Regular exercise, even at a light intensity, could dramatically reduce a woman's risk of breast cancer, according to new US research.
However, substantial weight gain could negate the benefits.
Researchers have long recognised that physical activity can lower the risk of breast cancer, but a new study from the University of North Carolina has found that even mild exercise has a positive impact.
Dr Lauren McCullough studied 1504 women with breast cancer and 1555 women without breast cancer. They were aged between 20 and 98.
She wanted to find out if there was a link between exercise at different ages and breast cancer.
Her study, published in the journal Cancer, showed that exercise during either the reproductive or post-menopausal years reduced women's breast cancer risk, provided they were a healthy weight.
And the more exercise, the better those who exercised for 90 minutes to 2.5 hours per day had a 30 percent reduced risk, regardless of whether it was high or low intensity.
"The observation of a reduced risk of breast cancer for women who engaged in exercise after menopause is particularly encouraging given the late age of onset for breast cancer," Dr McCullough said.
In Australia, about 75 percent of breast cancer diagnoses occur in women over the age of 50.
This study showed exercise was particularly helpful in reducing the risk of hormone receptor positive breast cancer, the most common tumour type in Australian women.
The National Breast Cancer Foundation (NBCF) predicts 42 women will be diagnosed with breast cancer in Australia every day in the next three years.
Alison Butt, the NBCF director of research investment, said this research is exciting for women.
"It's great to see some research coming out that's giving women lots of positive messages about what they can do to help reduce their risk of breast cancer," she said.
"It's an important message that you don't have to be pounding at the gym for hours on end. Keep active, go for a daily walk it really can have an enormously positive effect on women's wellbeing."
Butt says most of the existing research around exercise and breast cancer has looked at reducing the side effects of treatment.
"We have known for a while that exercise can really have a positive benefit on women who have already been diagnosed with breast cancer to help with some of the debilitating side effects," she said.
"This research confirms that a healthy diet and exercise has a lot of positive benefits for women’s health and breast cancer."