In making the decision, delegates at the association's annual meeting in Chicago overrode a recommendation against doing so by a committee that had studied the matter.
"Recognising obesity as a disease will help change the way the medical community tackles this complex issue that affects approximately 1 in 3 Americans," Dr Patrice Harris, a member of the association's board, said in a statement. She suggested the new definition would help in the fight against Type 2 diabetes and heart disease, which are linked to obesity.
To some extent, the question of whether obesity is a disease or not is a semantic one, since there is not even a universally agreed upon definition of what constitutes a disease. And the AMA's decision has no legal authority.
Still, some doctors and obesity advocates said that having the nation's largest physician group make the declaration would focus more attention on obesity. It could also help improve reimbursement for obesity drugs, surgery and counseling.
"I think you will probably see from this physicians taking obesity more seriously, counseling their patients about it," said Morgan Downey, an advocate for obese people and publisher of the online Downey Obesity Report. "Companies marketing the products will be able to take this to physicians and point to it and say, 'Look, the mother ship has now recognized obesity as a disease.'"
Two new obesity drugs - Qsymia from Vivus, and Belviq from Arena Pharmaceuticals and Eisai - have entered the market in the last year.
Qsymia has not sold well for a variety of reasons, including poor reimbursement and distribution restrictions imposed because of concerns that the drug can cause birth defects. Those restrictions are now being relaxed. Belviq went on sale only about a week ago, so it is too early to tell how it is doing.
Whether obesity should be called a disease has long been debated. The Obesity Society officially issued its support for classifying obesity as a disease in 2008, with Downey as one of the authors of the paper.
The Internal Revenue Service has said that obesity treatments can qualify for tax deductions. In 2004, Medicare removed language from its coverage manual saying obesity was not a disease.
The vote of the AMA House of Delegates went against the conclusions of the association's Council on Science and Public Health, which had studied the issue over the past year. The council said that obesity should not be considered a disease mainly because the measure usually used to define obesity, the body mass index, is simplistic and flawed.
Some people with a BMI above the level that usually defines obesity are perfectly healthy while others below it can have dangerous levels of body fat and metabolic problems associated with obesity.
The council summarised the arguments for and against calling obesity a disease.
One reason in favour, it said, was that it would reduce the stigma of obesity that stems from the widespread perception that it is simply the result of eating too much or exercising too little. Some doctors say that people do not have full control over their weight.
Supporters of the disease classification also say it fits some medical criteria of a disease, such as impairing body function.
Those arguing against it say that there are no specific symptoms associated with it and that it is more a risk factor for other conditions than a disease in its own right.
They also say that "medicalising'' obesity by declaring it a disease would define one-third of Americans as being ill and could lead to more reliance on costly drugs and surgery rather than lifestyle changes. Some people might be overtreated because their BMI was above a line designating them as having a disease, even though they were healthy.
The delegates rejected the conclusion of the council and voted instead in favour of a resolution pushed by the American Association of Clinical Endocrinologists, the American College of Cardiology and some other organizations.
This resolution argued that obesity was a "multimetabolic and hormonal disease state" that leads to unfavourable outcomes such as Type 2 diabetes and cardiovascular disease.
"The suggestion that obesity is not a disease but rather a consequence of a chosen lifestyle exemplified by overeating and/or inactivity is equivalent to suggesting that lung cancer is not a disease because it was brought about by individual choice to smoke cigarettes," the resolution said.
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