City of Hope researchers found that women who exercised three or more hours a week in the 10 years after their first period had 30 percent reduction in breast cancer risk compared to less-active women.
And women who exercised nearly four hours a week had a 58 percent reduction in risk.
Researchers say exercise has a two pronged effect on breast cancer. First, it helps boosts the immune system. Second, it helps women lose weight which has an effect on hormone production.
Cancer experts say about 65 to 75 percent of breast cancers are sensitive to estrogen.
"By doing exercise we will decrease the amount of fat in their body. Then we will decrease the amount of the estrogen effect on the receptors and hence the growth or recurrence of the breast cancer," said Dr. Nubar Boghossian, Glendale Adventist Medical Center.
"It makes me feel good that I am already exercising," said Verrengia.
Even though Verrengia goes to the gym every day, doctors say most women don't need a lot to make a difference.
"Women don't need to run the marathon to decrease their cancer, just mild to moderate exercise. Even for women who cannot perform three to five hours a week, which was in the City of Hope study, that leaves 30 minutes three times a week," said Dr. Boghossian.
When City of Hope researchers monitored the activity level of post-menopausal women specifically, they found over a lifetime, 2-1/2 hours of strenuous activity or three or more hours of moderate exercise a week appeared to drop breast cancer risk significantly.