Obesity is an epidemic, says the World Health Organization. The prevalence of adult obesity has exceeded 30% in the United States, is over 20% in most of Europe (5-23% in men, 7-36% in women), and is 40-70% in the Gulf states and Polynesian islands. Obesity is also present in low income countries, and low socioeconomic groups are affected most. In most countries the prevalence of obesity now exceeds 15%, the figure used by WHO to define the critical threshold for intervention in nutritional epidemics.

Obesity is increasingly affecting younger people, with warning signs for the future from the increasing prevalence of overweight in childhood

Obese people are at high risk of multiple health problems and need full medical management. The numbers are so great (and rising), however, that individual medical care becomes impractical and prohibitively expensive. Currently, the cost of obesity to a country's health service is estimated at up to 9%, and the overall social cost of the condition is seen as a major hindrance to economic development. An even larger proportion of the population is overweight, with increased morbidity. Virtually all the costs (personal, health, and economic) of obesity are met in adulthood and result from fat that has accumulated in adulthood, but there is a likely additional cost due to inactivity and overweight in childhood that should also be considered.