Exercise can lower women’s risk of breast cancer, but how much
exercise is enough and at what age do women have to be physically active
to benefit?

Those are the questions that Lauren McCullough, a doctoral candidate
in epidemiology at the University of North Carolina, Chapel Hill, and
her colleagues sought to answer in a new study published in the journal Cancer.
They found that women who exercised about two hours a day five days a
week were about 30% less likely to develop breast cancer than less
active women. The intensity of the exercise didn’t seem to matter; all
it took was moderate physical activity, which could include gardening,
walking or doing household chores, for the women to benefit.

McCullough’s study included 1,504 women with breast cancer
and 1,555 similar women without the disease, aged 20 to 98 years old,
enrolled in the Long Island Breast Cancer Study Project. As part of the
project, the women answered questions about their physical activity
over their lifetimes — any recreational exercise they did for at least
an hour per week for three months or more. The researchers then
calculated a lifetime composite score for physical activity that they
used to compare across the participants.

Overall, women who did any exercise had a 6% lower risk of breast
cancer than those who did not, but certain subgroups of women enjoyed
even larger benefits. The effect was strongest among women who had
children and exercised about 10 to 19 hours each week — either during
their reproductive years or after menopause; for them, exercise was
associated with a 30% lower risk of breast cancer during the study
period, compared with women who exercised less, or not at all.

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