A large prospective study of more than 13,000 healthy women at high risk of breast cancer identified several important lifestyle factors associated with cancer risk. The study reported that the risks of invasive breast, lung and colon cancers were significantly higher in women with long smoking histories, compared to women who did not smoke or had shorter smoking histories.

Investigators also found a significant association between low levels of physical activity and endometrial cancer risk. Use of alcohol, however, was not associated with increased cancer risk.

"The NSABP study was the first large study to prospectively examine the impact of smoking in women at high risk of breast cancer," said Stephanie Land, Ph.D., study author and Research Associate Professor in the Department of Biostatistics, Graduate School of Public Health, University of Pittsburgh. "Our results showed an even greater increase in risk than has been shown in previous studies, suggesting that for women who are at risk of breast cancer because of family history or other factors, smoking cigarettes is even more risky than for other women. It sends a very important message for women with family histories of breast cancer about the long-term risks of smoking, as well as the importance of staying physically active. We're seeing again that smoking cessation is one of the most effective tools we have for reducing risk of many cancers."

The study analyzed the risk of several common cancers in 13,388 women at increased risk for breast cancer (as defined by age, a diagnosis of lobular carcinoma in situ, family history, or other factors) who participated in the National Surgical Adjuvant Breast and Bowel Project Breast Cancer Prevention Trial, based on their baseline self-reported smoking, alcohol use, and physical activity.

The study found that the risk of invasive breast cancer was higher in smokers than in non-smokers, and increased with more years of cigarette smoking. Compared to women who never smoked, those who smoked at least 35 years had a 60 percent higher risk of invasive breast cancer, and those who smoked between 15 and 35 years had a 34 percent higher risk. Those who smoked less than 15 years had no increased risk of breast cancer. This is the third large, prospective study to report a strong association between smoking and breast cancer, and is the first to show further elevation of cancer risk in women already at high risk of breast cancer.

The incidence of colon cancer was also significantly higher for women with long histories of cigarette smoking. The risk of getting colon cancer was over four times higher for women who smoked more than 35 years versus those who had never smoked; risk was 7 percent higher for women who smoked for 15 to 35 years. This result confirmed findings of previous studies of women already at high risk of breast cancer.

Similarly, women who smoked had a significantly higher risk of lung cancer, a finding that confirms many previous studies. Those who smoked more than one pack of cigarettes per day for over 35 years had a risk that was 30 times higher than women who never smoked. Women who smoked less than one pack per day for over 35 years had a 13-fold increase in lung cancer risk.

Alcohol use was not associated with breast cancer risk in this study. Moderate alcohol consumption of up to one drink a day was associated with a 60 percent decreased risk of colon cancer compared to those who did not drink. Several factors might have been different in this study from past studies that have shown associations between alcohol use and cancer risk, Dr. Land said. In particular, there were fewer heavy drinkers enrolled in this study, compared to other studies. Also, the results of this study are based on a one-time self-report of alcohol drinking habits.

Low physical activity was not associated with breast, lung or colon cancer risk, though it was associated with a 70 percent increased risk of endometrial cancer. Investigators said this may be due to the association between fitness and obesity, also a risk factor for endometrial cancer.