Smoking or inhaling second-hand smoke can increase the risk of breast cancer, according to a large study, published in the current issue of Cancer Causes and Control. The work may help explain why previous studies of the link between tobacco and breast cancer came up with conflicting results.
Researchers have long puzzled over the hazy relationship between cigarettes and breast cancer. A few early studies hinted that smoking might actually protect women, a finding some researchers attributed to nicotine's suppression of estrogen, a hormone often linked to breast cancer. But most follow-up studies found no relationship between smoking and breast cancer; and still others showed that smoking, and second-hand smoke in particular, hiked the risk.
Kenneth Johnson and his colleagues at the Canadian government's Laboratory Centre for Disease Control in Ottawa gleaned information about 2317 women with breast cancer and 2438 healthy women from a Canadian cancer surveillance study. Participants had answered questions about risk factors such as alcohol consumption, physical activity, and age at menarche, along with their lifetime exposure to tobacco smoke at home and work. For premenopausal women, smoking or regular exposure to second-hand smoke doubled the risk of breast cancer; for postmenopausal women, those factors increased the risk by 50% and 30% respectively.
Johnson says the study may explain why studies consistently show that second-hand smoke raises breast cancer risk--his is the seventh one to do so--while others that examined smokers had mixed results. Perhaps, he says, it's because smoking and passive smoking carry almost the same risk. Most past studies compared active smokers to people who had never smoked; but many of the women in the latter group were exposed to passive smoke, "so you're really comparing exposed to exposed," Johnson says. In fact, when the smokers in the new study were compared to all nonsmokers--regardless of whether they were exposed to second-hand smoke or not--the risk of breast cancer was equal, just like in previous studies. The findings suggest that some women will develop breast cancer once they have been exposed to a certain amount of smoke, says Johnson--no matter how it enters their body.
Support for this theory came in 1996, when Christine Ambrosone, a biologist at the National Center for Toxicological Research in Jefferson, Arkansas, reported that women with a slow form of an enzyme that metabolizes a tobacco carcinogen had an added risk of breast cancer. Ambrosone says Johnson's study is solid, but she cautions that researchers still need to work out many details of the link between smoking and breast cancer. "My feeling is that breast cancer is a very heterogeneous disease, so this relationship is going to be complicated."