An American Cancer Society study found that gaining weight may be a serious risk factor for breast cancer. Women who gained 21 to 30 pounds after the age of 18 were 40 percent more likely to get breast cancer than women who gained five pounds or less. Women who gained more than 70 pounds had twice the risk of cancer of women who stayed within five pounds of what they weighed at 18. This supports the idea that body fat increases breast cancer risk by increasing levels of the hormone estrogen, and gives even further incentive to maintain a healthy body weight.
Cancer Epidemiology, Biomarkers and Prevention. February 2004;13(2):220-4