If exercise were a pill, its effectiveness in driving down a woman's breast cancer risk would occur fairly quickly, the new research says: When women reported at least this modest level of physical activity over the last four years, they were less likely to have developed malignancy in a breast.
But, like a pill, exercise must be continued for the effect to endure: Even if a woman had been physically active earlier in life, when her reported physical activity levels dropped below the equivalent of four hours of weekly walking, her risk of developing breast cancer went back up.
At the same time, the research found that engaging in more physical activity than 12 "metabolic equivalents" per week -- either via greater intensity or longer duration of exercise -- did not further drive down a woman's likelihood of breast cancer. The authors suggested that finding such a "dose response" might have required a more detailed record of participants' energy expenditures than was collected by the European researchers who designed the study.
The new findings were culled from the French component of the European Prospective Investigation Into Cancer and Nutrition. In all, 59,308 French women ages 43 to 78 -- most of them teachers -- started answering detailed surveys about their health and health behaviors in 1993 and did so every four years until 2005.
The researchers sought to clarify the role of physical activity in reducing breast cancer risk in postmenopausal women, a stage of life in which the probability of a woman developing breast cancer rises steadily.
The designers of the study used a very rough measure of physical activity: They asked women to quantify the average time they spent walking (including walking to work or shopping), cycling or engaging in sports during a typical two-week period in the winter and in the summer. They assigned walking a "metabolic equivalent task" value of three per hour, and cycling and sports a "MET" value of six.
For the study's postmenopausal women, weight gain was common: Roughly 69% reported a gain of more than a kilogram (2.2 pounds) over the previous two to three years. But the exercise effect was seen irrespective of weight gain, and a woman's body mass index (BMI) and waist circumference had no bearing on the cancer-reduction benefits she received from regular exercise.
The medications tamoxifen and raloxifene can also drive down the risk of breast cancer in those at higher than average risk. They come with side effects such as an increased risk of deep-vein thrombosis or pulmonary embolism, and their powers of risk reduction are actually pretty modest: If 1,000 women took either tamoxifen or raloxifene for five years, eight breast cancers would be prevented
By comparison, regular physical activity is powerful. But, as with a pill, you need to keep taking it.
If you're sedentary or don't exercise routinely, boosting your physical activity to at least four hours of walking weekly has a 1 in 10 chance of preventing the most common -- and second-most deadly -- cancer in women, at least in the near term.
Past research has already established that regular exercise will help drive down the risk of heart disease, which is a far more common killer of women.