The how and why likely go well beyond weight management, says Lee W. Jones, Ph.D., an associate professor at Duke Cancer institute. For starters, exercise is a powerful anti-inflammatory (many experts consider cancer an inflammatory disease); it also does wonders for your immune system, helping your built-in defenses wipe out suspicious cells.
In addition, working out speeds up your body's glucose-to-muscle transport system, meaning frequent exercisers have lower glucose and insulin levels in their bloodstreams—boons for reducing breast-cancer and type 2 diabetes risks. Another new study found vigorous sweat sessions might help the body break down estrogen in a cancer-preventing way. And research in Science found that exercise is so protective, it even benefits those living at very high risk: Among women who carry the BRCA mutations, those who exercise the most in their younger years tend to develop cancer later in life.
So exactly how much should you do? "More!" says Jones. "When it comes to exercise and breast cancer, there seems to be a dose response: The more you do, the greater the protection." Aim for an hour a day, four or five times a week, though the ideal would be an hour every day. The key, says Jones, is to vary your routine and exertion levels (some days, give 50 percent, others, 100) and to maintain an elevated heart rate for at least 20 minutes of each workout. In other words, says Bernstein, "you need to exert yourself so that you become out of breath. And you need to stick with exercise throughout your life. If you fall off, start again."