Obesity 165an agricultural society to an industrial and increasingly information-based
society. As a simple example, a telephone company in the US has suggested
that in the course of one year an extension phone saves an individual approx-
imately one mile of walking, which could be the equivalent of 2–3 lbs of fat
or up to 10,500 kcals (Stern, 1984). Think how much energy mobile phones
now save! Furthermore, at present only 20 percent of men and 10 percent
of women are employed in active occupations (Allied Dunbar, 1992) and
for many people leisure times are dominated by inactivity (Central
Statistical Office, 1994). Although data on changes in activity levels are pro-
blematic, there exists a useful database on television viewing which shows
that whereas the average viewer in the 1960s watched 13 hours of television
per week, in England this has doubled to 26 hours per week (General
Household Survey, 1994). This is further exacerbated by the increased use
of videos and computer games by both children and adults. It has therefore
been suggested that obesity may be caused by inactivity. To examine the
role of physical activity in obesity, research has asked, “Are changes in obesity
related to changes in activity?” “Do the obese exercise less?” “What effect
does exercise have on food intake?” and “What effect does exercise have
on energy expenditure?” These questions will now be examined.
Are changes in obesity related to changes in activity?
This question can be answered first by using epidemiological data on a
population, and second by using prospective data on individuals.
In 1995, Prentice and Jebb published a paper entitled “Obesity in Britain:
Gluttony or Sloth?” in which they presented epidemiological data on changes
in physical activity from 1950 to 1990 as measured by car ownership and
television viewing and compared these to changes in the prevalence of obe-
sity. The results from this study suggested a strong association between an
increase in both car ownership and television viewing and an increase in
obesity. These results are shown in figure 8.4. Prentice and Jebb (1995) argued
that “it seems reasonable to conclude that the low levels of physical activity
now prevalent in Britain must play an important, perhaps dominant role
in the development of obesity by greatly reducing energy needs.” However,
their data were only correlational. Therefore it remains unclear whether
obesity and physical activity are related (the third-factor problem – some
other variable may be determining both obesity and activity) and whether
decreases in activity cause increases in obesity, or whether in fact increases
in obesity actually cause decreases in activity. In addition, the data are at
the population level and therefore could miss important individual differences