Obesity prevalence around the world has been rising over time, with the sharpest increases occurring in the second half of the 20th century and into the early 21st. Prior to this time period, obesity was a rare phenomenon. According to the World Health Organization (WHO), obesity has reached "epidemic proportions" around the globe and is projected to continue rising. The sharp rise in the obesity rate in the majority of developed nations is attributed to the increased consumption of high calorie foods and low levels of physical activity. Due to the health risks associated with obesity, the WHO recommends that countries institute weight management programs.
For the majority of history, obesity was rarely seen; in the last century, however, it has become commonplace. The sharpest increases in obesity prevalence have occurred since the mid 1960s, with the rates in many countries doubling or tripling. In the United States for example, the rate of obesity in adults ages 20 to 74 has risen from 13.4 percent in 1962 to 35 percent in 2006, and with the levels rising even faster in children. The WHO estimates that over 300 million adults are clinically obese, which is defined as having a body mass index (BMI) over 30, and considers the problem to be an epidemic. Increases in obesity prevalence are present in developing nations as well as developed ones, in both rural and urban areas, and across all socioeconomic levels; rates are expected to continue rising unless large percentages of the population make lasting lifestyle changes.
Increased obesity prevalence is attributed to two major causes. One is the increased availability and consumption of high calorie foods at all levels of society. As populations worldwide shift to more urban lifestyles and experience rising incomes, foods with high levels of fat and sugar are consumed in larger amounts and displace healthier alternatives. Another cause is less physical activity that has accompanied the spread of technology. Smaller percentages of populations work in jobs that require physical exertion, and leisure activities have shifted to more sedentary forms of relaxation.
The increases in obesity prevalence around the world has been accompanied serious health issues. Obesity is associated with significantly higher incidences of medical conditions such as diabetes, osteoarthritis, heart disease, certain cancers, and respiratory difficulties. The rising rates of obesity and related health conditions significantly increases healthcare expenditures worldwide. The WHO recommends that countries make resources available for the medical care of affected individuals. It has also advised nations to implement public policy measures to encourage healthy lifestyles, including increasing the availability of foods with low fat and high fiber, and promoting opportunities for daily exercise.