The most common form of diabetes disappears in most obese diabetics after weight-loss surgery, researchers said on Tuesday in a study that strongly affirmed the benefits of the operations.

The researchers combined data from 621 studies worldwide with 135,246 patients and found that 78 percent of obese diabetics returned to normal blood sugar levels and had no symptoms of diabetes following weight-loss operations, also known as bariatric surgery.

Another 8 percent saw their diabetes symptoms improve, although the disease was not eliminated.

"This is the most comprehensive study of the effect of bariatric surgery on type 2 diabetes. It includes every major paper that's been written in this field," Dr. Henry Buchwald of the University of Minnesota, who led the research published in the American Journal of Medicine, said in a telephone interview.

"The greater the weight loss, the greater is the resolution of type 2 diabetes."

There are several different forms of weight-loss surgery that alter the digestive system's anatomy to cut the volume of food that can be eaten and digested. The study showed that some types of surgery led to greater weight loss and helped diabetics more.

Eighty percent of those who had the most common form, gastric bypass, eliminated their diabetes, compared with 57 percent for gastric banding and 95 percent for a procedure called "biliopancreatic diversion/duodenal switch surgery", the researchers said.

Weight-loss surgery is becoming more popular. In the United States, about 220,000 people had such an operation last year, the American Society for Metabolic and Bariatric Surgery said.

The operations, like any type of surgery, pose a risk to the patients. In the United States, bariatric surgery costs an average of $17,000 to $25,000, and insurance coverage varies.