Prior studies have shown that weight-loss surgery can reverse type 2 diabetes in patients who already have the condition. The latest findings offer evidence that the procedures can prevent the condition.

“We saw a marked delay (in the development of diabetes) over 15 years,” said Dr. Lars Sjostrom of the University of Gothenburg in Sweden, whose study appears in the New England Journal of Medicine.

“Some of those surgical patients will probably develop diabetes later. But over a lifetime, there will be a large difference.”

According to the World Health Organization, 346 million people worldwide have diabetes. Most of them, about 90%, have type 2 diabetes, the form of the disease linked with obesity and lack of exercise.

The link between obesity and diabetes is well-documented, and making lifestyle changes or taking weight-reducing drugs can cut the risk of diabetes by 40 to 45%.

The study, part of the larger Swedish Obese Subject study, was designed to see if the surgical weight loss would have the same effect. None of the patients included in the test had diabetes when the project began in 1987.

Participants chose whether or not to have surgery, and enrollment ended in February 2001. Stomach stapling was the most common procedure (69 percent), followed by gastric banding (19 percent) and gastric bypass (12%.)

When they compared the two groups over the course of the 15-year study, the team found that among the 1658 volunteers who underwent weight-loss surgery, the annual risk of developing diabetes was about 1 in 150.

That compared with an annual risk of 1 in 35 among the 1771 people in the control group, about four times higher than the treatment group.

The improvement was seen even though the people who underwent surgery initially were a bit heavier and had more risk factors than the control group at the start of the study.

“It’s favorable in spite of these differences,” Sjostrom said.

The improvements correlated with weight loss seen in the groups. In the surgery group, the average weight loss at the 15-year mark was 20 kilograms, or 44 pounds. The non-surgery group — which received standard care consisting of recommendations for healthier eating and more physical activity - stayed within three kilograms (7 pounds) of their starting weight.

Three patients died within 90 days of their surgery, and between 2% and 5% of patients had lung complications, vomiting, infections, bleeding or a blood clot.

According to the American Society for Metabolic and Bariatric Surgery, about 220000 people had bariatric surgery in 2009. Surgery costs range from $11500 to $26000.

Dr. Danny Jacobs of Duke University School of Medicine in Durham, North Carolina, said in a commentary in the journal that

“it remains impractical and unjustified to contemplate the performance of bariatric surgery in the millions of eligible obese adults.”

Sjostrom said more studies are needed and a cost analysis of the pros and cons of surgery, now underway, could be published in a year or so.