Exercise Protects Against Premenopausal Breast Cancer

Zosia Chustecka

May 14, 2008

May 14, 2008 — That exercise reduces the risk for postmenopausal breast cancer is well known, but a new study shows that it can also protect against premenopausal breast cancer.

The study, published online May 13 in the Journal of the National Cancer Institute, also provides some details on the amount of exercise that is involved. The most active women, who showed a 23% reduction in the risk for premenopausal breast cancer, reported exercise that was equivalent to running 3.25 hours a week or walking 13 hours a week.

The strongest association for a lowered risk for premenopausal breast cancer was with activity between the ages of 12 and 22 years; the next strongest association was with activity between the ages of 23 and 34 years. No association for reduced risk for premenopausal breast cancer was apparent after age 35. However, many other studies have shown that exercise during adulthood reduces the risk for postmenopausal breast cancer.

"We don't have a lot of prevention strategies for premenopausal breast cancer, but our findings clearly show that physical activity during adolescence and young adulthood can pay off in the long run by reducing a woman's risk of early breast cancer," commented lead investigator Graham Colditz, MD, DrPH, from the Siteman Cancer Center, Washington University School of Medicine, in St. Louis, Missouri. This is just 1 more reason to encourage young women to exercise regularly," he told journalists.

Dr. Colditz and colleagues analyzed data from 64,777 premenopausal women involved in the Nurses' Health Study II. During the 6 years of follow-up, 550 premenopausal women developed breast cancer.

The women completed detailed questionnaires about their occupational and leisure-time activities. Because self-reporting can be unreliable, the researchers used data from an earlier validation study on past-year adult activity to correct for measurement error. When they did this, Dr. Colditz and colleagues found a 39% lower breast cancer risk for total lifetime physical activity in the most active women compared with the least active women. This suggests that "our original estimate of a 23% lower risk was an underestimate," they comment.

The most important association with risk for premenopausal breast cancer was total activity. It did not seem to matter much what the activity was; the differences between strenuous, moderate, and walking activities were not statistically significant. "You don't have to be a marathon runner to get the risk-reducing effects of exercise," Dr. Colditz commented.

Exactly how exercise protects against breast cancer is unclear, but it is thought to work through hormone-related mechanisms. Physical activity can delay the start of menstruation and lengthen the time between periods, and strenuous exercise can stop ovulation. All of these effects would reduce a woman's lifetime exposure to estrogen and its mitogenic effects.

"Although the underlying mechanisms require further study, this research support the benefits of regular exercise during all ages among women," Dr. Colditz and colleagues comment. Physical activity is 1 of the very few risk factors for breast cancer that can be modified, and the finding that exercise appears to protect against breast cancer has public-health implications, they add.

More Evidence of Protective Effect of Exercise

Further evidence that exercise protects against breast cancer comes from a review of 62 studies published online May 13 in the British Journal of Sports Medicine.

Nearly half of these studies showed a dose-response effect, researchers Dr. Christine Friedenreich, from the Alberta Cancer Board, in Calgary, and Dr. Anne E. Cust, from the School of Population Health, University of Melbourne, in Australia, note and, overall, they show that exercise reduces the risk for breast cancer by about 25%.

Women who had undertaken a lot of physical activity throughout their life had the lowest risk for breast cancer, the researchers found. All types of activity reduced the risk for breast cancer, but recreational activity had a greater effect than occupational or household activity. Moderate and vigorous activity had equal benefits.

This review adds a few details to what is already known about the effect of exercise on reducing the risk for breast cancer, the researchers write. "Consistent and strong observational epidemiologic evidence" already exists to show that physical activity reduces the risk for breast cancer, and this evidence has been classed as "convincing," they point out.

J Natl Cancer Inst. 2008;100:728-737.
Br J Sports Med. Published online before print May 13, 2008.

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