Obesity is a medical condition, sometimes considered a disease,[8][9][10] in which excess body fat has accumulated to such an extent that it can potentially have negative effects on health.

[16] Obesity prevention requires a complex approach, including interventions at medical, societal, community, family, and individual levels.

[26] Medical organizations tend to classify people living with obesity as based on body mass index (BMI) – a ratio of a person's weight in kilograms to the square of their height in meters.

In recent years, Canada chose to allow both Chile & Ireland to adapt their obesity guidelines to suit both countries' health systems.

[89] The paradox was first described in 1999 in overweight and obese people undergoing hemodialysis[89] and has subsequently been found in those with heart failure and peripheral artery disease (PAD).

[95] Another study found that if one takes into account chronic obstructive pulmonary disease (COPD) in those with PAD, the benefit of obesity no longer exists.

According to one study, factors like these may play as big of a role as excessive food energy intake and a lack of physical activity;[104] however, the relative magnitudes of the effects of any proposed cause of obesity is varied and uncertain, as there is a general need for randomized controlled trials on humans before definitive statement can be made.

[105] According to the Endocrine Society, there is "growing evidence suggesting that obesity is a disorder of the energy homeostasis system, rather than simply arising from the passive accumulation of excess weight".

[114] The primary sources of these extra carbohydrates are sweetened beverages, which now account for almost 25 percent of daily food energy in young adults in America,[115] and potato chips.

In the United States, subsidization of corn, soy, wheat, and rice through the U.S. farm bill has made the main sources of processed food cheap compared to fruits and vegetables.

)[142] In people with early-onset severe obesity (defined by an onset before 10 years of age and body mass index over three standard deviations above normal), 7% harbor a single point DNA mutation.

[3] Certain medications may cause weight gain or changes in body composition; these include insulin, sulfonylureas, thiazolidinediones, atypical antipsychotics, antidepressants, steroids, certain anticonvulsants (phenytoin and valproate), pizotifen, and some forms of hormonal contraception.

In undeveloped countries the ability to afford food, high energy expenditure with physical labor, and cultural values favoring a larger body size are believed to contribute to the observed patterns.

[129] Time spent in motor vehicles, as opposed to active transportation options such as cycling or walking, is correlated with increased risk of obesity.

[175] The main treatment for obesity consists of weight loss via lifestyle interventions, including prescribed diets and physical exercise.

[190] Dietary and lifestyle changes are effective in limiting excessive weight gain in pregnancy and improve outcomes for both the mother and the child.

[210] When compared to placebo, all dosages of fluoxetine appeared to contribute to weight loss but lead to increased risk of experiencing side effects such as dizziness, drowsiness, fatigue, insomnia and nausea during period of treatment.

[210] When comparing, in the same review, the effects of fluoxetine on weight of obese adults, to other anti-obesity agents, omega-3 gel and not receiving a treatment, the authors could not reach conclusive results due to poor quality of evidence.

[210] Among antipsychotic drugs for treating schizophrenia clozapine is the most effective, but it also has the highest risk of causing the metabolic syndrome, of which obesity is the main feature.

[215][234] With the onset of the Industrial Revolution, it was realized that the military and economic might of nations were dependent on both the body size and strength of their soldiers and workers.

[115] Increasing the average body mass index from what is now considered underweight to what is now the normal range played a significant role in the development of industrialized societies.

[20] During the Renaissance some of the upper class began flaunting their large size, as can be seen in portraits of Henry VIII of England and Alessandro dal Borro.

[241]: 29  When compared to their ideal weight counterparts, workers with obesity, on average have higher rates of absenteeism from work and take more disability leave, thus increasing costs for employers and decreasing productivity.

The Court said that if an employee's obesity prevents them from "full and effective participation of that person in professional life on an equal basis with other workers", then it shall be considered a disability and that firing someone on such grounds is discriminatory.

[265] The US-based National Association to Advance Fat Acceptance (NAAFA) was formed in 1969 and describes itself as a civil rights organization dedicated to ending size discrimination.

The American legal system, however, has decided that the potential public health costs exceed the benefits of extending this anti-discrimination law to cover obesity.

[264] In 2015, the New York Times published an article on the Global Energy Balance Network, a nonprofit founded in 2014 that advocated for people to focus on increasing exercise rather than reducing calorie intake to avoid obesity and to be healthy.

[272] In 2006, the National Institute for Health and Clinical Excellence (NICE) issued a guideline on the diagnosis and management of obesity, as well as policy implications for non-healthcare organizations such as local councils.

[273] A 2007 report produced by Derek Wanless for the King's Fund warned that unless further action was taken, obesity had the capacity to debilitate the National Health Service financially.

[278] Brief weight management interventions in primary care (e.g. delivered by a physician or nurse practitioner) have only a marginal positive effect in reducing childhood overweight or obesity.