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, MD, senior clinical investigator at Greisinger 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.”

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.

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, MD, 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.

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.