NEW YORK — Every year, hundreds of thousands of obese Americans undergo weight-loss surgery in a last-ditch effort to shed pounds and control their Type 2 diabetes.

Now a new study suggests that bariatric surgery may also have other significant health benefits, cutting the overall risk of serious cardiovascular events and premature death by almost half.

The study, published in the medical journal JAMA on Monday (Sept 2), is not definitive. Though it compared the long-term outcomes of about 2,300 bariatric surgery patients with some 11,500 closely matched patients who had not undergone surgery, it was an observational study, not a randomised controlled trial of the kind considered the gold standard in medicine.

But the findings were so striking that an editorial accompanying the paper suggested that weight-loss surgery, rather than medications, should be the preferred treatment for Type 2 diabetes in certain patients with obesity.

“The new information here is the ability of bariatric surgery to control macrovascular events like strokes, heart attacks, heart failure and kidney disease,” not just improve weight and diabetes control, said Dr Edward H. Livingston, the editorial’s author. “That’s a big deal.”

A bariatric surgeon himself, Dr Livingston said he had long been known as a “curmudgeon” who was reluctant to make claims about the long-term health benefits of weight-loss surgery. “This is the first time I’ve come out publicly saying, ‘You know what, this may be a better way to go,’” he said, adding that insurers should cover the procedure more liberally.

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But other scientists were less certain, and even the study’s authors said the findings must be confirmed in clinical trials that randomly assign patients to have surgery or continue with regular care.

“This study needs to be taken with a giant grain of salt,” said Dr David M. Nathan, director of the Diabetes Centre and Clinical Research Centre at Massachusetts General Hospital and a professor at Harvard Medical School, who was not involved in the study.

“It will be interpreted as, ‘You see, the surgery reduces heart disease,’” he said. But, he added, “it doesn’t show that.”

A similar observational study last year that compared 5,301 obese patients with Type 2 diabetes who had bariatric surgery with 14,934 patients who served as controls also found improved outcomes in patients who underwent the operation.

Both Dr Nathan and Dr Livingston said the comparisons between patients who had surgery and those who did not were flawed, because people who elect to undergo weight-loss surgery are in many ways different from those who do not. Surgical patients are highly motivated, for instance, and healthy enough that surgeons do not turn them away.

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The results of the study of weight-loss surgery — known as bariatric surgery and sometimes as metabolic surgery — were presented on Monday at the European Society of Cardiology Congress in Paris. The study, carried out at the Cleveland Clinic, was partly funded by Medtronic, a company that makes medical equipment used in weight-loss surgery.

The researchers first combed through electronic medical records to identify 2,287 patients with obesity and Type 2 diabetes who had undergone one of four types of weight-loss surgery at the Cleveland Clinic. The majority of patients had undergone gastric bypass or sleeve gastrectomy, while a smaller number had adjustable gastric banding or a duodenal switch procedure.

The scientists then identified 11,435 control patients with obesity and diabetes for comparison — five times the number of surgery patients. Although the researchers made an effort to match the control patients closely to the surgery patients, there were differences between the groups.

The members of the control group were slightly older and had double the smoking rates of the surgery group; the surgery patients were slightly heavier to begin with, and had higher rates of high blood pressure and high cholesterol.

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The main question the scientists sought to answer was whether those who had surgery were less likely to experience death or one of five major complications associated with obesity and diabetes: coronary artery events (like heart attacks), cerebrovascular events (like strokes), heart failure, atrial fibrillation or kidney disease.

The investigators found that over a period of eight years, 30.8 per cent of patients who had weight-loss surgery either died or developed one of the conditions, compared with 47.7 per cent of patients who did not have surgery, a 40 per cent reduction.

Surgery patients were also 41 per cent less likely to die of any cause during the study period: 10 per cent died, compared with 17.8 per cent of the patients who did not have surgery.

“The differences were simply astonishing,” said Dr Steven Nissen, chief academic officer of the Heart & Vascular Institute at Cleveland Clinic and the study’s senior author.

“We struggle to make small incremental improvements in cardiovascular morality, and here’s an eight-year trial where the magnitude and absolute reduction are very large,” he added.

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Patients who had metabolic surgery also lost an average of 15 per cent more weight than those who did not, and they had lower blood-sugar levels. They needed less medication to control diabetes and less insulin after the operation than the comparison group, and required fewer drugs to control blood pressure and cholesterol.

The sustained weight loss means “you’ve taken away the burden on the heart of pumping blood to a large body mass,” Nissen said.

Dr Ali Aminian, a surgeon who was the study’s lead author, said the next project would be to carry out a randomised controlled clinical trial that randomly assigned similar patients either to weight-loss surgery or to regular care.

Over one-third of American adults have obesity, which is closely linked to Type 2 diabetes, heart disease and stroke. The vast majority of people with Type 2 diabetes die of heart disease. THE NEW YORK TIMES