"Exercise lowers risk of breast cancer after menopause," reports The Independent. This and similar headlines were sparked by a large study of postmenopausal teachers that found increased recreational activity was associated with a 10% decrease in the risk of breast cancer.
The risk reduction eroded among some women who became less active over the years, suggesting keeping up a certain level of activity might be important in maintaining the benefits.
The study used questionnaires to estimate the levels of walking, cycling and sport the women did outside of work.
It found women who did the equivalent of walking at least four hours a week or doing sport for two hours a week had a reduced risk of breast cancer. Factors such as body mass index (BMI) did not change the results.
However, the majority of women in the study had a healthy BMI and were teachers, so the results may not be applicable to all postmenopausal women.
Lack of physical activity and excess body fat have been linked to an increased risk of many cancers, including breast, colon, endometrial (lining of the womb) and prostate cancer, as well as heart disease, stroke and diabetes.
Despite the limitations of this study, taking regular exercise such as walking has been found to have wide-reaching benefits – the 30 minutes a day suggested in much of the news coverage is enough to get your recommended 150 minutes of exercise a week.
Where did the story come from?
The study was carried out by researchers from the Nutrition, Hormones and Women's Health team at the CESP Centre for Research in Epidemiology and Population Health, Université Paris Sud, Université Hospital and the Université d’Auvergne in France.
It was funded by the Institut National du Cancer, the Fondation de France and the Institut de Recherche en Santé Publique.
The study was published in the peer-reviewed medical journal Cancer Epidemiology, Biomarkers and Prevention.
The media reported the study accurately, but did not point out that the study only involved teachers, most of whom were a healthy weight.
What kind of research was this?
The researchers wanted to see whether exercise levels reduced the risk of breast cancer, and whether it mattered if the exercise was recent or several years before.
As this is a cohort study, it can only show an association between the two – it cannot prove that regular exercise can prevent or delay breast cancer.
What did the research involve?
The researchers used information gathered from a large prospective cohort study of female teachers in France conducted from 1993 to 2005.
The 59,308 postmenopausal women filled in questionnaires in 1993, 1997 and 2002 on their health status and levels of physical activity. The researchers verified the women's self-reported breast cancer by checking pathology reports and the national cause of death registry.
Physical activity level was assessed by asking the women to estimate the amount of time they spent in a typical week in both the summer and winter:
- walking (including walking to work, shopping and leisure time)
- cycling (including cycling to work, shopping and leisure time)
- doing sports
The level of activity was averaged over these two weeks and graded by metabolic equivalent task (MET). One hour walking was equivalent to three MET hours, while one hour cycling or doing any sport was given six MET hours.
Women were excluded if they had:
- cancer at the beginning of the study
- cancer before the menopause (other than basal cell carcinoma)
- never menstruated
- missing information on physical activity level
- been in the top 1% of reported physical activity
The researchers analysed the results according to the level of physical activity reported in each of the three questionnaires. These were adjusted to take into account:
- energy intake
- alcohol use
- family history of breast cancer
- history of benign breast disease
- age of starting their periods and the menopause
- use of HRT
- the number of children they had given birth to before and after the age of 30
What were the basic results?
The average length of follow-up was 8.5 years. During this time, 2,155 women developed breast cancer. Most of the women (73%) had a BMI between 18.5 and 25.
The researchers calculated that women with levels of recreational activity of more than 12 MET hours a week in the previous four years had a 10% lower risk of breast cancer than those with a lower level (hazard ratio [HR] 0.90, 95% confidence interval [CI] 0.82 to 0.99).
This remained the same after taking numerous other factors into account, including BMI, waist circumference, recent change in weight, sports activities from the age of 8 to 15 years, and the use of progestogen or oral contraceptives.
Women who had done more than 12 MET hours of exercise a week five to nine years ago, but who then became less active, had a 16% increased risk of breast cancer than those who remained active (HR 1.16, 95% CI 1.01 to 1.35).
If activity levels remained the same five to nine years earlier and in the last four years, the activity level during those five to nine years was not significantly associated with breast cancer risk (HR 1.04, 95% CI 0.92 to 1.18).
There was quite a high rate of change in reported levels of physical activity, with a fifth (21%) moving from more than 12 MET hours a week to less than 12 MET hours a week in at least two consecutive questionnaires, and a fifth (20%) moving from less than 12 MET hours a week to a higher level.
How did the researchers interpret the results?
The researchers concluded that, "Recent recreational physical activity, even at a modest level, was associated with a breast cancer risk reduction in postmenopause; this association seemed to attenuate a few years after activity stops."
This large study has shown that increased exercise is associated with a reduced risk of breast cancer for postmenopausal women. Strengths of the study include the large number of women and that self-reports of breast cancer were verified by a pathology report in 94% of cases.
However, as the authors point out, a limitation of this study is that it was conducted on a group of teachers who were mainly of a healthy weight. This means the results may not be applicable to women of a different weight with different occupations, including more or less sedentary jobs.
The study also relied on self-reported exercise levels, which may not be entirely accurate. It also only looked at recreational physical activity, so did not include any physical activity at work (for example, it didn't distinguish PE teachers from teachers of other subjects).
For the women who developed breast cancer, it is not clear whether the diagnosis occurred before or after the levels of physical activity reduced.
Lack of physical activity and excess body fat have been linked to an increased risk of many cancers, including breast, colon, endometrial (lining of the womb) and prostate cancer, as well as heart disease, stroke and diabetes. Regardless of the limitations of this study, it is still advisable to take regular exercise.