Cancer experts say they're increasingly confident that at least two lifestyle choices can affect a woman's risk of getting breast cancer: drinking alcohol and exercising.
Just one alcoholic drink each day is enough to boost breast cancer risk, according to a comprehensive new report published Tuesday. Vigorous exercise, by contrast, can decrease the risk in both pre- and postmenopausal women.
The American Institute for Cancer Research and the World Cancer Research Fund published their joint report, which includes data on 12 million women and 260,000 cases of breast cancer gathered in nearly 120 studies.
"The evidence is clear: Having a physically active lifestyle, maintaining a healthy weight throughout life and limiting alcohol — these are all steps women can take to lower their risk," said Anne McTiernan, a lead author of the report and a cancer prevention expert at the Fred Hutchinson Cancer Research Center in Seattle.
The study gives researchers "even greater confidence in the results," McTiernan said in an email.
Alcohol increases risks
Tuesday's report upholds earlier findings about the links between alcohol consumption and breast cancer risk. Yet McTiernan said she was surprised to find that just one drink a day on average was enough to raise a woman's risk.
In the U.S., a standard drink contains about 14 grams of pure alcohol, which is found in 5 ounces of wine, 1.5 ounces of distilled spirits, or 12 ounces of a 5-percent alcohol beer.
The analysis of premenopausal women included 10 large cohort studies, in which more than 4,000 women developed breast cancer. While the increase in risk for drinking an average of 10 grams of alcohol per day was relatively small — about 5 percent — it is still statistically significant.
The postmenopausal analysis included 22 large cohort studies, in which more than 35,000 women developed breast cancer. Researchers found a 9 percent increase in risk for drinking an average of 10 grams of alcohol per day, which is also statistically significant.
There are still many unknowns about how and why alcohol consumption affects breast cancer risk, Melissa Pilewskie, a surgical breast oncologist at the Memorial Sloan Kettering Cancer Center said in an interview.
Pilewskie was not involved in Tuesday's report but said its findings were consistent with a number of other studies.
She said it's unclear whether one drink per day is the same as having a few drinks here and there throughout the week. Alcohol consumption may also be a "surrogate" for other lifestyle factors that are the real risk culprits.
Whatever the case, our drinking habits are one of the few areas of cancer risk that we can actually control, she said. Genetics, family history, age, breast density — these are much greater risk factors for breast cancer, but we can't change them.
"For women who are at increased risk [of breast cancer], this is something we think likely could make a difference, even though it may be only a moderate difference," Pilewskie said.
Exercise decreases risks
The new report provided stronger evidence that moderate exercise can decrease the risk of post-menopausal breast cancer — the most common type of breast cancer. It also revealed, for the first time, that vigorous physical exercise can decrease the risk in premenopausal women as well.
Premenopausal women who were the most active had a 17 percent lower risk of developing breast cancer, compared to those who were the least active, the report found. Postmenopausal women had a 10 percent reduction in risk.
Alice Bender, a nutritionist at the American Institute for Cancer Research, said "vigorous" activity should be sufficiently intense that it's hard to carry on a sustained conversation. That could mean power walking, jogging, or cycling, depending on the person's fitness level.
Bender acknowledged that exercising more and drinking less are not surefire ways to prevent cancer, just like exercising less and drinking more won't condemn you to a diagnosis.
"There are no guarantees when it comes to cancer. We know a lot of things are out of our control," she said. But evidence increasingly suggests that healthier lifestyle choices can "move the needle" toward cancer prevention.
Findings from Tuesday's breast cancer report will included in the cancer institutes' forthcoming 2017 report on diet, weight, physical activity and cancer prevention. A global panel of experts will also use the research to update the World Cancer Research Fund's Recommendations for Cancer Prevention.