The National Cancer Institute says that weight gain is a major risk factor for developing breast cancer. Gaining weight in adulthood appears to increase the risk of breast cancer before and after menopause. One large study found that women who gained about 20 pounds after age 18 had a 15% higher risk of breast cancer compared to women who gained little or no weight. Women who gained 55 pounds or more had a 45% higher risk. It is important for both men and women to keep a healthy weight. Managing weight is a challenge for most people in the U.S. In Fact, according to the Centers for Disease Control and Prevention (CDC), about 69% of American adults are over weight.