How does physical exercise help prevent breast cancer?

The benefits of physical exercise for weight loss, general fitness, and mood enhancement are well-known to all of us. Less widely known, perhaps, is that physical exercise reduces the risk of getting breast cancer, improves survival in breast cancer patients and reduces breast cancer recurrence (1, 2, 3).

We look at why this is and consider whether exercise might also influence the levels of endocrine disrupting chemicals (EDCs) in our bodies.

How much exercise is needed to reduce breast cancer risk?

Exercise helps prevent breast cancer in women of all ages but is especially beneficial for post-menopausal women (3). Moderate exercise (defined by the World Health Organisation as 150 minutes per week) is estimated to reduce the incidence of breast cancer in post-menopausal women by 20-30% (4).

Physical activity also helps those who have breast cancer. Following diagnosis, moderate exercise reduces mortality by an estimated 24% (1). As little as one hour of walking per week is of some benefit, and the benefit increases with increasing duration and intensity (5). Exercise can also be psychologically helpful for post-treatment breast cancer survivors; one study found higher levels of exercise were associated with reduced stress and fatigue and reduced memory impairment (6).

What percentage of breast cancers are attributed to lack of exercise?

Around 3.4% of post-menopausal breast cancer cases in the UK have been attributed to a lack of physical activity (7). In other countries, estimates are higher; an Australian study suggests that 7.8% of post-menopausal breast cancers result from insufficient physical activity (8), and in the Netherlands, the figure is around 5.5% (9).

What is the scientific basis for the beneficial effect of physical exercise?

Although it is not fully understood how exercise decreases breast cancer risk, it is thought that exercise helps to reduce body fat and consequently decreases the levels of hormones (oestrogen, leptin and insulin) and growth factors (IGF-1) that are secreted by adipose (fat) cells. High levels of leptin, insulin and IGF-1 promote breast tumour growth and progression and independently increase oestrogen biosynthesis in adipose cells (10). A high concentration of endogenous oestrogen is associated with an increased risk of breast cancer (11). After menopause, adipose tissue becomes the main source of oestrogen for women. Reducing body fat will therefore reduce levels of oestrogen and other hormones and growth factors associated with increased breast cancer risk.

Exercise also reduces inflammation, enhances the immune system, reduces insulin resistance and decreases oxidative stress. This helps reduce the risk of breast cancer and at least 25 other medical conditions, including type 2 diabetes, osteoporosis, colon cancer and cardiovascular disease (12).

Does exercise help reduce the levels of endocrine disrupting chemicals inside our bodies?

Many EDCs (persistent organic pollutants such as chlorinated pesticides, dioxins, and polychlorinated biphenyls) accumulate inside adipose cells. Physical exercise reduces the size of adipose cells due to the breakdown and release of stored fat into the bloodstream. Adipose cells containing less fat will accumulate fewer EDCs.

It should be noted that during exercise, along with releasing stored fat into the bloodstream, any EDCs that have accumulated will also be released (13). Therefore it makes sense to reduce your exposure to concentrations of environmental EDCs too.

Are the preventative effects of exercise the same as those associated with weight loss?

Being overweight is also a risk factor for breast cancer. Numerous studies have shown that weight loss is beneficial in reducing risk. However, the positive effect of physical activity in reducing breast cancer risk is not due to weight loss alone. A recent study (14) found that in post-menopausal women, 6-7% weight loss induced mainly by exercise resulted in greater loss of body fat, better physical fitness and less free testosterone in the bloodstream, compared to a similar weight loss induced by diet alone.

Other studies have shown that even without a loss of body fat, exercise can reduce serum concentrations of insulin, IGF-1, oestrogen and leptin, and so reduce breast cancer risk (10). This highlights the positive benefits of exercise even in the absence of weight loss or loss of body fat.

We all knew exercise was good for us, and now we know why. Physical exercise reduces breast cancer risk by helping to reduce the levels of many of the chemicals in our bodies associated with the initiation and development of breast cancer. And it helps us feel good too.

References:

1. Schmid, D. and Leitzmann, M. F. (2014). Association between physical activity and mortality among breast cancer and colorectal cancer survivors: a systematic review and meta-analysis. Annals of Oncology 25: 1293–1311. https://www.ncbi.nlm.nih.gov/pubmed/24644304

2. Lahart, I. H. et al. (2015). Physical activity, risk of death and recurrence in breast cancer survivors: A systematic review and meta-analysis of epidemiological studies. Acta Oncologica, 54:5,

635-654. https://dx.doi.org/10.3109/0284186X.2014.998275

3. Gonçalves, A. K. et al. (2014). Effects of Physical Activity on Breast Cancer Prevention: A Systematic Review. Journal of Physical Activity and Health 11: 445-454. https://www.ncbi.nlm.nih.gov/pubmed/23416687

4. Theriau, C. F. et al. (2016). Voluntary physical activity abolishes the proliferative tumor growth microenvironment created by adipose tissue in animals fed a high fat diet. Journal of Applied Physiology 121: 139–153. https://www.ncbi.nlm.nih.gov/pubmed/27150834

5. Holmes, M. D. et al. (2005). Physical activity and survival after breast cancer diagnosis. The Journal of the American Medical Association 293: 2479–2486. https://jama.jamanetwork.com/article.aspx?articleid=200955

6. Phillips, S. M. et al. (2016). Relationship between self-reported and objectively measured physical activity and subjective memory impairment in breast cancer survivors: role of self-efficacy, fatigue and distress. Psycho-oncology 2016 Jul 8. doi: 10.1002/pon.4156 [Epub ahead of print]. https://onlinelibrary.wiley.com/doi/10.1002/pon.4156/abstract

7. Parkin, D. and Boyd, L. (2011). The Fraction of cancer attributable to lifestyle and environmental factors in the UK in 2010. British Journal of Cancer 105, S77-81. https://www.ncbi.nlm.nih.gov/pubmed/22158327

8. Olsen, C. M. et al (2014). Cancers in Australia in 2010 attributable to insufficient physical activity. Australian and New Zealand Journal of Public Health 39: 458-63. https://www.ncbi.nlm.nih.gov/pubmed/26437732

9. van Gemert, W. A. et al. (2015). The proportion of postmenopausal breast cancer cases in the Netherlands attributable to lifestyle-related risk factors. Breast Cancer Research and Treatment 152: 155–162. https://www.ncbi.nlm.nih.gov/pubmed/26044369

10. Schmidt, S. et al (2015). The integrative role of leptin, oestrogen and the insulin family in obesity-associated breast cancer: potential effects of exercise. Obesity reviews 16: 473–487. https://www.ncbi.nlm.nih.gov/pubmed/25875578

11. McTiernan A. (2008). Mechanisms linking physical activity with cancer. Nature Reviews Cancer 8(3): 205–211. https://www.ncbi.nlm.nih.gov/pubmed/18235448

12. Warburton, D. E. R. and Bredin, S. S. D. (2016). Reflections on Physical Activity and Health: What Should We Recommend? Canadian Journal of Cardiology 32(4): 407-409. https://www.ncbi.nlm.nih.gov/pubmed/26995692

13. Cheikh Rouhou et al. (2016). Adverse effects of weight loss: Are persistent organic pollutants a potential culprit? Diabetes Metabolism (2016), https://dx.doi.org/10.1016/j.diabet.2016.05.009

14. van Gemert, W. A. et al. (2015). Effect of weight loss, with or without exercise, on body composition and sex hormones in postmenopausal women: the SHAPE-2 trial. Breast Cancer Research 17 (1): 120. https://breast-cancer-research.biomedcentral.com/articles/10.1186/s13058-015-0633-9

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