BAKERSFIELD, Calif. -- Research has suggested that obesity causes 90,000 cancer deaths in the United States each year. But so far there has been little direct proof that losing weight can affect an individual's cancer risk.

On Wednesday at the American Society for Metabolic & Bariatric Surgery meeting, doctors from McGill University reported the first convincing evidence that reversing obesity can dramatically cut the odds of cancer.

Researchers compared cancer rates in more than 1,000 obese patients who had obesity surgery to cancer rates in nearly 5,800 similar patients who did not have surgery.

They found that surgery reduced the risk of cancer by 80 percent, with the largest decreases seen in breast, colon, and pancreatic cancers.

Patients in the study were morbidly obese -- meaning they were more than 100 pounds overweight.