Aggressive Breast Cancer May Be Triggered by Obesity

Obesity in menopause boosts a woman's risk for a type of breast cancer tied to the hormone estrogen. Those excess pounds may also increase the risk for an aggressive breast cancer that's not influenced by hormones.

The aggressive type of breast cancer is called "triple-negative," because it lacks not only estrogen, but also progesterone and the protein HER2, which is involved in other types of breast cancer. Although triple-negative breast cancer is less common than other types, it's deadlier because it is so aggressive and because few treatments are available for it.

Researchers at the Fred Hutchinson Cancer Research Center in Seattle assessed data on 155,723 women in the Women's Health Initiative, a large-scale study of postmenopausal women that started in 1993. The women ranged in age from 50 to 79 years old.

Assessing data

The data included participants’ exercise habits, weight, height, and body mass index (BMI). During the follow-up, which lasted an average of eight years, 2,610 women developed estrogen receptor-positive breast cancer, which is fueled by estrogen, and 307 women developed triple-negative breast cancer.

The women were divided into four groups, from lowest to highest BMI. Those in the highest group had a BMI of 31 or higher. (A BMI of 30 or higher is categorized as obese.) Women with the highest BMIs had a 35 percent increased risk of developing triple-negative breast cancer, compared with those who had the lowest BMIs. The highest BMI group also had a 39 percent increased risk of developing other breast cancers.

Lead author Amanda Phipps, Ph.D., says the association between triple-negative breast cancer and obesity is "borderline" statistically significant, but that’s probably because of the relatively low numbers of cases in the study.

More fat, more estrogen

Researchers have long recognized the link between obesity and estrogen-positive breast cancer, which has been blamed on higher exposure to estrogen in fat tissue. After menopause, most of a woman's estrogen comes from fat tissue. "The more fat, the more estrogen exposure," Phipps says.

But triple-negative cancers aren't responsive to hormones, she says. "The fact that we see such a similar association between the two subtypes [and obesity] tells us it may not just be the hormonal effect of the obesity [driving the risk]."

The bottom line for women? Maintaining a healthy weight is vital as women age.

"I think people assume fat is inert," says Joanne Mortimer, M.D., at City of Hope Comprehensive Cancer Center in Duarte, Calif. But researchers are finding that fat cells "do more than take up space," she says.

Always talk with your health care provider to find out more information.