Exercise for breast cancer prevention

How can I use exercise to help prevent breast cancer, and how much will it decrease my risk?

It is well appreciated that exercise can help prevent or delay an enormous range of health problems from cardiovascular disease, to neurodegenerative disease, to cancer.  In fact it would appear that all our bodily systems benefit from routine physical exertion.  Unfortunately, generic health recommendations such as “eat better”, “exercise more”, or “relax” tend to be less than motivational.  For myself, taking a closer look at the underlying research can help me stay motivated.  For breast cancer, here’s where the research on exercise and prevention stands:

A number of studies have shown that the risk of breast cancer declines as physical activity levels increase.  One large prospective study (ref 1) followed over 75,000 postmenopausal women, documenting cancer incidence and physical activity levels.  Their results were exciting.  Compared with women who did no current physical exercise, those who exercised (specifically, 5.1- 10, 10.1-20, 20.1-40 and > 40 ‘metabolic equivalent’ task-hours per week [MET-h/wk]), had reductions of breast cancer rates (18%, 11%, 17% and 22%, respectively).  Metabolic equivalent is an approximate measure of energy expenditure per hour (for example yoga = 3 MET, walking at 4km/hr = 2.9 MET, jogging = 7 MET).  So in other words, the researchers found that exercising more than 5.1 MET-h per week (equivalent to about 1 hour and 45 min of brisk walking) correlated with an 18% decreased risk of breast cancer.  Wow!

In fact, the World Health Organization reports that exercising 3-5 days per week may reduce the risk of breast cancer by 20-40% (ref 2).  That’s huge.  As I write this I’m starting to think about my running shoes at the front door.  But correlation does not necessarily suggest causation.  What if it’s simply coincidence that people who tend to exercise also tend to not get cancer?  There is actually a large body of additional supporting evidence, including animal studies, suggesting that exercise does in fact cause a reduction in cancer rates, but to push me into my MEC running shorts I need to build a stronger argument.  In particular, is there a plausible underlying biological mechanism to explain how exercise could help prevent breast cancer specifically?

Yes there is.  A number of potential biological mechanisms exist to explain why exercise could reduce the risk of breast cancer.  In particular, there is a large body of research that shows physical activity can alter levels of hormones, growth factors, and other signaling molecules, many of which have known roles in breast cancer.  This includes both estrogen and insulin-like growth factor 1 (IGF-1).  See figure 1 for a brief overview of some of the biological mechanisms by which exercise may decrease risk of breast cancer.

Figure 1: Underlying biological mechanisms explaining how exercise could reduce risk of breast cancer (BMI: body mass index, SHBG: sex hormone-binding globulin, TNF-a: tumor necrosis factor alpha, IL-6: interleuking 6, IGF-1: insulin-like growth factor 1) (ref 3)

Not only is the body of evidence supporting exercise for breast cancer prevention substantial, there are known biological mechanisms that may explain its considerable positive effects.

So how much exercise do I need in order to reduce my risk of breast cancer by 20-40%?  The best guidance is that women should participate in at least 150 min per week of moderate to vigorous exercise to substantially reduce their risk of breast cancer (ie at 50-75% VO2peak or 60-80% of maximum heart rate).  Given the correlation that has been observed between amount of exercise and reduction of risk, more exercise is probably better.

Time to grab my runners and get out there!

References:

1. McTiernan A., Kooperberg C., White E., Wilcox S., Coates R., Adams-Campbell L.L., Woods N., Ockene J. Recreational physical activity and the risk of breast cancer in postmenopausal women. The women’s health initiative cohort study. JAMA. 2003; 290(10):1331–6.
2. www.who.int
3. Neilson H.K., Friedenreich C.M., Brockton N.T., Millikan R.C. Physical activity and postmenopausal breast cancer: proposed biologic mechanisms and areas for future research. Cancer Epidemiol Biomarkers Prev. 2009; 18(1):11–27.