Energy Metabolism 47ity, some studies suggest that a low level of energy
expenditure predicts the development of obesity, and
others do not support this hypothesis.
Physical activity is hypothesized to protect people
from the development of obesity through several
channels. First, physical activity, by defi nition, results
in an increase in energy expenditure owing to the cost
of the activity itself, and is also hypothesized to
increase RMR. These increases in energy expenditure
are likely to decrease the likelihood of positive energy
balance. However, the entire picture of energy balance
must be considered, particularly the possibility that
increases in one or more components of energy
expenditure can result in a compensatory reduction
in other components (i.e., resting energy expenditure
and activity energy expenditure). Secondly, physical
activity has benefi cial effects on substrate metabo-
lism, with an increased reliance on fat relative to
carbohydrate for fuel utilization, and it has been
hypothesized that highly active individuals can main-
tain energy balance on a high-fat diet.
Cross-sectional studies in children and adults have
shown that energy expenditure, including physical
activity energy expenditure, is similar in lean and
obese subjects, especially after controlling for differ-
ences in body composition. Children of obese and
lean parents have also been compared as a model of
preobesity. Some studies show that children of obese
parents had a reduced energy expenditure, including
physical activity energy expenditure, whereas another
study did not. A major limitation of the majority of
studies that have examined the role of energy expen-
diture in the etiology of obesity is their cross-sectional
design. Because growth of individual components of
body composition is likely to be a continuous process,
longitudinal studies are necessary to evaluate the rate
of body fat change during the growing process. Again,
some longitudinal studies support the idea that
reduced energy expenditure is a risk factor for the
development of obesity, whereas others do not.
Finally, intervention studies have been conducted to
determine whether the addition of physical activity
can reduce obesity. These studies tend to support
the positive role of physical activity in reducing
body fat.
Several possibilities could account for such dis-
crepant fi ndings. First, the ambiguous fi ndings in the
literature may be explained by the possibility that
differences in energy expenditure and physical activ-
ity and their impact on the development of obesity
are different at the various stages of maturation. This
hypothesis is supported by previous longitudinal
studies in children, showing that a reduced energy
expenditure is shown to be a risk factor for weight
gain in the fi rst 3 months of life, but not during
the steady period of prepubertal growth. Secondly,
there could be individual differences in the effect of
altered energy expenditure on the regulation of energy
balance. Thus, the effect of energy expenditure on the
etiology of obesity could vary among different sub-
groups of the population (e.g., boys versus girls, dif-
ferent ethnic groups) and could also have a differential
effect within individuals at different stages of devel-
opment. It is conceivable that susceptible individuals
fail to compensate for periodic fl uctuations in energy
expenditure. Third, explanations related to the meth-
odology can also be offered because of the complexity
of the nature of physical activity and its measure-
ment. The success of controlled exercise interven-
tions in improving body composition indicates an
extremely promising area for the prevention of
obesity. However, further studies are required to elu-
cidate the specifi c effects of different types of exercise
on the key features of body weight regulation.3.8 Perspectives on the future
Much is known about how the body balances energy
intake and expenditure. There are, however, areas
that need further research. The technology to deter-
mine total energy expenditure with doubly labeled
water has been standardized. Most of the data from
using this method have been obtained in populations
living in industrialized countries. More studies on
infants, children, adolescents, adults, pregnant and
lactating women, and the elderly living in developing
countries are indicated. Doubly labeled water is an
expensive method. There is a need to develop more
cost-effective methods that can be used in fi eld studies
and to determine the energy cost of specifi c activities
of people throughout the life cycle in developing
countries. Obesity has recently been defi ned as a
disease by the World Health Organization. The
growing problem of obesity worldwide, and in chil-
dren and in people who were previously food inse-
cure and malnourished, needs to be addressed with