SEATTLE, United States (AP) — Obesity surgery may dramatically lower the danger of heart attacks and strokes in patients with diabetes, new research suggests, reinforcing evidence that benefits extend beyond weight loss.

The study tracked about 20,000 severely obese patients with Type 2 diabetes. Those who had weight loss surgery had a 40 per cent lower chance of developing a heart attack or stroke in the five years following surgery compared to those who got usual care with diabetes medicines or insulin.

For every 1,000 patients in the study who had surgery there were roughly 20 heart attacks or strokes compared to 40 such events per 1,000 who got regular care.

More than 30 million Americans have diabetes, mostly Type 2 where the body loses the ability to produce or use insulin to turn food into energy.

Other research has shown obesity surgery can reverse and even prevent diabetes. Taken together, it means doctors should discuss weight loss surgery more often, said study co-author Dr David Arterburn of Kaiser Permanente Washington Health Research Institute in Seattle.

Doctors usually mention insulin and pills, “but it’s not always brought up that weight loss surgery is another available treatment option,” Arterburn said.

Researchers analysed records from four US health care systems: HealthPartners in Minnesota and Kaiser Permanente in Washington state, Northern California and Southern California. Results were published Tuesday in the Journal of the American Medical Association.

The study can’t prove cause and effect because patients weren’t randomly assigned to get surgery. The researchers tried to match patients for gender, age, blood sugar levels and other factors. But other things they didn’t account for could have contributed to the surgery patients’ better results.

Everyone in the study had a BMI, or body mass index, of at least 35. For instance, someone who is five-foot-eight-inches and weighs 230 pounds has a BMI of 35.

Of the more than 5,300 who had surgery, most had gastric bypass, the most common type of stomach-shrinking operation. Some had gastric sleeve or gastric band procedures. The rest, nearly 15,000 people, had usual care.

Obesity surgery can cost US$20,000 to US$25,000. Insurers are increasingly covering it, but some impose strict limits.

The new findings suggest insurance coverage should be expanded for the right patients, Dr Sayeed Ikramuddin of the University of Minnesota wrote in an accompanying editorial.

Surgery is thought to help by affecting hormones, gut bacteria and other substances that affect how the body handles insulin and blood sugar. Weight loss without surgery also helps, but is difficult for many people to achieve.

Most weight loss surgery today is done through small incisions. The dangers are similar to other surgeries, including a small chance of life-threatening complications, and some people need to have their surgeries repeated.