Contributors to Obesity 127obesity in the Pima Indians of rural Mexico is less than half
that of their Arizona counterparts. Although the two groups
share the same genetic makeup, they differ dramatically in
their lifestyles. The Pimas in rural Mexico consume a diet
with less animal fat and more complex carbohydrates, and
they expend a greater amount of energy in physical labor than
do their cousins in Arizona (Ravussin, Valencia, Esparza,
Bennett, & Schultz, 1994). Thus, environments that foster
appropriate food consumption and energy expenditure can
limit the development of obesity even in the presence of a
strong genetic predisposition.
Alternatively, environments that offer unlimited access to
high-calorie foods and simultaneously support low levels of
physical activity can promote obesity even in the absence of
a speci“c genetic predisposition. As several authors (Hill &
Peters, 1998; Poston & Foreyt, 1999) have noted, the human
gene pool has not changed in the past quarter century. Conse-
quently, the increased prevalence of obesity in the United
States and other Western countries must be due to the in”u-
ence of environmental factors on energy consumption and/or
energy expenditure.
Are Americans eating more food and taking in more calo-
ries? Research on the trends in energy intake has been incon-
clusive (Ernst, Sempos, Briefel, & Clark, 1997; Nestle &
Woteki, 1999). Some surveys (e.g., Norris et al., 1997) show
that energy intake has been declining, whereas others (e.g.,
Centers for Disease Control and Prevention, 1994) suggest
that energy intake has been rising. Because surveys of self-
reported food consumption are susceptible to response bi-
ases, alternative methods of gauging population trend in
energy intake are worth examining. The data from food sup-
ply and disappearance studies show a consistent pattern.
Between 1970 and 1994, per capita energy availability in-
creased by 15% (Harnack, Jeffery, & Boutelle, 2000), an
amount suf“cient to help explain the increased prevalence of
overweight in the United States.
Americans are surrounded by a •toxicŽ environment
that promotes the overconsumption of energy-dense,
nutrient-poor food (Battle & Brownell, 1996; Kant, 2000).
The temptation to eat is virtually everywhere. Tasty, low-
cost, high-calorie items are readily available not only at
fast-food restaurants, but also in supermarkets, food courts,
vending machines, and even 24-hour service stations. In ad-
dition, larger portion sizes, •supersizing,Ž •value meals,Ž
and •2-for-1Ž deals, all provide increased opportunities for
excess consumption. Americans are eating more meals out-
side the home and in doing so they are consuming larger
portions of food. In the early 1970s, about 20% of the
household food dollar was spent on food outside the home
but by 1995 that amount had doubled to 40% (Putnam &
Allshouse, 1996). Importantly, eating away from home, par-
ticularly at fast-food restaurants, is associated with higher
energy intake and with higher fat intake (French, Harnack,
& Jeffery, 2000). Thus, it is not surprising that studies have
shown •eating outŽ to be a signi“cant contributor to weight
gain and the increasing prevalence of overweight (Binkley,
Eales, & Jekanowski, 2000; McCrory et al., 1999).
Physical inactivity also appears to be a signi“cant contrib-
utor to overweight and obesity. Few occupations now require
vigorous levels of physical activity. Moreover, labor-saving
devices such as cars, elevators, escalators, motorized walk-
ways, and remote controls, have had a signi“cant cumulative
impact in decreasing daily energy expenditure (Hill, Wyatt, &
Melanson, 2000; James, 1995). In addition, energy expended
in leisure-time activities has decreased as people spend more
time sitting passively in front of televisions, VCRs/DVD
players, and computers rather than participating in physical
activities that require movement and greater amounts of en-
ergy expenditure. According the Surgeon General (U.S.
Department of Health and Human Services, 1996), 54% of
the U.S. population engages in little or no leisure-time physi-
cal activities and fewer than 10% of Americans regularly par-
ticipate in vigorous physical activity.
Cross-sectional population studies typically show an in-
verse relationship between physical activity and body weight
(DiPietro, 1995). Lower body weights and lower BMIs are as-
sociated with higher levels of self-reported physical activity.
The “ndings appear strongest for high-intensity physical ac-
tivities (presumably due to more accurate reporting of vigor-
ous activities such as jogging). However, in cross-sectional
studies, it is sometimes dif“cult to determine the direction of
cause-and-effect relationships. While physical activity may
affect body weight, it is also likely that body weight impacts
physical activity via increased discomfort associated with
higher body weight, including higher levels of breathlessness
and sweating, and general dif“culty in negotiating body move-
ment. Many obese individuals also report embarrassment at
being seen exercising (Ball, Crawford, & Owen, 2000).
Longitudinal cohort studies may provide a better perspec-
tive on the cause-and-effect relationship between physical
activity and body weight. For example, in the Male Health
Professionals Study, Coakley et al. (1998) examined the im-
pact of changes in activity on body weight in a prospective
cohort study of 19,478 men. The researchers found that over
the course of a four-year period, vigorous activity was asso-
ciated with weight reduction, whereas sedentary behavior
(TV/VCR viewing) and eating between meals were associ-
ated with weight gain. Men who increased their exercise,
decreased TV viewing, and stopped eating between meals,
lost an average weight of 1.4 kg compared to a weight gain of