If Tim Byers could convince women of one thing regarding breast cancer risk, it is that 20 percentof all breast cancers, regardless of other factors, are caused by being overweight.

"Not only do we know that being overweight increases your risk, we think we know why that is," says Byers, a professor in the School of Public Health at the University of Colorado Denver. "It’s because it causes higher levels of estrogen in your body."

Fat makes estrogen. In addition, overweight women often have elevated levels of insulin, which suppresses hormones that normally would remove excess, freely circulating estrogen from the body.

All of this adds up to a cancer-friendly environment -- estrogen stimulates breast-cell division and supports the growth of estrogen-positive tumors -- particularly for post-menopausal women. "After menopause, the major source of estrogen is body fat," Byers says.

Byers, who specializes in cancer prevention research, is involved in several studies that are looking at dietary factors connected to cancer risk and recurrence.

"We’re testing this idea that losing weight will improve outcomes," he says. "We have good reason to believe that it will."

Not surprisingly, women who are physically active also have a lower breast cancer risk. "In large part, this is because of how it affects their weight," Byers says.

No. 2 in terms of risk factors?

Byers says it is unquestionably alcohol consumption: Two or more drinks on average per day increases risk.

"It’s a message that’s complicated, though," he says.


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"Drinking alcohol is heart-healthy, and so it’s a trade-off. I think it’s why we don’t hear as much about it, because it’s one of those rare cancer risks that’s good for some things but not for others."

He advises that women limit their intake to less than two drinks on average per day, and that those who are at a very high risk for breast cancer because of other factors - women with a first-degree family history (mother, sister, daughter), gene-mutation carriers or those who have already had the disease - limit intake to rarely or not at all.

"We don’t understand the mechanism for why there is this connection with alcohol, but there very clearly is," he says.

Beyond that, he recommends what he calls the "b" foods - blueberries, broccoli, Brussels sprouts - and other foods high in antioxidants, and says that there are several things that are on his list of "maybes."

"We’re looking at night-shift work, disturbances of circadian rhythms," he says. "But those are in the early stages of study. Also, different vitamins may play a part, but again, these are things that we are just now starting to consider."

But in terms of the most preventable causes of breast cancer today?

"Not being physically active and being obese. Hands down," he says.