Obese & postmenopausal women have higher risk for breast cancer

A new study suggests that women who are obese and postmenopausal may have a higher risk for developing breast cancer compared to thinner women. 

The study looked at data from more than 67,000 postmenopausal women who participate in a study from 1993 to 1998 called the Women's Health Initiative. The women were followed for an average of thirteen years. Over the course of that time, more than 3,300 of the women developed breast cancer.

The risk for breast cancer among the most obese women in the study was 58 percent higher than the risk for women of normal weights. The researchers said the most obese women had a body mass index of 35 or higher. Their findings showed that very obese women were at risk for breast cancers driven by estrogen and progesterone. 

Obese women were also more likely to have large tumors and cancer that spread beyond the breast and into the lymph nodes. The study also showed that women who gained more than fivepercent of their body weight over the course of the study also had an increased risk for breast cancer. 

Among the women in the study who were already overweight or obese, there was no change in the risk for breast cancer whether they lost or gained weight over the thirteen year follow-up.

Other than obesity being linked to a higher risk for breast cancer, it was also associated with worse outcomes. Even if women were a normal weight at the beginning of the study, women who gained weight in early postmenopause had an increased risk for breast cancer. 

The researchers did not find a cause-and-effect link between obesity and the increased risk of breast cancer. However, lead researcher of the study Marian Neuhouser says that the increased risk for breast cancer is likely due to an increase in estrogen. "Obesity is known to increase estrogens in the postmenopausal women because estrogen is made by fat tissue. Fat tissue also secretes inflammatory factors and is associated with insulin resistance -- all of which may increase breast cancer risk." Neuhouser is a professor of epidemiology at the Fred Hutchinson Cancer Research Center in Seattle.

Neuhouser said, "Obesity is a risk factor for breast cancer that is modifiable, making a healthy weight very important for prevention. We can't change our genes or family history, but we can change our lifestyle habits and aim to maintain a healthy weight to lower breast cancer risk.” Women who lost weight during the study reduced their risk for breast cancer. 

The study was published in June in the online journal JAMA Oncology.