changes suggested above. These
reduced-obese individuals (who have maintained an average weight loss
of 67 pounds for an average of 5 years) are at high risk for weight
gain. They report that their success in weight maintenance is due to
consumption of a low-fat diet, low total energy intake, and high levels
of regular physical activity (20). The failure of many
people to avoid obesity may be due to a failure to adopt and maintain
these behaviors in our current environment.
Altering the environment to encourage behaviors that prevent
obesity may appear to be an insurmountable challenge. Changing the
environment to reduce cigarette smoking must have seemed equally insurmountable in the 1960s, yet partnerships among educators, government, and industry have led to substantial reductions in the
number of people who smoke. Currently, the rate of HIV infection is
declining as a result of public health education that increased awareness of the specific behaviors that were leading to the spread of
AIDS. Dealing with obesity may require a similar strategy. We must
begin now to form the necessary partnerships to develop cost-effective
strategies to stem the obesity epidemic. It may be decades before such
efforts realize success, but the dramatic increase in obesity
prevalence suggests that we are rapidly losing the opportunity to
prevent this public health threat.
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We thank A. Kriketos for help in preparing this
manuscript. Supported by NIH grants DK42549, DK38088, and
DK48520. Because of space limitations, it was not possible to
include a comprehensive list of references for all of the work
discussed. References (1) and (6) provide a
more comprehensive reference list. The Procter and Gamble Company
markets both regular and low-fat food
products.
Volume 280, Number 5368
Issue of 29 May 1998,
pp. 1371 - 1374
©1998 by The American Association for the Advancement of Science.
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Copyright © 1998 by the American Association for the Advancement of Science.