Weight-loss surgery cut the long-term risk of heart failure by more than half in obese patients without a history of heart disease or stroke, a new study shows.

It was a dramatic finding even the researchers hadn’t expected.

“We were surprised,” said Peter N. Benotti, M.D., senior clinical investigator at Geisinger Obesity Institute in Danville, Pennsylvania, and the study’s lead researcher. “Ours is the first published study to show that bariatric surgery impacts favorably on the risk of congestive heart failure.”

People with a body mass index of 30 or higher are considered obese. Bariatric surgery is generally recommended for people with a body mass index of 35 or higher. But guidelines issued in January from the American Diabetes Association recommend bariatric surgery for Type 2 diabetes patients with a BMI of 30 or higher if their diabetes is poorly controlled.

In the 3,448-patient study, published Tuesday in the Journal of the American Heart Association, half of patients received a type of bariatric surgery called gastric bypass and half did not. The surgery group saw their BMI drop from an average 46.5 to 32.5 after five years. BMI remained stable at around 46 in the non-surgery group.

Eight years after the weight-loss surgery, 24 people had been diagnosed with heart failure. Twice as many — 55 patients — in the non-surgery group had developed heart failure. The surgery group also had fewer heart attacks and strokes, but not enough to be significant by research standards.

Heart failure is on the rise, affecting more than 6 million U.S. adults as of 2014. By 2030, that number is expected to surpass 8 million. The condition is expensive, with total costs estimated at $30.7 billion. It’s also a drain on quality of life, causing symptoms such as shortness of breath and fatigue.

Although obesity is not known to cause heart failure, obese patients with a BMI above 35 are at higher risk of developing stiff or clogged arteries, high blood pressure, high cholesterol and diabetes — all risk factors for heart failure, said Philip Schauer, M.D., professor of surgery at the Cleveland Clinic Lerner College of Medicine and director of the Cleveland Clinic Bariatric and Metabolic Institute.

“Most likely, reducing obesity and those secondary risk factors is what likely led to reduction in developing heart failure,” said Schauer, who was not involved in the study.

Despite being the first published evidence of bariatric surgery’s ability to prevent heart failure, the new study is not the first to show the connection. Preliminary results of a Scandinavian study presented at a medical conference in November found bariatric surgery cut the risk of heart failure by almost half after four years.

During the procedure, surgeons reduce the stomach to a pouch about the size of an egg, then bypass the top section of the small intestine and attach a lower part of the intestine to the new pouch.

Experts prefer the term “metabolic surgery” because the procedure changes the metabolism beyond the benefits of weight loss, Schauer said.

Such changes, he said, include greater satiety — the feeling of fullness after eating — and greater loss of belly fat, known to contribute to inflammation and clogged arteries more than other types of fat. The surgery can also put diabetes into remission for some patients, according to Schauer’s landmark 2014 study.

Bariatric surgery has been around since the 1970s, Benotti said. But it didn’t enter the mainstream until a 1991 National Institutes of Health consensus statement classified it as a reimbursable procedure.

By 2011, bariatric surgery had become relatively common, with about 158,000 surgeries done that year in the United States, according to the American Society for Metabolic and Bariatric Surgery.

In 2015, there were about 196,000 surgeries. Less than a quarter of them were gastric bypass and more than half were gastric sleeve, in which surgeons reduce the size of the stomach but don’t bypass part of the small intestine.

Despite the sleeve procedure’s growing popularity, Geisinger Obesity Institute and the Cleveland Clinic continue to perform more gastric bypass operations, according to Benotti and Schauer.

“The patient perception is that it’s much less invasive than bypass, but that’s not borne out by the data,” Schauer said.

The sleeve may offer modest benefits in terms of fewer complications, but it results in less weight loss and fewer metabolic improvements, he said.

Overall, Schauer said, complications from bariatric surgery have fallen so low thanks to better training and the use of small, laparoscopic incisions that the procedure is no risker than an appendectomy, hysterectomy or gallbladder removal.

There are 24 million Americans with severe obesity, according to the ASMBS. Yet despite the proven benefits of bariatric surgery, only about 1 percent of patients who qualify actually get the procedure, Benotti said.

Insurance coverage is partly to blame.

Private insurers might cover a hip replacement for someone who weighs 400 pounds, but not bariatric surgery, “which is really strange and ironic,” Schauer said.

“This is not an expensive operation. Insurance companies need to get caught up with the literature and modify their policies accordingly,” he said.

Depending on the type of procedure, bariatric surgery costs between $12,000 and $30,000, according to research.

“Obviously in an ideal world, the best way to lose weight would be with diet and exercise, because there’s no risk,” Schauer said.

But research has shown that’s not enough for most obese patients. A look at 10 studies found that obese people undergoing surgery lost an average 57 pounds more than people trying to lose weight the old-fashioned way. Surgery combined with lifestyle changes has a success rate of up to 85 percent for five years or more, Schauer said.

Even so, bariatric surgery shouldn’t be taken lightly, Benotti said.

“It’s certainly not a quick fix or a simple solution,” he said. “It’s a long-term, lifelong commitment.”

Editor’s Note: This post was updated on June 1 to correct the spelling of Geisinger Obesity Institute.