Six or more hours per week of strenuous
recreational activity may reduce the risks of invasive
breast cancer by 23 percent, according to researchers
from the University of Wisconsin Paul P. Carbone Comprehensive
Cancer Center (UWCCC).
Their report in the February issue of Cancer Epidemiology
Biomarkers & Prevention, based on a survey of over
15,000 women, shows that exercise has a protective effect
against invasive breast cancer throughout a woman's
The results provide further evidence that for most women
physical activity may reduce the risk of invasive breast
cancer, the researchers concluded.
To gain further insights into the mechanisms of risk
reduction for breast cancer, the researchers investigated
the relationship between physical activity and breast
cancer risk in a population-based case control study
in Massachusetts, New Hampshire, and Wisconsin.
During structured telephone interviews, the researchers
questioned 7,630 women without breast cancer, 1,689
survivors of in situ, or non-invasive, breast cancer
and 6,391 survivors of invasive breast cancer, all between
the ages of 20 and 69. They asked detailed questions
about physical activity, occupation, family history
of breast cancer, menopausal status, and body mass index.
According to the researchers, women who exercised had
a reduced risk of developing invasive breast cancer
provided they didn't have a family history of breast
cancer. This reduction in risk was apparent whether
the physical activity took place early in life, in the
postmenopausal years, or in the recent past.
"A woman's hormone levels naturally fluctuate
throughout her life, and we have found that exercise
likely offers protection against breast cancer regardless
of a woman's stage in life," said Brian Sprague,
a UWCCC research assistant and lead author of the study.
"The take-home message for women should be that
it is never too late to begin exercising."
Previous research has linked high levels of estrogen
to an increased risk for developing breast cancer. Women
who exercise heavily are, in general, older at the time
of the first period, and tend to have irregular periods
and a shortened estrogen-producing phase, which translates
in a lower body exposure to estrogen, the researchers
Similarly, postmenopausal women who are physically
active have also been shown to have lower levels of
estrogen. This reduction may explain why increased physical
activity reduces the risk of breast cancer, according
to Amy Trentham-Dietz assistant professor at the University
of Wisconsin-Madison and member of the University of
Wisconsin Paul P. Carbone Comprehensive Cancer Center.
Other potential mechanisms include prevention of weight
gain, regulation of insulin sensitivity and alterations
in immune function.
Taking all these factors into consideration, "intervention
studies assessing the effect of physical activity on
estrogen and other hormone exposure, and other biomarkers
of risk would provide valuable insights on the mechanisms
of physical activity in reducing breast cancer risk,"
"Further studies of population subgroups are necessary
to gain a better understanding of the relation of physical
activity to breast cancer risk, and to identify the
groups most likely to gain benefit from it," said
Trentham-Dietz. "Future research should also consider
household activity in addition to recreational and occupational