Women who walk for an hour a day can cut their chance of breast cancer by 14%, experts have said.
While it is well known that being active cuts risk of the disease, this is the first study to specifically look at the effect of walking.
The American Cancer Society study involved 73,615 postmenopausal women, of whom 4,760 were diagnosed with breast cancer during a 17-year follow-up.
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The researchers found that moderate activity in the form of walking an hour a day was associated with a 14% lower risk of the disease compared to women who were least active.
Among all women in the group, 47% said walking was their only recreational activity and women were far more likely to take part in walking, dancing or aerobics than vigorous activities like running, swimming, and tennis.
Among those women who reported walking as their only activity, those who walked at least seven hours per week had a 14% lower risk of breast cancer compared to those who walked three or fewer hours per week.
The study also found that women who took part in more vigorous activities for an hour a day had a 25% lower risk of breast cancer than the least active, which echoed findings in other studies.
The results were unaffected by factors such as a woman's weight or whether she used hormone replacement therapy.
Dr Alpa Patel, senior epidemiologist at the American Cancer Society in Atlanta, Georgia, said: "Our results clearly support an association between physical activity and postmenopausal breast cancer, with more vigorous activity having a stronger effect.
"Our findings are particularly relevant, as people struggle with conflicting information about how much activity they need to stay healthy.
"Without any other recreational physical activities, walking on average of at least one hour per day was associated with a modestly lower risk of breast cancer.
"More strenuous and longer activities lowered the risk even more."
Dr Patel said the majority of women reported some daily walking and therefore promoting walking could be an effective strategy for increasing activity.
"We were pleased to find that without any other recreational activity, just walking an average of one hour per day was associated with lower risk of breast cancer in these women."
The authors said this was the first study to report a lower risk for breast cancer among postmenopausal women specifically related to walking.
Baroness Delyth Morgan, chief executive of Breast Cancer Campaign, said: "This study adds further evidence that our lifestyle choices can play a part in influencing the risk of breast cancer and even small changes incorporated into our normal day to day activity can make a difference.
"We know that the best weapon to overcoming breast cancer is the ability to stop it occurring in the first place.
"The challenge now is how we turn these findings into action and identify other sustainable lifestyle changes that will help us prevent breast cancer."
Public Health England should conduct an audit of existing lifestyle information on preventing the disease, she added.
There are an estimated 570,000 people living with breast cancer in the UK.
Around 50,000 women and 400 men are diagnosed with breast cancer each year and around 12,000 women and 80 men die from it.
Dr Hannah Bridges, senior information officer at Breakthrough Breast Cancer, said: "We've known for some time that being active can reduce a woman's risk of breast cancer.
"Breakthrough Breast Cancer advises that 30 minutes of moderate physical activity a day (or 3.5 hours a week) can reduce breast cancer risk by at least 20%.
"Any activity that raises your pulse reduces your risk - so regular brisk walks are an easy and free way to get active."
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