Obesity is a condition in which excess body fat has accumulated to such an extent that health may be negatively affected. It is commonly defined as a body mass index (BMI = weight divided by height squared) of 30 kg/m2 or higher. This distinguishes it from being overweight as defined by a BMI of between 25–29.9 kg/m2.

Excessive body weight is associated with various diseases, particularly cardiovascular diseases, diabetes mellitus type 2, obstructive sleep apnea, certain types of cancer, and osteoarthritis. As a result, obesity has been found to reduce life expectancy.The primary treatment for obesity is dieting and physical exercise. If this fails, anti-obesity drugs and (in severe cases) bariatric surgery can be tried.

Obesity arises from too much energy intake compared with a person's basal metabolic rate and level of physical exercise. Excessive caloric intake and a lack of physical activity in genetically susceptible individuals is thought to explain most cases of obesity, with purely genetic, medical, or psychiatric illness contributing to only a limited number of cases. With rates of adult and childhood obesity increasing, authorities view it as a serious public health problem.

Although obesity is often stigmatized in the modern Western world, it has been perceived as a symbol of wealth and fertility at other times in history.

Risk factors and Comorbidities

The presence of risk factors and diseases associated with obesity are also used to establish a clinical diagnosis. Coronary heart disease, type 2 diabetes, and sleep apnea are possible complications that would indicate a need to commence or intensify treatment for obesity. Smoking, high blood pressure, age and family history are other risk factors that, in combination with obesity, may indicate an additional reason for treatment.


Obesity is one of the leading preventable causes of death. Mortality risk varies with BMI; the lowest risk is found at a BMI of 22–24 kg/m2 and increases with changes in either direction. A BMI of over 32 is associated with a doubling of risk of death[21] and obesity is estimated to cause an excess 111,909 to 365,000 death per year in the United States. Obesity on average reduces life expectancy by 6–7 years. Severe obesity (BMIs >40) reduces life expectancy by 20 years for men and 5 years for women.


A large number of physical and mental conditions have been associated with obesity.

Health consequences can be categorized by the effects of increased fat mass (osteoarthritis, obstructive sleep apnea, social stigmatization) or by the increased number of fat cells (diabetes, cancer, cardiovascular disease, non-alcoholic fatty liver disease). Increases in body fat alter the body's response to insulin, potentially leading to insulin resistance. Increased fat also creates a proinflammatory state, increasing the risk of thrombosis.

Central obesity, characterized by its high waist to hip ratio, is an important risk for metabolic syndrome. Metabolic syndrome is a combination of medical disorders which often includes diabetes mellitus type 2, high blood pressure, high blood cholesterol, and triglyceride levels.

Obesity is related to a variety of other complications. Some of these are directly caused by obesity and others are indirectly related through mechanisms sharing a common cause such as poor diet or a sedentary lifestyle. The strength of the link between obesity and specific conditions varies. One of the strongest is the link with type 2 diabetes. Excess weight is behind 64% of cases of diabetes in men and 77% in women.

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Obesity survival paradox

Although the negative health consequences of obesity in the general population are well supported by the available evidence, health outcomes in certain subgroups seem to be improved at an increased BMI, a phenomenon known as the obesity survival paradox. The paradox was first described in 1999 in overweight and obese patients undergoing hemodialysis. Since then it has been found in a few other subgroups and explanations for its occurrence have been put forward.

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In people with heart failure, those with a BMI between 30.0–34.9 had lower mortality then those with a normal weight. This has been attributed to the fact that people often lose weight as they become progressively more ill. Similar findings have been made in other types of heart disease.

People with class I obesity and heart disease do not have greater rates of further heart problems than people of normal weight who also have heart disease. In people with greater degrees of obesity, however, risk of further events is increased. Even after cardiac bypass surgery, no increase in mortality is seen in the overweight and obese. One study found that the increased survival could be explained by the more aggressive treatment obese people receive after a cardiac event.


Most researchers agree that a combination of excessive calorie consumption and a sedentary lifestyle are the primary causes of obesity. In a minority of cases, increased food consumption can be attributed to genetic, medical, or psychiatric illness. Generally however the rising prevalence of obesity is attributed to the availability of an easily accessible and palatable diet, car culture and mechanized manufacturing. A 2006 review identifies ten other possible contributors to the recent increase of obesity: (1) insufficient sleep, (2) endocrine disruptors—food substances that interfere with lipid metabolism, (3) decreased variability in ambient temperature, (4) decreased rates of smoking as smoking suppresses appetite, (5) increased use of medication that leads to weight gain, (6) increased distribution of ethnic and age groups that tend to be heavier, (7) pregnancy at a later age, (8) intrauterine and intergenerational effects, (9) positive natural selection of people with a higher BMI, (10) assortative mating, heavier people tending to form relationships with each other.


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