Obesity is associated with many diseases, particularly heart disease, type 2 diabetes, breathing difficulties during sleep, certain types of cancer, and osteoarthritis. Obesity is most commonly caused by a combination of excessive dietary calories, lack of physical activity, and genetic susceptibility, though a limited number of cases are due solely to genetics, medical reasons, or psychiatric illness. Obesity is a leading preventable cause of death worldwide, with increasing prevalence in adults and children, and authorities view it as one of the most serious public health problems of the 21st century.

Obesity in Children
The healthy BMI range varies with the age and sex of the child. Obesity in children and adolescents is defined as a BMI greater than the 95th percentile or to put it another way the 5% with the greatest BMI. The reference data that these percentiles are based on is from 1963 to 1994 and thus has not been affected by the recent increases in rates of obesity. Childhood obesity has reached epidemic proportions in 21st century with rising rates in both the developed and developing world. Rates of obesity in Canadian boys have increased from 11% in 1980s to over 30% in 1990s, while during this same time period rates increased from 4 to 14% in Brazilian children. As with obesity in adults many different factors contribute to the rising rates of childhood obesity. Changing diet and decreasing physical activity are believed to be the two most important in causing the recent increase in the rate of obesity. Activities from self propelled transport, to school physical education, and organized sports has been declining in many countries. Treatments used in children are primarily lifestyle interventions and behavioral techniques. Medications are not FDA approved for use in this age group.

Causes
At an individual level, a combination of excessive caloric intake, lack of physical activity, and genetic susceptibility is thought to explain most cases of obesity, with a limited number of cases due solely to genetics, medical reasons, or psychiatric illness. In contrast at a societal level increasing rates of obesity are felt to be due to an easily accessible and palatable diet, increased reliance on cars, and mechanized manufacturing. A 2006 review identified ten other possible contributors to the recent increase of obesity: (1) insufficient sleep, (2) endocrine disruptors (environmental pollutants that interfere with lipid metabolism), (3) decreased variability in ambient temperature, (4) decreased rates of smoking, because smoking suppresses appetite, (5) increased use of medications that can cause weight gain (e.g., atypical antipsychotics), (6) proportional increases in ethnic and age groups that tend to be heavier, (7) pregnancy at a later age (which may cause susceptibility to obesity in children), (8) epigenetic risk factors passed on generationally, (9) natural selection for higher BMI, and (10) assortative mating leading to increased concentration of obesity risk factors (this would not necessarily increase the number of obese people, but would increase the average population weight). There is substantial but not conclusive evidence for these mechanisms, and the authors specify that they are probably less influential than the ones discussed in the previous paragraph (but still important).

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