How Much Does Lifestyle Affect Breast Cancer Risk?

June 24, 2011 -- Women who are struggling to exercise regularly, maintain a healthy weight, or turn down a nightly glass of wine just got a bit of breathing room, at least as far as breast cancer is concerned.

A new study shows that these modifiable risk factors, even when taken together, account for a relatively small portion of a woman's overall breast cancer risk.

"I was surprised, a little bit, that you really do get a really different view of what is achievable by looking at absolute risks as opposed to relative risks, or attributable risks. The absolute risk reductions are kind of small it seems to me," says study researcher Mitchell H. Gail, MD, PhD, a senior investigator in the division of cancer epidemiology and genetics at the National Cancer Institute in Bethesda, Md.

Gail is no lightweight in the field of risk prediction. His research led to the development of a widely used tool that calculates a woman's five-year risk of breast cancer based on things that can't be controlled, like age, family history, ethnicity, how old she was when she had her first child, and her age at her first period. The tool calculates a number that's known as the "Gail score."

His new study, which is published in the Journal of the National Cancer Institute, shows that non-modifiable risk factors may be more important to determining overall breast cancer risk than things that are controllable, like body mass index (BMI), leisure time physical activity, or alcohol consumption.

Analyzing Cancer Risk

To look at the risks, the study compared 2,569 breast cancer patients to 2,588 women of the same ages who did not have cancer. The women interviewed in detail about a host of things that could contribute to cancer risk like their socioeconomic status, education, occupation, smoking habits, physical activity, alcohol and coffee consumption, dietary habits, and their weights at various ages.

Based on their answers, researchers developed models to try to tease out the relative contributions of physical activity, body weight, and consumption of alcohol to their risk of breast cancer.

Here's an example. The study predicts that a woman who is 65 years old with no other risk factors will have an average risk of developing breast cancer within the next 20 years of about 6.5%. If that woman stopped drinking alcohol, started exercising for at least two hours each week, and maintained a normal body weight, her absolute risk of getting breast cancer would drop about 1.6%, to 4.9%.

Here's the part that often ends up in news headlines. Exercising, staying slim, and not drinking alcohol accounted for about 24% of her overall risk of getting breast cancer. So by doing all those things, she's reducing her risk for breast cancer by 24%, which sounds like a sizeable chunk. But it's only a part of an already small number.