“It’s not yet time to have a parade,” said Dr. David M. Nathan, the director of the Diabetes Center and Clinical Research Center at Massachusetts General Hospital. But he noted, “It has finally entered into the consciousness of our population that the sedentary lifestyle is a real problem, that increased body weight is a real problem.”

Even here in Alabama, which has the highest prevalence of diabetes in the country, the rate of new cases has begun to taper. According to figures for 2013, the most recent available by state, 12.7 percent of Alabama’s residents had the disease.

In the tiny town of Eutaw, Lynette Carpenter got serious about staving off diabetes around the time her cousin’s leg was amputated because of the disease. She started to make a dish she called Sexy Pork Chops, involving a bell pepper, an onion and the oven. She weaned herself off Coca-Cola, going from about 50 cans a week to fewer than seven. And she started walking, leaving rubber bands in her mailbox to pull onto her arm — one for each mile walked — to remind her how many miles she had gone. The result made her doctor proud: She lost 42 pounds, and two years later has still not developed full-blown diabetes.

Diabetes “has got my respect and attention,” she said. “I take it real serious.”

Robin Williams, a music teacher in Birmingham, lost 33 pounds by cutting out candy, packing her lunch and not buying fast food during long bus trips to band practice, habits she learned in a Y.M.C.A. program on diabetes prevention. She started watching “My 600-lb Life,” a television show about people losing weight, with her teenage daughter. The show frightened her, as did her father’s leg amputations two years ago. She is down to 222 pounds, and is determined to keep going.

“I want to see my child grow up,” Ms. Williams said. She uses an app on her phone to track her calories. “It made me much more conscious of what I was putting in my mouth.”

Diabetes has been particularly devastating here in what is known as the Black Belt, a strip of counties that originally got its name for its fertile soil. Its majority black populations were once slaves, and later, after the Civil War, they were sharecroppers. The area had some of the worst racial violence of the Jim Crow era, said Hasan Kwame Jeffries, an associate professor of history at Ohio State University, and today is among the poorest in the country.