"This suggests there is no level of alcohol use that is completely safe in terms of breast cancer," said Anne McTiernan, a cancer-prevention researcher at Fred Hutchinson Cancer Research Center in Seattle and one of the report's lead authors. "If a woman is drinking, it would be better if she kept it to a lower amount."
The review, by the American Institute for Cancer Research (AICR) and the World Cancer Research Fund, evaluated research in 119 studies encompassing data on 12 million women from around the world. It is the first such review since 2010, the groups said.
For the first time, researchers concluded evidence is strong that vigorous exercise reduces breast-cancer risk. Pre-menopausal women who were the most active had a 17 percent lower risk of developing malignancies compared to the least active women, while post-menopausal women had a 10 percent decreased risk.
The researchers noted that many things influence cancer risk and that women can't control factors such as a family history of cancer. But, McTiernan said, "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."
At the same time, she said, a healthy lifestyle is not a guarantee. Rather, it's more like wearing a seat belt. Many women will do everything they can to reduce their risks of breast cancer but still get diagnosed. "That's unfortunate, but that's what happens," she said.
Researchers were not able to calculate the degree to which vigorous exercise might cut the risk of alcohol consumption. What happens, for example, if a regular drinker also runs daily?
"The mechanism suggests that it could be helpful," McTiernan said. Alcohol increases estrogen, which is linked to increased breast-cancer risk, while physical activity reduces it. "But I can't say that for someone who drinks five drinks and then runs, that the exercise is going to negate the adverse effects of the alcohol."
The report found women who are overweight or obese have a higher risk of post-menopausal disease.
"If women lose just 10 percent of their weight, it's linked to reduced blood estrogen, inflammation" and other factors associated with breast cancer, McTiernan said.
The report also found limited evidence linking dairy foods, diets high in calcium and foods containing carotenoids to a lower risk of some breast cancers. Carotenoids include such fruits and vegetables as kale, apricots and carrots.
About 252,000 women in the United States are expected to be diagnosed with breast cancer this year. AICR estimates that 1 in 3 cases could be prevented if women did not drink alcohol, were physically active and maintained a healthy weight.