An article published this afternoon in the Archives Internal Medicine sheds some interesting light on the ongoing question of whether or not cigarette smoking increases the risk of breast cancer. And guess what? According to this research, for some women the answer is yes, for others no and for some-believe it or not-the risk of breast cancer may be decreased.
That smoking could actually decrease the risk of breast cancer is one of those “believe it or not” moments in evidence-based medicine, but I wouldn’t go around cheering that smoking is good for your health. It isn’t, and nothing about the findings in this study should change anyone’s opinions about the risks of tobacco. [more]
The research, which was done by very highly regarded scientists from Harvard, reviewed the relationship between smoking and breast cancer in a large group of nurses who have been carefully studied prospectively for the effects of active smoking (from 1976-2006) and passive smoking (1982-2006). There are 111,140 women in the active smoking study and 36,017 in the passive group.
The scientists looked as carefully as possible at a number of related items, such as the number of years smoked, the number of packs per day, when (and if) a woman in the study stopped smoking among others. They also looked at a number of factors that may have influenced a woman’s risk of getting breast cancer and made adjustments to the data to account for those as well.
They found that-in general-cigarette smoking “marginally” increased the incidence of breast cancer compared to women who had never smoked. Every increase of 20 pack years of smoking (a “pack-year” is one year of smoking one pack of cigarettes, so 20 pack years could be 2 packs a day for 10 years or one pack a day for 20 years) beyond the onset of menstrual periods as also associated with a “marginal” increase in the incidence of breast cancer as well.
In one analysis of their data, the researchers combined several “smoking” factors, and found that women who were “heavy smokers” (defined as more than 35 years with more than 25 cigarettes a day) and who started smoking before the age of 18 had a 25% higher risk of breast cancer compared with women who never smoked.
In a somewhat unusual twist, the researchers report that as pre-menopausal women smoked more cigarettes, their risk of breast cancer increased. However-and this is where things get interesting-for every 20 pack year increase AFTER menopause there was a “marginally decreased incidence of breast cancer” amounting to about 7%.
For women who smoked and developed breast cancer, there was a slight preponderance of hormone sensitive breast cancers as women consumed more cigarettes.
Exposure to passive smoke in women who were active smokers showed that passive smoking increased breast cancer risk compared to actively smoking women who reported little passive smoke exposure. In non-smokers, exposure to passive smoking under a number of scenarios did not influence breast cancer risk.
The authors conclude:
“Results from the present study suggest that the initiation of smoking before menopause and particularly before the first birth was most strongly associated with an increased risk of breast cancer…Smoking after menopause was positively associated with a slightly decreased breast cancer risk. This difference suggests an antiestrogenic effect of smoking among postmenopausal women that may further reduce their already low endogenous (internal) estrogen levels.”
In other words, if you are premenopausal, and are a heavy smoker, your risk of breast cancer is increased. In postmenopausal women, smoking may decrease circulating hormone levels (which are already low in these women) and have a slight protective effect, while at the same time blocking the positive effects of estrogen and potentially explain some of the increased risk of osteoporosis seen in postmenopausal women.
In another conclusion, the authors write:
“Our results combined with the evidence from previous prospective cohort studies collectively suggest that passive smoking may not play an important role in the etiology of breast cancer. Nonetheless, we found that regular exposure to passive smoking may magnify the effect of active smoking.”
“Despite the extensive research on cigarette smoking in relation to breast cancer risk, this association remains controversial. Results from the present study suggest that the potential effect of active smoking on breast cancer risk is modest. The risk of breast cancer may increase with younger age at smoking initiation and longer duration of smoking. Antiestrogenic effects of smoking, which may convey a reduced risk of breast cancer, are suggested to be more dominant among postmenopausal women.”
What should we make of these findings?
As pointed out by Dr. Michael Thun–who is one of my American Cancer Society colleagues, and an acknowledged expert on these issues–others have studied the relationship between cigarette smoking and breast cancer risk and concluded that the evidence is limited for active smoking, and debatable regarding passive smoking. And while everyone would like to have more definitive guidance, the reality is that answers to the question of breast cancer in relation to tobacco smoke have to be put into the larger context that smoking cigarettes does lots of bad things to your body.
We can have academic arguments about the impact of cigarette smoking on breast cancer risk, but even the current paper points out some of the difficulties in coming to definitive conclusions. And even when the relationship appears to be statistically significant, from a practical standpoint (as noted by the authors several times in their report) the impact is “modest” or “marginal.” That translates into an increase of a couple of percentage points in risk, but not the huge magnitudes of increased risk that have been seen for other cancers, especially lung cancer.
So while this research is important and well done, no one should take any comfort that smoking can decrease the risk of breast cancer in postmenopausal women anymore than they can grab onto the findings that smoking increases the risk of breast cancer in premenopausal women.
The message remains the same today as it was yesterday and will be tomorrow and for years to come: smoking is just plain bad for your health, no ifs and or butts.