TUESDAY, May 13 (HealthDay News) -- New research has found both bad news and good news on breast cancer risk.

The bad news is a risk factor you can't change: Women whose sisters were diagnosed with breast cancer face an increased risk of breast cancer throughout their lives, regardless of their sister's age at diagnosis, according to a study in the May 13 issue of the Journal of the National Cancer Institute (JNCI).

The good news comes from a risk factor you can do something about: Women who exercise are much less likely to develop breast cancer, according to two new research studies -- one from the same issue of JNCI, and the other from the 2008 online first edition of the British Journal of Sports Medicine.

The first study from JNCI compared the rate of breast cancer in nearly 24,000 sisters of women with breast cancer to the rate of cancer in nearly 1.8 million women with sisters who didn't have breast cancer. All of the women were from Sweden, and the data collection for the study spanned from 1958 to 2001.

The researchers found that women between the ages of 20 and 39 who had a sister who'd been diagnosed with breast cancer faced a sixfold higher risk of breast cancer than did women whose sisters didn't have breast cancer. The excess risk declined as the women aged but didn't disappear. Women who were older than 50 with a sister with breast cancer had about a twofold risk of developing the disease, according to the study. And, it didn't matter what age the sister was when she was diagnosed.

"After the diagnosis of breast cancer in a family, the other sisters -- especially the youngest -- have an increased risk of breast cancer that persists for 20 years," said one of the study's authors, Marie Reilly, a professor of biostatistics at the Karolinska Institute in Stockholm, Sweden. "This suggests that sisters of breast cancer patients, especially the young sisters, should be intensely screened, independent of the screening recommendations for women their age."

Dr. Julia Smith, director of the Lynne Cohen Breast Cancer Preventative Care Program at the New York University Cancer Institute and Bellevue Hospital in New York City, called the findings "interesting and troubling." She added, "Sisters have to worry about increased risk no matter when their sister was diagnosed."

The second study from JNCI relied on data from the Nurse's Health Study II and included information from almost 65,000 women who completed questionnaires about their physical activity from age 12 until age 35. During the six-year follow-up period, 550 women from that group were diagnosed with breast cancer.

Women who walked about 13 hours a week or ran 3.25 hours a week had a 23 percent reduced risk of developing premenopausal breast cancer than women who were less active. The incidence rates of breast cancer were 194 per 100,000 "person-years" for the least active women, compared to 136 cases per 100,000 "person-years" for the most active women.

"These results suggest that consistent physical activity during a woman's lifetime is associated with decreased breast cancer risk. Unlike many risk factors for breast cancer, physical activity is an exposure that can be modified," wrote the study's authors, who were led by Dr. Graham Colditz, of Washington University School of Medicine in St. Louis.

Smith said: "For many reasons, women should continue to exercise and try to be in shape." It makes sense that exercise might reduce breast cancer risk, she said, adding, "Women who are exercising regularly are decreasing body fat and estrogen."

The third study on breast cancer risk was an analysis of 62 other studies that looked at the impact of physical activity and breast cancer risk. This review, published online ahead of the print version of the British Journal of Sports Medicine, found that women who are physically active have a 25 percent decreased risk of breast cancer.

The researchers found that both recreational or on-the-job activity could reduce risk, and that moderate and vigorous exercise caused a similar reduction in risk. This review also found that activity performed after menopause was more effective in reducing risk.

Smith recommends that women exercise at least 20 minutes, three times a week, and preferably more. She said that during those 20 minutes of moderate to vigorous exercise, the heart rate should consistently be above baseline.

More information

To learn more about breast cancer risk factors, visit the American Cancer Society.

SOURCES: Marie Reilly, Ph.D., professor of biostatistics, Karolinska Institute, Stockholm, Sweden; Julia Smith, M.D., director, Lynne Cohen Breast Cancer Preventative Care Program, New York University Cancer Institute and Bellevue Hospital, New York City; May 13, 2008, Journal of the National Cancer Institute; 2008 online, British Journal of Sports Medicine

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