Purpose: The term "epidemic" of obesity implies that obesity is a character
istic of populations, not only of individuals. The purpose of this paper is
to review evidence on obesity in populations and to identify future resear
ch issues. Methods: To examine recent increases in the population prevalenc
e of overweight or obesity, a literature search was undertaken. Results: Tr
ends in overweight or obesity among adults showed considerable variability
internationally. Some the United Kingdom; the United States, and Western Sa
moa showed large increases in prevalence (>5 percentage points), whereas se
veral other countries showed smaller or no increases. Overweight is also in
creasing among children and adolescents, at least in some countries. It is
not clear what the expected prevalence of overweight or obesity might be in
the current environment, and these findings may be most usefully viewed as
shifts in the distribution of a population characteristic. The reasons for
these shifts are not clear. The health implications of these shifts are al
so not clear, in part because trends in cardiovascular risk factors do not
always parallel trends in obesity. Of the classic epidemiologic triad of ho
st, agent, and environment, the environment has often received the least at
tention. Conclusions: The economic, social, and cultural factors that influ
ence the distribution of body mass index in a population are nor well under
stood. Future research needs include continued monitoring of trends in obes
ity and in related health conditions and observational studies to examine t
he causes of these trends. Public health research should aim at defining re
alistic goals and strategies to improve health in an environment conducive
to high levels of overweight and obesity.