Researchers at the Netherlands Cancer Institute in Amsterdam compared the physical activity histories of 918 women aged 20 to 54 who had been diagnosed with invasive breast cancer with those of 918 women who did not have cancer.

In the study, the researchers interviewed women about their lifetime recreational histories, taking into account the type and duration of activity. Types of exercise included walking, gardening, running, and competitive sports.

The authors found the following:

• Active women had a 30 percent lower risk of breast cancer than those who were inactive.
• The benefits were greatest in women who kept their weight in proportion to their height.
• A woman?s age at the time of exercise wasn?t as important as exercising regularly.

"The basic message of this data is that women should be more active if they want to reduce their risk of breast cancer," said Janneke Verloop, MS, lead author of the study.

Colleen Doyle, MS, RD, director of nutrition and physical activity for the American Cancer Society (ACS), agreed. "It doesn?t mean women have to run marathons or hike mountains to reduce risk," she said. "They just have to get up, and keep moving. That?s the bottom line."

Regularity Matters More Than Age

The researchers found women who started exercising before age 20 were more likely to keep exercising later in life. But regularity mattered more than the age at which a woman started exercising. "It?s like that old Chinese proverb," said Doyle. "What?s the best time to plant a tree? Fifty years ago. When is the next best time? Right now."

Regular physical activity gave the most benefit to women who managed to keep their weight in proportion to their height. "Breast cancer is hormone-related," said Doyle, "and fatty tissue produces more estrogen, and more insulin, both of which may increase risk. So, it may be very wise for women to try to maintain a lean body weight throughout their lifetime."

Vigorous activity in early life can reduce estrogen by delaying onset of the regular ovulation cycle. It can help keep fat from accumulating around the abdominal area, where it is known to increase breast cancer risk.

Many earlier studies found a link between physical activity and lowered breast cancer risk, but some did not. "Some earlier studies looked only at activity in one period of life. Our study confirms the conclusions of the only other study that looked at lifetime activity," Verloop said.

There are other good reasons for women to be physically active, Doyle said. "Women are more likely to develop heart disease than breast cancer, and we know exercise decreases that risk," she said.

The ACS estimates 182,800 new cases of breast cancer will be diagnosed in the US this year, and 40,800 deaths from the disease are expected, making it the No. 2 cancer killer of American women.