Smoking raises the risk of breast cancer for healthy-weight and overweight women but not for those who are obese, new research suggests.
It's a first-of-its-kind finding, and even if other studies confirm it, it doesn't mean that smoking is safe for women who weigh way too much, researchers say.
"Smoking is a strong risk factor for many other diseases other than breast cancer," including lung cancer and heart disease, said Juhua Luo, a West Virginia University scientist.
She led the study and presented results Sunday at a meeting of the American Association for Cancer Research in Orlando, Fla.
Obesity has long been recognized as a risk factor for breast cancer, but research tying smoking to breast cancer is much weaker. In March, Luo published results of a study that found a 16 percent higher risk of breast cancer among postmenopausal women who smoke or used to smoke versus those who never did.
The 76,628 women were in a government-funded, decades-long study called the Women's Health Initiative, and 3,378 breast cancer cases occurred.
Luo's new study looks closer at these same women according to body mass index, a measure of height and weight.
Those who were healthy-weight or merely overweight, with BMIs under 30, were more likely to develop breast cancer if they smoked; the risk was 16 percent higher for those smoking for 10 to 29 years and 25 percent higher for those who smoked 30 to 49 years.
However, researchers saw no added breast cancer risk in obese smokers (BMI of 30 or above) compared to nonsmokers who weighed that much.
"That needs to be interpreted cautiously because it's the first time that anyone's examined whether the relationship of smoking with breast cancer is different by level of obesity," said Susan Gapstur, vice president of epidemiology at the American Cancer Society. "Other studies would need to confirm this relationship."
Researchers had no explanation for the results. Many breast cancers are fueled by estrogen, and fat tissue makes that hormone. So it could be that obesity is contributing so much risk already that a smaller risk from smoking is less apparent in these women.
But there's no way to tell that from this research. "We cannot separate the effect of smoking from the effect of obesity," Luo said.
It also could be that obese women died of other smoking-related problems such as heart disease before they even had a chance to develop breast cancer. In that case, they would not have even been counted in a study like this, she said.
The frustrating bottom line: More research is needed.