46 Introduction to Human Nutrition
Etiology of obesity: excess intake or
decreased physical activity
Stated simply, obesity is the end-result of positive
energy balance, or an increased energy intake relative
to expenditure. It is often stated, or assumed, that
obesity is simply the result of overeating or lack of
physical activity. However, the etiology of obesity is
not as simple as this, and many complex and interre-
lated factors are likely to contribute to the develop-
ment of obesity; it is extremely unlikely that any
single factor causes obesity. Many cultural, behav-
ioral, and biological factors drive energy intake and
energy expenditure, and contribute to the homeo-
static regulation of body energy stores, as discussed
earlier in the chapter. In addition, many of these
factors are infl uenced by individual susceptibility,
which may be driven by genetic, cultural, and hor-
monal factors. Obesity may develop very gradually
over time, such that the actual energy imbalance is
negligible and undetectable.
Although there are genetic infl uences on the various
components of body-weight regulation, and a major
portion of individual differences in body weight can
be explained by genetic differences, it seems unlikely
that the increased global prevalence of obesity has
been driven by a dramatic change in the gene pool. It
is more likely and more reasonable that acute changes
in behavior and environment have contributed to the
rapid increase in obesity, and genetic factors may be
important in the differing individual susceptibilities
to these changes. The most striking behavioral changes
that have occurred have been an increased reliance on
high-fat and energy-dense fast foods, with larger
portion sizes, coupled with an ever-increasing seden-
tary lifestyle. The more sedentary lifestyle is due to an
increased reliance on technology and labor-saving
devices, which has reduced the need for physical activ-
ity for everyday activities. Examples of energy-saving
devices are:
● increased use of automated transport rather than
walking or cycling
● central heating and the use of automated equip-
ment in the household, e.g., washing machines
● reduction in physical activity in the workplace
due to computers, automated equipment, and elec-
tronic mail, which all reduce the requirement for
physical activity at work
● increased use of television and computers for enter-
tainment and leisure activities
● use of elevators and escalators rather than using
stairs
● increased fear of crime, which has reduced the like-
lihood of playing outdoors
● poor urban planning, which does not provide
adequate cycle lanes or even pavements in some
communities.
Thus, the increasing prevalence, numerous health
risks, and astounding economic costs of obesity clearly
justify widespread efforts towards prevention.
The relationship between obesity and lifestyle
factors refl ects the principle of energy balance. Weight
maintenance is the result of equivalent levels of energy
intake and energy expenditure. Thus, a discrepancy
between energy expenditure and energy intake de-
pends on either food intake or energy expenditure,
and it is becoming clear that physical activity provides
the main source of plasticity in energy expenditure.
In addition, lifestyle factors such as dietary and activ-
ity patterns are clearly susceptible to behavioral mod-
ifi cation and are likely targets for obesity prevention
programs. A second, yet related, reason that control
of the obesity epidemic will depend on preventive
action is that both the causes and health consequences
of obesity begin early in life and track into adulthood.
For example, both dietary and activity patterns
responsible for the increasing prevalence of obesity
are evident in childhood.Role of physical activity and energy
expenditure in the development of obesity
Although it is a popular belief that reduced levels of
energy expenditure and physical activity lead to the
development of obesity, this hypothesis remains con-
troversial and has been diffi cult to prove. There are
certainly good examples of an inverse relationship
between physical activity and obesity (e.g., athletes
are lean and nonobese individuals), as well as good
examples of the positive relationship between obesity
and physical inactivity (obese individuals tend to be
less physically active). However, not all studies provide
supporting evidence. For example, several studies
suggest that increased television viewing (as a marker
for inactivity) increases the risk of obesity, whereas
others do not. Similar to the results for physical activ-