ORLANDO, Fla. – When the American Medical Association last week declared obesity a disease — a move that instantly labeled one-third of Americans as sick — it launched a controversy not seen since alcoholism received the disease designation.

Hailed by some obesity experts as a long-overdue victory, the news from the nation’s largest and most respected medical group was denounced by others who say the move fuels the stigma against obese Americans.

Fat activists promptly started the #IAmNotADisease hashtag on Twitter, and a petition demanding that the AMA reverse its position, which had nearly 1,200 signatures by Friday.

Calling obesity a disease will open doors to better treatment and better reimbursements, said Dr. Steve Smith, president-elect of The Obesity Society, which has referred to obesity as a disease since 2008.

“It adds legitimacy to the problem, will help raise public awareness and will get doctors engaged in treating the condition,” said Smith, also scientific director for the Florida Hospital-Sanford Burnham Translational Research Institute for Metabolism and Diabetes, in Orlando.

The AMA’s decision “is a defining moment,” said Joe Nadglowski, president of the Obesity Action Coalition, a national nonprofit based in Tampa, Fla., that helps those struggling with obesity.

“It puts obesity on the same path as treatments for addictions to alcohol or tobacco, and mental health problems, such as depression.”

A few decades ago, those conditions were also perceived as behavioral problems, said Nadglowski. “Once we realized they involved a disease process, that drove better coverage, better treatment, and real change.”

In making the call, the AMA aims to reduce the incidence of obesity-related diseases, such as cardiovascular disease and type 2 diabetes, said AMA board member Dr. Patrice Harris in a statement accompanying the announcement.

More than 35 percent of Americans are currently obese, according to the Centers for Disease Control and Prevention.

But that doesn’t necessarily mean they’re sick, many argue.

“We don’t see ourselves as diseased,” said Peggy Howell, spokeswoman for the National Association to Advance Fat Acceptance, a 44-year-old nonprofit that works to improve the quality of life for people of large size.

“To label a whole segment of society as diseased without any knowledge of their health is unacceptable,” she said. “It directly fuels discrimination. This is a step backward.”

Howell, who is 65, said, “I have been fat my entire life. If being fat were so horrific, why am I not wracked with problems? I have slightly high blood pressure, like a lot of thin people, but other than that, I perceive myself as a healthy woman, far healthier than a lot of people I know. But two days ago I was declared diseased.”

Linda Bacon, a nutritionist at University of California at Davis, said, “I’m appalled that the AMA chose to ignore science and name obesity a disease.”

Bacon, author of “Health at Every Size,” joins other critics in noting that the definition of obesity — basically how one’s weight and height ratio stacks up on a BMI chart — is imprecise, and only defines size, not health.

“The AMA just determined that some people are sick based on how they look,” Bacon said. “What’s next? Will they pronounce being black as a disease because there are higher rates of cardiovascular disease in black communities?”