by Dr. C.H. Weaver M.D. updated 1/2022
There is a growing body of evidence to indicate that exercise is crucial to reducing the risk of cancer.1-6 Research suggests that women who meet the aerobic exercise guidelines spelled out by the Centers for Disease Control and Prevention are 41.5 percent less likely to die of breast cancer than their more sedentary counterparts— even after adjusting for body mass index (BMI).1
Here is the kicker: those guidelines are fairly attainable. They recommend two and a half hours of moderate activity (such as brisk walking) or 75 minutes of vigorous activity (such as running) per week.2 To be clear, that is nearly half the risk of breast cancer for less than three hours of effort each week.
Breast cancer claims the lives of approximately 40,000 women annually in the United States alone, with over 200,000 diagnoses. Since breast cancer is such a common malignancy, research efforts have been focused on the discovery of risk factors associated with an increased or decreased incidence of breast cancer. Discovery of these associations may assist in helping to prevent some women from developing breast cancer. Body weight, diet and exercise have all been implicated in the development of breast cancer, and researchers continue to study these factors in association with breast cancer in order to define their role.
The study included 79,124 women (32,872 walkers and 46,252 runners) from the National Walkers’ and Runners’ Health Studies. All the women were cancer-free at the beginning of the study. They were followed for 11 years, during which time 111 women (57 walkers, 54 runners) died from breast cancer. The women reported the distances they ran or walked each week, as well as their bra cup size, body weight, and height.
Running and walking offered the same level of protection. One activity is not better than the other—it is all about energy expenditure. At higher intensities— such as those achieved with running— it simply takes less time to expend the necessary amount of energy. At lower intensities—such as when walking— you will need to spend twice the amount of time for the same results. Choose your intensity level and put in the time, and you are on your way to a significantly reduced risk of breast cancer.
The researchers noted that other than age and menopause, baseline bra cup size was the strongest predictor of breast cancer mortality—the risk of dying from breast cancer increases with bra cup size. Meeting the exercise recommendations decreased the risk of breast cancer, however, regardless of cup size.
What is the relationship between exercise and breast cancer? It might come down to estrogen, which is known to make breast cancer grow. Researchers have theorized that exercise reduces the effects of estrogen on cancer by changing how the body breaks it down. What’s more, exercise can reduce the level of fatty tissue, which secretes estrogen.
In another study researchers at the University of North Carolina analyzed data from women who participated in the Long Island Breast Cancer Study Project, which was designed to investigate possible environmental causes of breast cancer. The analysis included 1,504 women with breast cancer and 1,555 women without breast cancer who were between the ages of 20 and 98. They found that women who exercised either during their reproductive or postmenopausal years had a reduced risk of developing breast cancer. Those who exercised 10 to 19 hours per week experienced the greatest benefit, with an almost 30 percent reduced risk of developing breast cancer. The risk reduction was observed across all levels of intensity—so even mild physical activity proved worthwhile.7
This research is consistent with other studies that have suggested that physical activity may reduce the risk of breast cancer, particularly among postmenopausal women.3 Researchers conducted a review of nearly 50 studies that evaluated the relationship between physical activity and the incidence of breast cancer. Results were as follows:
- Physical activity reduced the risk of breast cancer by 20–80% among postmenopausal women.
- The relationship between premenopausal breast cancer and physical activity was weak.
Is More Exercise Better?
Over half of the studies reported a relationship between increasing exercise and decreasing risk of breast cancer, suggesting a 6% decrease in the risk of breast cancer for each additional hour of exercise per week.6
According to one study published in the Journal of the National Cancer Institute, women with the highest levels of physical activity had a roughly 20% decreased risk of breast cancer. The reduction in breast cancer risk was observed in both white women and black women.
To assess the relationship between physical activity and risk of breast cancer, as well as to determine whether this relationship varies by race or family history of breast cancer, researchers evaluated 4538 women with breast cancer (1605 black and 2933 white) and 4649 women without breast cancer (1646 black and 3033 white).
Study participants were between the ages of 35 and 64 years of age, and were enrolled in five locations: Atlanta, Detroit, Los Angeles, Philadelphia, and Seattle. Information about lifetime physical activity was collected by in-person interviews; the focus was on activities in which the woman had participated for at least 1-2 hours per week for at least four months.
Roughly a quarter of study participants (25% of women with breast cancer and 23% of the women without breast cancer) reported no exercise between the age of 10 and the present. Black women were more likely to report being inactive than white women-roughly 33% of black women reported being inactive, compared to 18-20% of white women. Among women who did report exercise, the most common activities were walking (54% of subjects), aerobics (27% of subjects), and bicycling (21-22% of subjects).
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Women who had a lifetime average of at least 1.3 hours per week of exercise had a roughly 20% reduction in risk of breast cancer compared to women who were inactive. The benefit of exercise did not vary by race (the benefit was similar for both white women and black women), nor did it vary by age, parity, body mass index, menopausal status, use of postmenopausal hormones, use of oral contraceptives, estrogen receptor status, or cancer stage. Family history of breast cancer, however, did modify the relationship between exercise and breast cancer: Among women with a first-degree family history of breast cancer, increasing levels of exercise did not decrease the risk of breast cancer.
Does Exercise Intensity Reduce Risk Further?
According to one study published in the Archives of Internal Medicine, women who engage in long-term strenuous exercise activity have a significantly decreased risk of developing estrogen receptor-negative invasive breast cancer.9
Few studies have evaluated an association between physical activity and breast cancer risk with different stages and subgroups of breast cancer. Researchers from California evaluated data involving 110,599 women, aged 20 to 79 years, enrolled in the California Teachers Study. Information was collected about their participation in strenuous and moderate recreational activities beginning in high school through their current age (or 54 years if they were older).
Patients involved in long-term strenuous activity (greater than five hours versus less than half an hour per week per year) had significantly reduced rates of both in situ (small cancers that have not spread from the site of origin and are highly curable) and more advanced breast cancers. However, the risk reduction of invasive breast cancers was confined to ER-negative breast cancers.
The researchers concluded that long-term strenuous activity significantly reduces the risk of developing invasive ER-negative breast cancers as well as in situ breast cancers. These results further support the protective effect of exercise against the risk of developing breast cancer.
These results provide additional evidence that for many women, regular exercise throughout life reduces the risk of breast cancer. This is one of the few studies of exercise and breast cancer that specifically evaluated black women.
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Can Exercise Reduce the Risk of Developing Other Cancers?
Results from a recent study analyzing the relationship of physical activity and cancer indicated that physical activity results in a risk reduction of 13 types of cancer. The results of this study were published in JAMA Internal Medicine.
Scientists and numerous cancer research facilities have conducted significant research on the relationship of health conditions and exercise. As a result, there has been a consensus that exercise has a role in risk reduction in colon cancer, breast cancer, and uterine cancer.
In this study, researchers evaluated cancer incidence and self-reported physical activity in 1.44 million individuals from 1987-2004. Study participant data was analyzed based on whether or not an individual developed cancer as well as whether or not self-reported physical activity was considered high level versus low level. Approximately 187,000 participants were diagnosed with cancer during the study.
Examples of self-reported physical activity included walking, running, swimming, and others. The time duration for these activities was on average 150 minutes per a week at a moderate intensity level.
The researchers concluded that high versus low levels of physical activity were associated with a risk reduction of 13 different cancers. The cancers that recorded the highest risk reduction were esophageal cancer, liver cancer, cancer of the gastric cardia, kidney cancer, and myeloid leukemia. In addition, there was also a trend in risk reduction for multiple myeloma and cancers in the head, bladder, neck, lungs, and rectum; however the data for these types of cancers was not as strong. Lastly, data also showed that those who engaged in more activity reduced their chances of having colon cancer, breast cancer, and uterine cancer, which are significantly more common forms of cancer.
A co-author from the American Cancer Society states, “For years, we’ve had substantial evidence supporting a role for physical activity in three leading cancers: colon, breast, and endometrial cancers, which together account for nearly one in four cancers in the United States. This study linking physical activity to 10 additional cancers shows its impact may be even more relevant, and that physical activity has far reaching value for cancer prevention.”
The bottom line: exercise can significantly reduce your risk of dying from breast cancer. So, choose your intensity level and get moving! All it takes is a few hours per week to put a major dent in your risk level.
- Williams PT. Breast cancer mortality vs. exercise and breast size in runners and walkers. PLoS ONE. 2013;8(12): e80616. doi:10.1371/journal.pone.0080616.
- How much physical activity do adults need? Centers for Disease Control and Prevention website. Available here. Monninkhof E, Elias S, Vlems F, et al. Physical activity and breast cancer: a systematic review. Epidemiology. 2007;18:137-157.
- McTiernan A, Kooperberg C, White E, et al. Recreational Physical Activity and the Risk of Breast Cancer in Postmenopausal Women. Journal of the American Medical Association.2003;290:1331-1336.
- Rintala P, Pukkala E, Laara E, et al. Physical Activity and Breast Cancer Risk Among Female Physical Education and Language Teachers: A 34-year follow-up. International Journal of Cancer. 2003;107:268-270.
- Bernstein L, Patel AV, Ursin G et al. Lifetime Recreational Exercise Activity and Breast Cancer Risk Among Black Women and White Women. Journal of the National Cancer Institute. 2005;97:1671-9.
- LE, Eng SM, Bradshaw PT, et al. Fat or fit: The joint effects of physical activity, weight gain, and body size on breast cancer risk. Cancer. Published early online June 25, 2012. DOI: 10.1002/cncr.27433.
- Moore SC, et al. Leisure-time physical activity and risk of 26 types of cancer in 1.44 million adults. JAMA Internal Medicine. May 16, 2016. DOI:10.1001/jamainternmed.2016.1548.
- Dallal C, Sullivan-Halley J, Ross R, et al. Long-term recreational physical activity and risk of invasive and in situ breast cancer (The California Teachers Study). Archives of Internal Medicine. 2007;167:408-415.