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Obesity is a condition in which a mammal's natural energy reserve, stored in fat, is expanded far beyond usual levels to the point where it is believed to pose a health risk to the animal. Obesity in animals is relatively rare in the wild, but common in domestic animals like barrows and household pets who may be castrated, spayed, overfed and underexercised. Americans have the highest obesity rate in the world.

'Obesity' is a concept that is being continually redefined. In humans, the current measurement of obesity is the body mass index (BMI).

A person with a BMI over 25 is considered overweight; a BMI over 30 is considered obese. The American Institute for Cancer Research considers a BMI between 18.5 and 25 to be an ideal target for a healthy individual (although several sources consider a person with a BMI of less than 20 to be underweight). The BMI was created in the 19th Century by the Belgian statistician Adolphe Quetelet, and remained largely intact until June 1998 when the BMI was revised downward. This had the effect of changing one's status from "ideal" weight to "overweight" in one day.

Obesity is alleged to pose a severe health risk, being often correlated (in population studies) with an increased risk of heart disease, more so when it is combined with high blood pressure and high cholesterol and triglyceride levels in the blood. It is also alleged to increase the risk of other diseases like carpal tunnel syndrome. It is, however, unknown whether it is the obesity itself that causes the health risk, other lifestyle and dietary factors associated with it, or the stress of coping with society's disapproval. It is possible to be obese and physically healthy, and studies have found that physical activity levels and good nutrition are more important in maintaining good health than an arbitrary measurement such as a BMI.

Although many people have a genetic propensity towards obesity, it is only with the reduction in physical activity and high-calorie diets of modern society that it has become widespread, with significant fractions of the population in advanced countries now obese. In times where food is scarce, the ability to take advantage of rare periods of abundance and use such abundance efficiently was undoubtedly an evolutionary advantage. This is precisely the opposite of what is required in a sedentary[?] society, where high-energy food is available in abundant quantities.

Obesity can sometimes be reduced by increasing physical activity and reducing calorie intake. However, people with a genetic disposition towards obesity find it very difficult to reduce their weight, and more drastic treatments such as appetite-suppressing drugs and even surgery to limit stomach capacity (and thus food intake) are sometimes used. However, starvation itself is a strain on the cardiovascular system, and in some people, if the metabolism switches to "starvation mode", it never switches back, making weight regain impossible to avoid. Huge amounts of research continue into new drugs to combat obesity, which many public health authorities regard as the biggest health problem facing technologically-advanced societies.

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