Postmenopausal women who are obese have a 35 percent higher risk than those who aren't obese of developing "triple-negative" breast cancer, according to a new study. This type of aggressive cancer lacks the three hormone receptors common in other breast cancers.
Obese women also had a 39 percent increased risk of developing estrogen receptor-positive breast cancer, which is fed by the hormone estrogen, the study said.
The researchers were surprised to find obesity linked with triple-negative breast cancer, said study researcher Amanda Phipps, a researcher at the Fred Hutchinson Cancer Research Center in Seattle.
Obesity is a known risk factor for cancers that respond to hormones; the link stems from the fact that fat tissue can increase estrogen levels in the body, Phipps said.
Obesity's influence on triple-negative breast cancer reveals that factors other than hormones must be at play, she said.
"Maybe it's not just estrogen that affects cancer risk, maybe it's not the hormonal factors," Phipps told MyHealthNewsDaily. "There could be other pathways that don't affect hormones" that also influence cancer risk.
The study was published today (March 1) in the journal Cancer Epidemiology, Biomarkers & Prevention.
Reviewing the risks
Phipps and her colleagues analyzed the health data of 155,723 postmenopausal women for about 10 years as part of the Women's Health Initiative study. They looked at the women's body mass indices (BMIs) and levels of physical activity.
Of the women in the study, 307 developed triple-negative breast cancer and 2,610 developed estrogen receptor-positive breast cancer.
The women with the highest BMIs had a 35 percent increased risk of developing triple-negative breast cancer, and a 39 percent increased risk of developing estrogen receptor-positive breast cancer, the study said.
Physical activity decreased the risk of both cancers. Women who said they had high rates of physical activity had a 23 percent lower risk of developing triple-negative breast cancer, and a 15 percent lower risk of developing estrogen receptor-positive breast cancer, than women who reported the lowest rates of activity.
Other factors at play
Obesity has been linked to several common human cancers — some that are associated with hormones and some that are not, said Dr. Cliff Hudis, chief of the Breast Medicine Service at Memorial Sloan Kettering Cancer Center in New York.
That means there must be other obesity-related factors that influence the development of cancers that lack hormone receptors, like triple-negative breast cancer, said Hudis, who was not involved with the study.
"Based on a narrow view of things, that obesity equals estrogen equals cancer risk, then you'd scratch your head over triple-negative" types, Hudis told MyHealthNewsDaily.
But not all triple-negative breast cancers are alike, he said.
"We're calling something 'triple-negative' because of its lack of three specific [hormone] targets, but that doesn't mean triple-negative breast cancer is best thought of as a single entity," Hudis said. "It could be a collection of different things."
Triple-negative breast cancer accounts for only 10 to 20 percent of breast cancers, so it's difficult to conduct a large study of the disease, Phipps said.
Next, Phipps said she hopes to study triple-negative breast cancer risks in younger women, because the cancer disproportionally affects them.
Pass it on: Being obese is associated with an increased risk of triple-negative and estrogen receptor-positive breast cancers.
Follow MyHealthNewsDaily staff writer Amanda Chan on Twitter @AmandaLChan.
This story was provided by MyHealthNewsDaily, a sister site to LiveScience.