The American Medical Assn. voted Tuesday to declare obesity a disease, a move that effectively defines 78 million American adults and 12 million children as having a medical condition requiring treatment.

The nation's leading physicians organization took the vote after debating whether the action would do more to help affected patients get useful treatment or would further stigmatize a condition with many causes and few easy fixes.

In the end, members of the AMA's House of Delegates rejected cautionary advice from their own experts and extended the new status to a condition that affects more than one-third of adults and 17% of children in the United States.

"Recognizing obesity as a disease will help change the way the medical community tackles this complex issue that affects approximately 1 in 3 Americans," said Dr. Patrice Harris, an AMA board member.

Tuesday's vote is certain to step up pressure on health insurance companies to reimburse physicians for the time-consuming task of discussing obesity's health risks with patients whose body mass index exceeds 30. It should also encourage doctors to direct these patients to weight-loss programs and to monitor their often-fitful progress.

The federally funded Medicare program, which insures an estimated 13 million obese Americans who are over 65 or disabled, already covers the costs of "intensive behavioral therapy" for obese patients, as well as bariatric surgery for those with additional health conditions. But coverage for such obesity treatments has been uneven among private insurers.

Insurers who are members of the California Assn. of Health Plans cover many services to treat medical conditions associated with obesity, including bariatric surgery and diabetes, said President and Chief Executive Patrick Johnston.

The AMA's decision essentially makes diagnosis and treatment of obesity a physician's professional obligation. As such, it should encourage primary care physicians to get over their discomfort about raising weight concerns with obese patients. Studies have found that more than half of obese patients have never been told by a medical professional they need to lose weight — a result not only of some doctors' reluctance to offend but of their unwillingness to open a lengthy consultation for which they might not be reimbursed.

Past AMA documents have referred to obesity as an "urgent chronic condition," a "major health concern" and a "complex disorder." The vote now lifts obesity above the status of a health condition, disorder or marker for heightened risk of disease — as high cholesterol is for heart disease, for instance.

"As things stand now, primary care physicians tend to look at obesity as a behavior problem," said Dr. Rexford Ahima of University of Pennsylvania's Institute for Diabetes, Obesity and Metabolism. "This will force primary care physicians to address it, even if we don't have a cure for it."

The new designation follows a steep 30-year climb in Americans' weight — and growing public concern over the resulting tidal wave of expensive health problems. Treatment of such obesity-related illnesses as cardiovascular disease, Type 2 diabetes and certain cancers drives up the nation's medical bill by more than $150 billion a year, according to the Centers for Disease Control and Prevention.

Projected increases in the obesity rate could boost that figure by an additional $550 billion over the next 20 years, a recent Duke University study concluded.

In laying out the case for and against the redefinition of obesity, the AMA's Council on Science and Public Health argued that more widespread recognition of obesity as a disease "could result in greater investments by government and the private sector to develop and reimburse obesity treatments."

The Food and Drug Administration, which has approved just two new prescription weight-loss medications since 1999, would probably face increased pressure to approve new obesity drugs, spurring new drug development and more widespread prescribing by physicians, the council noted.

"The greater urgency a disease label confers" also might boost support for obesity-prevention programs such as physical education initiatives and reforms to school lunch, the council added. In addition, it speculated that "employers may be required to cover obesity treatments for their employees and may be less able to discriminate on the basis of body weight."

But the council also said that making obesity a disease could deepen the stigma attached to being overweight and doom some patients to endless nagging — even if they were otherwise healthy or had lost enough weight to improve their health.

It might also shift the nation's focus too much toward expensive drug and surgical treatments and away from measures to encourage healthy diets and regular exercise, the council wrote in a background memo for AMA members.

Dr. Daniel H. Bessesen, an endocrinologist and obesity expert at the University of Colorado Anschutz Medical Campus, called the AMA's shift "a double-edged sword." Though the semantic change may reflect "a growing awareness that obesity is not someone's fault," he worried that "the term disease is stigmatizing, and people who are obese don't need more stigmatizing."