Epidemiology of obesity

Obesity has been observed throughout human history. Many early depictions of the human form in art and sculpture appear obese.^[2] However, it was not until the 20th century that obesity became common — so much so that, in 1997, the World Health Organization (WHO) formally recognized obesity as a global epidemic^[3] and estimated that the worldwide prevalence of obesity has nearly tripled since 1975.^[4] Obesity is defined as having a body mass index (BMI) greater than or equal to 30 kg/m², and in June 2013 the American Medical Association classified it as a disease.^[5]

In countries of the Organisation for Economic Co-operation and Development (OECD), one child out of five is overweight or obese.^[6] Once considered a problem only of high-income countries, obesity rates are rising worldwide. In 2014, the mean BMI in 128 countries was above the threshold for overweight.^[7] Globally, there are now more people who are obese than who are underweight, a trend observed in every region over the world except parts of sub-Saharan Africa and Asia.^[8] In 2013, an estimated 2.1 billion adults were overweight, as compared with 857 million in 1980.^[9] Of adults who are overweight, 31% are obese.^[8] Increases in obesity have been seen most in urban settings.^[10]

Since body fat can be measured in several ways, statistics on the epidemiology of obesity vary between sources. While BMI is the most basic and commonly used indicator of obesity, other measures include waist circumference, waist-to-hip ratio, skinfold thicknesses, and bioelectrical impedance.^[11] The rate of obesity increases with age at least up to 50 or 60 years old.^[12] Sex- and gender-based differences also influence the prevalence of obesity. Globally there are more obese women than men, but the numbers differ depending on how obesity is measured.^[13][14]