March 1, 2011 -- Obesity and a sedentary lifestyle appear to increase the risk for an uncommon but aggressive breast cancer that is not fueled by the hormone estrogen, a surprising new study shows.
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.
They are referred to as triple-negative tumors because they do not express the hormones estrogen and progesterone or HER2 protein.
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.”
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.