CHICAGO — New research gives clear proof that weight-loss surgery can reverse and possibly cure diabetes, and doctors say the operation should be offered sooner to more people with the disease — not just as a last resort.

The two studies released Monday are the first to compare stomach-reducing operations to medicines alone for “diabesity” — Type 2 diabetes brought on by obesity. Millions of Americans have this and can’t make enough insulin or use what they do make to process sugar from food.

Both studies found that surgery helped far more patients achieve normal blood-sugar levels than medicines alone did.

The results were dramatic: Some people were able to stop taking insulin as soon as three days after their operations. Cholesterol and other heart risk factors also greatly improved.

Doctors don’t like to say “cure” because they can’t promise a disease will never come back. But in one study, most surgery patients were able to stop all diabetes drugs and have their disease stay in remission for at least two years. None of those treated with medicines alone could do that.

“It is a major advance,” said Dr. John Buse of the University of North Carolina at Chapel Hill, a leading diabetes expert who had no role in the studies. Buse said he often recommends surgery to patients who are obese and can’t control their blood-sugar through medications, but many are leery of it. “This evidence will help convince them that this really is an important therapy to at least consider,” he said.

There were signs that the surgery itself — not just weight loss — helps reverse diabetes. Food makes the gut produce hormones to spur insulin, so trimming away part of it surgically may affect those hormones, doctors believe.

Weight-loss surgery “has proven to be a very appropriate and excellent treatment for diabetes,” said one study co-leader, Dr. Francesco Rubino, chief of diabetes surgery at New York-Presbyterian Hospital/Weill Cornell Medical Center. “The most proper name for the surgery would be diabetes surgery.”

The studies were published online by the New England Journal of Medicine, and the larger one was presented Monday at an American College of Cardiology conference in Chicago.

More than a third of American adults are obese, and more than 8 percent have diabetes, a major cause of heart disease, strokes and kidney failure. Between 5 million and 10 million are like the people in these studies, with both problems.

For a century, doctors have been treating diabetes with pills and insulin, and encouraging weight loss and exercise with limited success. Few very obese people can drop enough pounds without surgery, and many of the medicines used to treat diabetes can cause weight gain, making things worse.

Surgery offers hope for a long-term fix. It costs $15,000 to $25,000, and Medicare covers it for very obese people with diabetes. Gastric bypass is the most common type: Through “keyhole” surgery, doctors reduce the stomach to a small pouch and reconnect it to the small intestine.