Obesity 167gain. The results showed that those who reported no physically active leisure
time both at baseline and follow-up, and those who showed reduced activity
by follow-up, were twice as likely to have gained more than 5 kg over the
10 years than those who reported two sessions of vigorous activity per week.
In a similar vein, Williamson et al. (1993) examined the relationship between
activity and weight gain in over 9,000 US men and women. The results
from this study also showed that low levels of physical activity predicted
weight gain at follow-up. A link between sedentary lifestyle and weight gain
in children has also been shown, using television viewing as a measure of
activity (Gortmaker et al., 1996). Furthermore, research has used the doubly
labeled water method to assess energy expenditure. For example, Ravussin
et al. (1993) reported that low levels of minor physical movements (fidget-
ing) predicted future weight gain, and Roberts et al. (1988) reported low
energy expenditure at baseline in six babies who later became overweight
by the time they were one year old. In a recent study, the authors analyzed
data from 146 twin pairs as a means to assess the relative contribution of
genetics and physical activity over a 30-year period (Waller et al., 2008).
The results showed that persistent physical activity across the 30 years of
the study was related to smaller waist circumference and a decreased
weight gain as the active twin showed less weight gain than the inactive
twin even though they shared the same genetic makeup and childhood
environment. However, although all these studies were prospective, it is still
possible that a third factor may explain the relationship (i.e., those with
lower levels of activity at baseline were women, and the women had children
and therefore put on more weight). Unless experimental data are collected,
conclusions about causality remain problematic.
Do the obese exercise less?
Research has also examined the relationship between activity and obesity
using a cross-sectional design to examine differences between the obese and
nonobese. In particular, several studies in the 1960s and 1970s examined
whether the obese exercised less than the nonobese. Using time-lapse pho-
tography, Bullen, Reed, and Mayer (1964) observed obese and normal-weight
girls at a summer camp. They reported that during swimming the obese
girls spent less time swimming and more time floating, and while playing
tennis the obese girls were inactive for 77 percent of the time compared to
the normal-weight girls, who were inactive for only 56 percent of the time.
This approach indicated that the obese exercise less than the nonobese. Other
early studies observed who used the stairs versus the escalators on public