I read an article today that summarized research presented to the 9th European Breast Cancer Conference (EBCC-9) by Professor Mathieu Boniol who reported that “practicing sport for more than an hour day reduces the risk of contracting breast cancer, regardless of age and weight.”  This report was the result of a meta-analysis of 37 studies published between 1987 and 2013, representing over four million women of all ages. It went on to say that compared with the least active women, those with the highest level of physical activity reduced their risk of breast cancer by 12%.
When I first got into training at the ripe old age of 22, I had no idea that training could improve or protect my health. I thought weight training was for looking strong and sexy. Over the years, these fabulous scientists have made me look smart for a decision that honestly was borne out of not wanting to look like my mother. Sound familiar, ladies? It didn’t work. I look so much like her I could be her daughter which, 30 years later, makes me happy. My mother, who quit exercising when Jack LaLane went off the air and who later added about 15 pounds to her 5’2″ frame after menopause, was one of those women who developed breast cancer later in life.
The EBBC-9 report referred to an hour a day of recreational physical activity (“sport”), which I suspect included physical activity such as walking the the dog or mowing the lawn. My assumption is in light of the time frame of the study and given recent discoveries regarding the cumulative effects of even light exercise. There are different qualities of training, so that time commitment, 7 hours a week, 365 hours a year, 14,600 hours over 40 years (which seems like a lot,) could be mitigated. The simple math of exercise is the higher the intensity, the less time spent doing it to acquire the same benefits. So I suspect as cancer research continues, it will enjoin the terminology and criteria of exercise physiologists for a clearer understanding of the intensity, volume and frequency of exercise necessary to solicit cancer prevention.
In any event, “physical activity” may prevent tumor development by lowering hormone levels, improving the immune response, and assisting with weight maintenance. And according to The National Cancer Institute, there seems to be “a decreasing risk of breast cancer as the frequency and duration of physical activity increase.”
The National Cancer Institute states that the according to over 60 published studies, physically active women have a lower risk of developing breast cancer than inactive women, “however, the amount of risk reduction achieved through physical activity varies widely (between 20 to 80 percent).” 
Lauren McCullough, of the University of North Carolina Gillings School of Global Public Health in Chapel Hill, and her colleagues looked for a link between recreational physical activity, done at different time points in life, and the risk of developing breast cancer and found.  She found that:
“In this large population-based study that included a life-course assessment of RPA [recreational physical activity] and body size, we found that frequent episodes of RPA (10-19 hours/week) at any intensity level during the reproductive and postmenopausal years may have the greatest benefit for reducing the risk of breast cancer. Our data indicate, however, that substantive postmenopausal weight gain may eliminate the benefits of regular RPA. Collectively, these results suggest that women can still reduce their breast cancer risk later in life by maintaining their weight and engaging in moderate amounts of activity. Future investigations should include populations with wide distributions of PA to confirm the nonlinear dose-response found in this study.”
Again, the volume of exercise suggested sounds very high and impractical for today’s working woman.
Researchers writing the open access journal BMC Cancer investigated the link between breast cancer and exercise. “Vigorous exercise has been hypothesized to reduce cancer risk for some time. However, this new study is one of the first prospective investigations to look at the importance of various intensities of exercise at different stages in an individual’s life.” The study concluded “Post-menopausal women engaging in moderate to vigorous exercise reduce their risk of breast cancer”  This without a history of exercise, so it’s never too late to start!
Higher BMI as Negating Factor
As indicated above, higher BMI is associated with a higher risk of breast cancer, and a higher BMI seems to negate the prophylactic effects of physical activity. In a 2010 study, more than 72,000 women between 55 and 74 years old including 3,677 cases of postmenopausal breast cancer.
“Compared with women who maintained approximately the same BMI, those who had an increase of 5 kg/m2 or more between age 20 and study entry had a nearly twofold increased risk of breast cancer,” Laura Sue, M.P.H., cancer research fellow at the National Cancer Institute. 
“In addition, women reporting increases of 5 kg/m2 or more between ages 20 and 50 were at an 88 percent increased risk of developing post menopausal breast cancer, compared to their counterparts with a stable BMI. (Source: Presented at the 2010 annual meeting of the American Association for Cancer Research).” 
Another report again from EBCC-9 reports that both obesity and diabetes have adverse effects on outcomes in breast cancer patients who receive chemotherapy as primary treatment before surgery (neoadjuvant chemotherapy). “Although a high body mass index (BMI) is known to have a negative impact on cancer development and prognosis, until now there has been uncertainty as to whether having a high BMI had an equal effect on patients with different types of breast tumors.”
So there is something women can do proactively to reduce the risk breast cancer: keep your weight under control and move your body. The less time you have, the higher the intensity should be. Again, that’s the simple math of training, and it applies in this case.
By the way, things worked out okay for Mom as far as her cancer battle, and she went on to live a long (87 years) life, albeit, not the healthiest one. She was plagued with osteoporosis, diabetes, and rheumatoid arthritis, and all the icky consequences therefrom. Exercise, of course, has been shown to reduce the risk of development of those diseases. Mom only saw exercise as a means of looking good so she stopped in her 40s when she said that she didn’t have to keep impressing Dad because he would love her no matter what. That’s kind of heartwarming.
Oh, one last interesting tidbit. Did you know that non-invasive breast cancers do not respond to exercise? What’s up with that?
3. McCullough LE, Eng SM, Bradshaw PT, Cleveland RJ, Teitelbaum SL, Neugut AI, Gammon MD: Fat or fit: the joint effects of physical activity, weight gain, and body size on breast cancer risk. Cancer 2012, 118:4860-4868