The analysis of data from a health study involving postmenopausal women revealed that the heaviest women were 35% more likely to develop so-called triple-negative breast cancers than the thinnest women.
Triple-negative breast cancers make up 10% to 20% of all cancers of the breast. They have a poorer prognosis than other tumors, in part because there are no targeted hormonal therapies to treat them.
Fat tissue is a significant source of estrogen production in women and obesity is a known risk factor for estrogen-sensitive tumors.
The finding that obesity also appears to raise the risk for triple-negative tumors, which are not fueled by estrogen, was unexpected, study researcher Amanda I. Phipps, PhD, tells WebMD.
Phipps is a postdoctoral fellow at Seattle’s Fred Hutchinson Cancer Research Center.
“Hormones are one pathway by which obesity can impact cancer growth, but there are others,” Phipps says. “The fact that we see this association with triple-negative tumors suggests that these other pathways are important.”
Exercise, Body Mass Index, and Cancer Risk
The analysis included 155,723 participants enrolled in the Women’s Health Initiative (WHI), which followed postmenopausal women for 15 years starting in the early 1990s to assess their risk for cancer, heart disease, and osteoporosis.
During about eight years of follow-up, 307 of the study participants were diagnosed with triple-negative breast cancers and 2,610 were found to have estrogen-sensitive breast cancers.
The women in the study were divided into four groups according to body mass index (BMI).
Compared to women with the lowest BMIs, those with the highest were 39% more likely to be diagnosed with estrogen-sensitive tumors and 35% more likely to have triple-negative tumors.
Compared to women who exercised the least, those who exercised the most were 15% less likely to develop estrogen-sensitive tumors.
The study appears in the March 1 issue of Cancer Epidemiology, Biomarkers & Prevention.
African-Americans, Younger Women Have Higher Risk
Triple-negative breast cancers are common among women who have a genetic predisposition known as BRCA1, and they also occur more often in African-American women and tend to occur in younger women.
While none of these risk factors is modifiable, Phipps says the new study suggests two potential interventions that may lower a woman’s risk for developing the disease.
“There are already hundreds of reasons for women to maintain a healthy weight and remain physically active,” she says. “This may be one more.”
University of Wisconsin associate professor Amy Trentham-Dietz, PhD, agrees.
“These findings suggest that avoiding obesity and staying active could lower a woman’s risk for all types of breast cancer, not just those that are estrogen-receptor positive,” she tells WebMD. “That is a very positive message.”
Number of Childbirths and Cancer Risk
Triple-negative breast cancers were first identified less than a decade ago, and Phipps says they are still something of a mystery to researchers and doctors.
Findings from a separate analysis of the WHI data, reported last week by Phipps and colleagues, also surprised the researchers. That study was published Feb. 24 online in the Journal of the National Cancer Institute.
The analysis suggested that the more children a woman has delivered, the higher her risk for the cancer. Women in the study who had never given birth had a 40% lower risk for triple-negative cancers than women who had.
Childbirth is known to be protective against estrogen-receptor positive breast cancers.