updated 6/23/2009 7:08:57 PM ET 2009-06-23T23:08:57

Women who have their stomachs stapled not only lose weight, they also may reduce their cancer risk by up to 40 percent, new research says.

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In a study of more than 2,000 fat people who had surgery to reduce the size of their stomachs, Swedish researchers found women who had the procedure were less likely to get cancer than those who did not.

But for some reason, the surgery didn't have the same effect in men; there was virtually no difference in the cancer rates in men who had the surgery and those who did not.

The research was published online Wednesday in the medical journal, Lancet Oncology.

A previous study has shown that stomach stapling surgery can prolong the lives of men and women by up to 10 years compared to those who don't have it. Two other studies have suggested women in particular benefit from a lower cancer risk after getting the weight loss operation.

Scientists have long thought fat people have a higher cancer risk, possibly because fat cells produce hormones that might lead to the disease. But doctors haven't been able to prove that losing weight in other ways, including dieting, reduces that risk.

"This is one more piece of evidence in a complex puzzle," said Dr. Len Lichtenfeld of the American Cancer Society, who was not linked to the Lancet Oncology study. "There seems to be a relationship between weight and cancer, but there is a missing link we don't understand."

Swedish researchers followed 2,010 obese patients from 1987 after they had their stomachs stapled, for about 10 years. Men and women were considered obese if they had a body mass index above 34 and 38 respectively. Experts say that a normal body mass index ranges from 19 to 25.

Researchers also tracked 2,037 fat people who did not have the surgery. For patients who got their stomachs stapled, most lost about 20 kilograms (44 pounds). In people who did not have the surgery, most gained a little over 1 kilogram (2 pounds, 3 ounces).

Of the women who had the surgery, 79 got cancer. In the non-surgery group, 139 women got cancer. Various types were seen, including breast, skin and blood cancer.

Among the men, 38 of the men who had the surgery got cancer versus 39 men in the non-surgery group.

The study was paid for by the Swedish Research Council and others, including drug makers Hoffman La Roche, Astrazeneca and Sanofi-Aventis, whose products include diet drugs.

Experts were baffled why only women appeared to have a lower cancer risk after the weight loss surgery.

Lars Sjostrom of Sahlgrenska University Hospital in Sweden and the paper's lead author, said it was possible there weren't enough men in the study to see an effect — men only made up about a quarter of the participants.

Sjostrom and colleagues also found neither weight loss without surgery or reduced calorie intake appeared to affect cancer rates. He added that other possibilities to explain the smaller cancer risk, including genetics, were now being considered. "There is an unknown factor behind this effect, but we have no idea what it is," he said.

Lichtenfeld hypothesized that the stomach surgeries might have different effects on hormones or some other substance in the body that ultimately reduced the chances of developing cancer.

Others said the study underlined the importance of losing weight for cancer prevention. About half of all cancers are thought to be preventable via lifestyle changes, including weight loss.

"It's good news if these risks can be reversed," said Julie Sharp of Cancer Research United Kingdom. "People don't have to do something as dramatic as having surgery, but the message is that individuals can change their own risk factors."

© 2013 The Associated Press. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

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