The Women's Cancers Program at City of Hope aims to uncover how women can cut down their risk of cancer, and eventually to prevent the disease altogether. Leslie Bernstein, Ph.D., professor and director of cancer etiology, leads these prevention efforts. The award-winning researcher recently sat down to talk about past breakthroughs, future directions and what women can do to reduce their cancer risk.

Q: What are some of your key findings about lifestyle and cancer risk?

A: In 1994, I published the first paper that shows physical activity reduces breast cancer risk. Over the years there have been more than 50 studies of physical activity and breast cancer risk, looking at it many different ways. There are consistent findings showing that women who have exercised for most of their lives have reduced risk of breast cancer.

In addition, one of the first things we ever found was that physical activity reduced colon cancer risk. We showed it in men, but initially, we saw no effect in women. It turns out that hormone replacement therapy, which many women were using, reduces risk of colon cancer so much that exercise doesn't provide an added benefit. There's a tradeoff, since hormones slightly increase risk for breast cancer. So we had to look only at women who hadn't used hormone therapy, and we were able to clearly see the effect of physical activity in reducing colon cancer risk in these women.

Q: What questions are you still seeking to answer?

A: We're beginning to look at the genetics behind who benefits from physical activity. There will be women who exercise and still get breast cancer, and we want to know why. Do women who get breast cancer and exercise differ somehow from women who do the same exercise and don't get breast cancer? People haven't looked at those kinds of genetic questions.

Q: What recommendations would you offer for women who are looking to lower their cancer risk?

We know that people should exercise four to five days a week, for about three hours total. A half-hour a day almost gets them to that three hours; 45 minutes would definitely do it.

If you can exercise three hours a week, you can lower your risk of breast and colon cancers. The evidence is beginning to be more conclusive that physical activity also reduces risk of cancer of the uterus and possibly the ovary. We're even seeing a little benefit in cutting thyroid cancer risk.

Your age and your physical health will define what you can do as exercise. Physically stress your body according to its limits (and make certain you consult your physician if you have any health problems). If you're 20, you may run or jog three to five miles. But if you're 70, you might walk a mile or two at a pace that makes you become out of breath.

And I almost want to couch the question in the opposite direction. What increases your risk of breast cancer? Inactivity increases your risk. Hormone therapy can increase your risk moderately. Drinking alcohol gives you a modest increase in risk.

These are choices you can make and balance. If you hadn't used hormones but over that same time had been drinking two glasses of wine every night, you'd see about the same increase in risk. And the risk in either case is not as significant compared to having a family history of breast cancer or being inactive all your life.

Some women say, 'I'm in my 50s and I haven't really exercised. Is it too late?' Well, we're not positive that it will lower your risk of breast cancer if you become active now. But exercise has many health advantages. So my answer always is 'It's never too late.'