Women were divided into four groups, from lowest to highest BMIs. Those in the highest group had BMIs of 31 or higher; 30 and above is termed obese.
The association between triple-negative breast cancer and obesity was what scientists call "borderline" statistically significant, but Phipps said that is probably because of the relatively low numbers of cases in the study.
The researchers also found a modest link between greater physical activity and reduced risk of both kinds of breast cancer, suggesting -- but not proving -- that increased exercise might moderate a woman's risk of these cancers, they said.
When evaluating the risks linked with obesity, Phipps and her colleagues accounted for such factors as age, education, income, family history and ethnicity.
The long-known link between obesity and estrogen-positive breast cancers has been attributed to 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 said.
But triple-negative cancers aren't responsive to hormones, she said. "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]," Phipps said.
The exact reasons are unknown, but Phipps said obesity increases markers of inflammation and levels of insulin and insulin-like growth factor. "All of those things could contribute to the growth of the breast tumor," she said.
The study "confirms that obesity [and cancer risk] is not just an estrogen problem," said Dr. Joanne Mortimer, director of the women's cancers program at the City of Hope Comprehensive Cancer Center in Duarte, Calif., who reviewed the study but was not involved in it.
"It's not just the problem of the estrogen feeding the cancer cells," she said.
The fat cells may set up an environme
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