Gastric bypass surgery – a treatment for obesity that is already known to reduce heart disease and diabetes – decreases the incidence of cancer by 80 percent over the five years following the procedure, Canadian researchers reported Wednesday.

The incidence of two of the most common tumors, breast and colon, were reduced by 85 percent and 70 percent respectively, Dr. Nicolas Christou of McGill University in Toronto said.

The study confirms the findings of two papers last August that showed the surgery reduced overall deaths from cancer. The new study goes a step further by showing reductions in the incidence of several specific types of cancer, said Dr. Philip Schauer of the Cleveland Clinic Lerner College of Medicine, who was not involved in the study.

“This is really powerful information,” said Schauer, who is the immediate past president of the American Society of Metabolic & Bariatric Surgery. “It reaffirms that obesity is a profound risk factor for cancer” and shows that “weight loss does seem to affect the development of new cancers.”

But Dr. Edward H. Phillips, a bariatric surgeon at Los Angeles’ Cedars-Sinai Medical Center, was skeptical about the findings because cancer takes a long time to develop and the patients were only studied for five years.

He noted that it is now common practice for surgery candidates to undergo mammograms, colonoscopies and endoscopies to screen for cancer before the weight loss surgery.

“It could be that we are selecting people out of the population who don’t have cancer,” biasing the results, he said. He believes losing weight will reduce the incidence of cancer, but it will take longer than five years for the effects to surface.

Christou countered that such screening “is not the standard of care” in Canada, where the subjects were drawn from.

Furthermore, many of the patients had undergone surgery as much as 15 years before the start of the study, leaving plenty of time for cancers to develop.

There are two main types of bariatric surgery.

The simplest is banding, in which an inflatable silicone band is placed around the stomach to reduce its capacity, allowing the patient to feel full after eating much less food.

In a gastric bypass, the stomach is sewn shut to reduce its capacity to three or four ounces, and the intestines are connected directly to the newly created pouch, bypassing part of the area where food absorption occurs.

This is generally a more invasive surgery but produces greater weight loss.

An estimated 205,000 Americans underwent the surgery last year, according to the American Society for Metabolic & Bariatric Surgery, and the number is expected to rise by 5 percent this year.

But only about 1 percent of those eligible for the surgery actually receive it, the society said.