Smoking seems to increase the risk of breast cancer in many women, but a new study suggests it doesn't further influence breast cancer risks in women who are obese.

Women who smoke and have a body mass index (or BMI, a measure of height and weight) of less than 30 have an increased risk of developing breast cancer , compared with non-smokers, and the risk goes up with smoking years, said study researcher Juhua Luo, assistant professor in the department of community medicine at West Virginia University.

But researchers didn't find the same association in obese women, who had a BMI higher than 30, Luo said.

"We do not yet understand the reasons for obesity's potential 'protective' effect against breast cancer from smoking, [though] we speculate it may be related to estrogen," Luo told MyHealthNewsDaily. "Smoking has been reported to lower the level of estrogen. The anti-estrogenic effects of smoking may be more pronounced in obese women."

However, the new study shows only that smoking doesn't further affect breast cancer risk in obese women, beyond risks that come solely from being obese.

Past research, including a study published last month in the journal Cancer Epidemiology, Biomarkers & Prevention, showed that obese women have a 35 percent higher risk of developing aggressive triple-negative breast cancer than non-obese women. That's because obesity is linked with an increase in estrogen, which can spur the growth of estrogen-responsive breast tumors, according to the National Cancer Institute.

The new study relating smoking and obesity to breast cancer risk was presented April 3 at the annual meeting of the American Association for Cancer Research.

Increased risks

Luo and his colleagues analyzed the health data of 76,628 women ages 50 to 79 with no cancer history, who were part of the Women's Health Initiative study. The women were recruited between 1993 and 1998, and followed until 2009.

Compared with non-smokers, researchers found that non-obese women who smoked 10 to 29 years had a 16 percent increased risk of breast cancer; those who smoked 30 to 49 years had a 25 percent increased risk of breast cancer; and those who smoked 50 years or more had a 62 percent increased risk of breast cancer.

But none of these breast cancer risks existed when researchers compared obese non-smokers with obese smokers, the study said.

However, the number of women who smoked 50 years or more was very small (50 non-obese women and 6 obese women smoked 50 years or more) so it's possible that the risk could be due to chance, Luo said.

Smoking still bad

While the findings are interesting, they do not at all mean that obese women should take up smoking to protect themselves from breast cancer, said Dr. Charles Shapiro, professor of medicine and director of breast medical oncology at the Ohio State Medical Center and James Cancer Hospital, who was not involved with the study.

"What about the lung cancers and all the other cancers you get from smoking?" Shapiro told MyHealthNewsDaily.

Obesity is a well-known risk factor for breast cancer, he said, so while smoking may not affect cancer risks for obese women, the finding does not suggest that obese people take up smoking to mitigate their breast cancer risk, Shapiro said.

Pass it on: Smoking doesn't further increase breast cancer risk in obese women, but that doesn't mean obese women should take up smoking to protect themselves from developing breast cancer.

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