Why Obesity May Not Raise Breast Cancer Risk in Mexican Americans

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For a disease as complex as breast cancer, pinpointing its risk factors, genetic or otherwise, is challenging enough. But an intriguing new study suggests that the task may be even harder when ethnicity is thrown into the mix.

The research, conducted by Krystal Sexton, a fellow at University of Texas Health Science Center in Houston, focused on a cohort of Mexican-American women enrolled in the Binational Breast Cancer Study (or Ella trial) investigating differences in ethnic risk factors in breast cancer. When Sexton compared the women’s breast cancer rates to their body weight across their lifetime, she found that being heavy or obese did not correspond to a higher risk of breast cancer. What’s more, gaining weight seemed to lower women’s risk of cancer; for every 5 kg (11 lbs.) of weight they gained on average, they decreased their breast cancer risk by 8%. (More on Time.com: Special Report: Advances for Breast Cancer Patients).

That finding runs contrary to previous studies, which had hinted that overweight and obesity are associated with an increased risk of breast cancer, particularly among postmenopausal women, because fat cells release a form estrogen that can promote tumors in breast tissue. Obesity also hinders the absorption of estrogen circulating in the body, thus leaving more of the hormone to float around and contact breast tissue. But those studies, says Sexton, involved primarily Caucasian women, and there may be race-related differences in the way women develop breast cancer.

Because breast cancer is related to estrogen, a woman’s reproductive history may influence her risk of developing cancer. Hispanic women, for example, tend to have less time between key reproductive events, such as the onset of menstruation and their first live birth, compared with white women. The Mexican-American women in the study also experienced menopause earlier than previous data suggest Caucasian women do, and this trend may have spared the study volunteers from about two years of additional exposure to estrogen. Previous studies have also found that Hispanic women are more likely to have breast cancer that is estrogen receptor–negative, meaning that the tumors are less responsive to the cancer-causing effects of estrogen. Taken together, says Sexton, it’s likely that some counterbalance may be at work, with the lower overall estrogen exposure negating, to some extent, the estrogen released by adipose tissue following menopause. (More on Time.com: Photos: The Landscape of Cancer Treatment).

That may explain in part why the Mexican-American women did not show higher rates of cancer even when they gained weight, which many of them did: by the time they were in their 30s or later, 80% of the women, regardless of whether they were breast cancer patients or not, were overweight or obese.

The results, stresses Sexton, are only the first to explore how weight can impact breast cancer in an ethnic population, and more research needs to be done on Hispanic women to determine how their cancer risk factors may differ from those of women of other races. “We need a lot more study to be able to determine the [cause] of breast cancer in Mexican-American women,” says Sexton. “At the moment, we just don’t know enough about the risk factors in that population.” (More on Time.com: Revisiting the Debate: Mammogram Benefit for Women in Their 40s?).

And that means that it’s too early to say that obesity does not play a role in contributing to breast cancer, despite the study’s findings. “Another thing that is important to note is that regardless of the association with breast cancer, obesity is a risk factor for other chronic diseases such as heart disease and diabetes,” says Sexton. “So obesity under any context is still not a good thing for your health.”

More on Time.com:

Another Study Questions the Benefit of Routine Mammograms

Pregnant Women with Breast Cancer Should Do Chemo

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