Purpose: The term "epidemic" of obesity implies that obesity is a characteristic of populations, not only of individuals. The purpose of this paper is to review evidence on obesity in populations and to identify future research issues.
Methods: To examine recent increases in the population prevalence of overweight or obesity, a literature search was undertaken.
Results: Trends in overweight or obesity among adults showed considerable variability internationally. Some countries, including Canada, Finland (men), New Zealand, the United Kingdom, the United States, and Western Samoa showed large increases in prevalence (>5 percentage points), whereas several other countries showed smaller or no increases. Overweight is also increasing 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 also not clear, in part because trends in cardiovascular risk factors do not always parallel trends in obesity. Of the classic epidemiologic triad of host, agent, and environment, the environment has often received the least attention.
Conclusions: The economic, social, and cultural factors that influence the distribution of body mass index in a population are not well understood. Future research needs include continued monitoring of trends in obesity and in related health conditions and observational studies to examine the causes of these trends. Public health research should aim at defining realistic goals and strategies to improve health in an environment conducive to high levels of overweight and obesity.