Reporting in the Feb. 26 issue of the Archives of
Internal Medicine, researchers at the University
of Southern California, Los Angeles, studied more than
110,000 women, aged 20 to 79, who took part in the California
At the start of the study, the women were asked about
their participation in moderate exercise (such as brisk
walking, golf or volleyball) and strenuous exercise
(such as swimming laps, aerobics, and running) from
high school up until their current age and within the
previous three years.
The study authors also collected information about
the women's breast cancer risk factors, including race,
family history and use of hormone replacement therapy.
Between the start of the study in 1995-96 and 2002,
2,649 of the women were diagnosed with invasive breast
cancer and 593 were diagnosed with in situ (noninvasive)
Women with a long-term history of doing more than five
hours per week of strenuous exercise were 20 percent
less likely to develop invasive breast cancer and 31
percent less likely to develop in situ breast cancer
than those who did less than 30 minutes of strenuous
exercise per week, the researchers found.
Strenuous exercise within the previous three years
did not offer the same benefit, they noted.
Long-term or more recent moderate exercise also
did not appear to affect the risk for invasive or in
situ breast cancer, the study found.
The researchers also found that strenuous exercise
affected the risk of different types of cancer in different
ways. Specifically, strenuous activity may be associated
with a lower risk of estrogen-receptor-negative breast
cancers, but not estrogen-receptor-positive breast cancers.
"In summary, these results provide additional evidence
supporting a protective role for long-term strenuous
recreational physical activity on risk of invasive and
in situ breast cancer, whereas the beneficial effects
of moderate activity are less clear," the authors wrote.