Most women would be willing to do just about anything to avoid breast cancer, a sometimes disfiguring, potentially fatal disease.
One way to significantly reduce breast-cancer risk is as close as the sneakers in your closet.
Nearly 30 studies have shown that women who exercise at moderate to vigorous levels for three or more hours per week reduce their risk of getting breast cancer by 30 percent to 40 percent.
That's about as much reduction in risk as provided by the drug tamoxifen, but without the side effects.
The American Cancer Society and the World Health Organization's International Agency for Research on Cancer recently developed guidelines for cancer prevention focusing on physical activity, weight control and nutrition.
They both conclude that the evidence linking exercise to a reduced risk for breast cancer is convincing, and both recommend that women engage in moderate to vigorous exercise for at least 30 minutes per day, preferably more for optimal benefit.
Hormones Are the Link
Scientists believe that hormones are the link between exercise and breast-cancer protection. Exercise reduces levels of estrogen, testosterone, insulin, and growth factors, all of which may either cause breast cancer or prompt it to grow faster and larger. Women with high levels of these hormones in their blood have a high risk of developing breast cancer.
Research recently presented at the American Association for Cancer Research annual meeting in San Francisco may shed some additional light. Dr. Celia Byrne and colleagues (from Harvard University) studied the relationship between C-peptide, a marker of insulin production, and risk for breast cancer in the Nurses' Health Study, which involves more than 32,000 women.
They measured C-peptide in blood from the 463 women who developed breast cancer and compared it with blood C-peptide levels in a sample of women who did not develop the disease.
They found that those who had the highest C-peptide levels had a statistically significant, 70 percent greater chance of developing breast cancer compared with women who had the lowest levels.
In the same study, they found that C-peptide levels were higher in overweight and obese women and were lower in women who were physically active. Other studies have shown that insulin levels drop right after an exercise session, so the beneficial effect is immediate.
Just 30 Minutes a Day
To reduce the risk of breast cancer, women should engage in aerobic exercise for at least 30 minutes every day, preferably more. Walking is an excellent aerobic activity and can be done easily by most women.
Small things count — walking down the hall to talk with a colleague rather than e-mailing, walking or biking to work, taking physically active vacations with family, using stairs rather than elevators.
Those who are already active can get more benefit by doing more exercise — as long as they don't overdo it. A good rule of thumb is to increase the time spent exercising no more than 10 percent per week.
Those who embark on an intense training program (such as preparing for a marathon) should make sure they get enough rest including one day of rest from training every week.
Many of the benefits of exercise in breast-cancer prevention may also apply for women who have had breast cancer.
Overweight and obese breast-cancer patients have poorer survival than lighter-weight women. Exercise can therefore be an important part of the breast-cancer recovery process.
Dr. McTiernan, an internist and epidemiologist, is a member of the Cancer Prevention Research Program at the Fred Hutchinson Cancer Research Center in Seattle, Washington. She is also the author of Breast Fitness: An Optimal Exercise and Health Plan for Reducing Your Risk of Breast Cancer (St. Martins Press 2001).