Up to a third of breast cancer cases in Western countries could be avoided if women ate less and exercised more, researchers at a breast cancer conference said.
Experts said the focus should shift to changing behaviors like diet and physical activity.
Breast cancer is the most common cancer in women. A woman's lifetime chance of getting breast cancer is about one in eight. Obese women are up to 60 percent more likely to develop any cancer than normal-weight women.
Many breast cancers are fueled by estrogen, a hormone produced in fat tissue. So experts suspect that the fatter a woman is, the more estrogen she's likely to produce, which could in turn spark breast cancer. Even in slim women, exercise can help reduce the cancer risk by converting more of the body's fat into muscle.