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Researchers at the American Cancer Society have found an increased breast cancer risk among women who smoke, especially those who start smoking before they have their first child.
Although previous studies have looked at the possible relationship between breast cancer and smoking, they have not settled the controversy within the scientific community. Part of the reason is that while many studies have found a slight increase in risk overall, they have not found that smoking more cigarettes per day or smoking for more years increases the risk further, which is what might be expected.
In addition, findings about alcohol have complicated many of these studies. This is because alcohol is a known risk factor for breast cancer, and women who smoke are also more likely to drink alcohol. (The link between alcohol and breast cancer is one of the reasons the American Cancer Society recommends that women who drink limit themselves to no more than 1 drink per day.)
In the new study, published online February 28, 2013 in the Journal of the National Cancer Institute, researchers analyzed data from 73,388 women in the American Cancer Society’s Cancer Prevention Study II (CPS-II) Nutrition Cohort. A cohort is a study that follows a large group of people over time. During more than 13 years of follow-up, researchers counted 3,721 cases of invasive breast cancer. The rate of new cases was 24% higher in smokers than in nonsmokers and 13% higher in former smokers than in nonsmokers.
When the researchers looked specifically at risk based on alcohol status, they found that the increased risk in smokers was limited to current or former alcohol drinkers, and was not seen in those who never drank.
The risk of invasive breast cancer was highest in women who began smoking at an earlier age. When compared to women who never smoked, those who started smoking before their first menstrual cycle had a 61% higher risk, while those who started smoking after their first cycle, but 11 or more years before having a child, had a 45% higher risk.
The researchers also found that these results were supported by the findings of earlier cohort studies. When combining the results of 9 studies (including this one), they found a 12% increase in breast cancer risk among women who started smoking at a younger age, and a 21% increase in risk among women who started before the birth of their first child.
Mia Gaudet, PhD, American Cancer Society director of genetic epidemiology, said breast tissue is not fully developed until after a woman has her first child, and that makes it more sensitive to the harmful effects of tobacco.
Gaudet said, “The key message from this study should be additional motivation to young women to not start smoking.”
“From a biological standpoint, what is really intriguing is that early life exposures are important,” said Gaudet. “In the Cancer Prevention Study-3 we are recruiting younger women, which will allow us to further examine risk factors at an earlier age.”
The study did not look at whether quitting affects breast cancer risk in women who already smoke. However, there are many good reasons to quit. In the US, tobacco use is responsible for nearly 1 in 5 deaths; this equals about 443,000 early deaths each year. About half of all people who continue to smoke will end up dying from a tobacco-related disease.
Overall, the increase in breast cancer risk found in this study is not nearly as large as the known increase for some other cancers, such as lung cancer (where smoking is estimated to increase the risk in women by about 13 times). Most people know that smoking causes lung cancer, but tobacco use also increases the risk for cancers of the mouth, lips, nose and sinuses, voice box, throat, esophagus, stomach, pancreas, kidney, bladder, uterus, cervix, colon/rectum, and ovary, as well as acute myeloid leukemia. It also raises the risk of many other health problems, including heart and lung diseases.