About 100,500 new cases of cancer are caused by obesity every year, according to the most comprehensive attempt ever to estimate the cancers attributed to extra weight.

The analysis, released today by the American Institute for Cancer Research, is based on updated cancer data and a report released earlier this year by a panel of experts. Among the types of cancer most strongly linked to excess body fat:

*Breast, 33,000 cases a year caused by obesity.

*Endometrial, 20,700 cases a year.

*Kidney, 13,900 cases.

*Colorectal, 13,200 cases.

*Pancreas, 11,900.

*Esophagus, 5,800.

*Gallbladder, 2,000.

"The list of cancers affected by obesity will almost certainly increase as more research is completed," says Michael Thun, emeritus vice president of epidemiological research at the American Cancer Society. "Several other types of cancer -- liver, multiple myeloma and certain leukemias -- have been linked to obesity in some studies, but this needs confirmation."

Overall, about one-third of U.S. adults are obese, roughly 30 or more pounds over a healthy weight. Obesity also increases a person's risk of diabetes, heart disease and other diseases.

Excess weight raises cancer risk in different ways, says Tim Byers, director of the University of Colorado Cancer Center in Denver. For breast and endometrial cancer, it seems to be an estrogen problem, he says. Women who are overweight after menopause have more estrogen circulating in their blood, which increases cancer risk. For esophageal cancer, obesity leads to acid reflux, which damages the lower part of the esophagus.

Donna Ryan, an oncologist and president of the Obesity Society, says several hormones and factors involved in obesity may lead to cancer. "Insulin is a prime suspect," she says. "Insulin resistance and elevated insulin levels frequently occur in obese people. Insulin is a very powerful cellular growth factor, and it affects cancer cells."

To lower your risk for cancer: Lose weight, increase physical activity and eat healthier, says Anne McTiernan, director of the Prevention Center at Fred Hutchinson Cancer Research Center in Seattle.

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