Weight-loss surgery may help obese women lower their risk of developing cancer, Swedish researchers said on Tuesday.

They found women who had weight-loss surgery were 42 percent less likely to develop cancer during a 10-year study published in the journal Lancet Oncology.

Men in the study did not benefit, possibly because many cancers are driven by female hormones such as estrogen, they said, or simply because fewer men get weight-loss surgery.

Obesity has long been known to raise the risk of cancer, and the evidence continues to mount.

A study released on Tuesday in the Journal of the American Medical Association found people who were obese as young adults had twice the risk of developing pancreatic cancer, an especially aggressive kind.

Weight-loss operations -- in which doctors change the digestive system's anatomy to cut the volume of food a person can eat -- have been shown to reverse diabetes and reduce the risks of dying from heart disease.

The Swedish study, led by Lars Sjostrom of Sahlgrenska University Hospital in Gothenburg, compared 2,010 obese patients who had weight-loss surgery with 2,037 obese patients who got standard diet and exercise treatment.

Overall, they found the surgery helped people maintain an average weight loss of 19.9 kg or about 43 pounds over 10 years. People in the diet and exercise group gained an average of 1.3 kg or nearly 3 pounds during the study period.

The surgery cut the rates of cancer by a third, but women enjoyed most of that benefit. Among women, there were 79 first-time cancers in the surgery group, and 130 among those who got standard treatment.

Dr. Andrew Renehan of the University of Manchester in the United Kingdom said in a commentary the absence of a benefit in men could simply reflect the number of men who were in the study.

He said for women, the greatest cancer prevention effects were likely to be post-menopausal breast and endometrial cancers -- cancers sensitive to hormone levels.

Renehan said the effects of weight-loss surgery may take longer to show up in men, who are more prone to colon, rectal and kidney cancers, which take longer to become apparent.

Dr. Leena Khaitan, a bariatric surgeon at University Hospitals Case Medical Center in Cleveland, said the fact that men did not benefit likely reflected who gets the surgery.

"We know 80 percent of patients who undergo weight-loss surgery tend to be women. I suspect if we had larger numbers of men, we would probably see a difference," Khaitan said.

She said the study and others like it suggested bariatric, or weight-loss, surgery may be an important way to prevent costly, chronic diseases like cancer.

"It's a strong argument for preventive medicine."

She said many patients who qualify for the surgery do not get it because of problems with insurance coverage, but that may be short-sighted.

A study last year in the American Journal of Managed Care funded by Johnson & Johnson, a maker of bariatric surgery instruments, found insurers recoup the costs of weight-loss surgery within two to four years as obese patients become healthier and have fewer medical problems.