“Women who walk for an hour a day can cut their chance of breast cancer by 14%,” The Guardian reports. A new study has found that even moderate regular exercise is associated with lower risk of cancer.
This US cancer prevention study involved over 73,000 postmenopausal women who were tracked for 17 years.
During this time 6% of the women developed breast cancer. The researchers then looked back to see whether reported time spent walking, sitting or in recreational physical activity was linked to risk of developing breast cancer.
The researchers found that the most physically active women had 25% reduced cancer risk compared to the least active.
Nearly half the women in the study said walking was their only form of exercise. And for these women, those who walked seven or more hours a week had reduced risk, estimated to be around 14% compared to those who walked three or less hours. The link persisted even with adjustment for other hormonal factors and body mass index (BMI) or weight gain.
The study does not prove that exercise alone is directly responsible for the decreased risk, as other lifestyle factors may be involved. For example, women who do regularly exercise may also adopt other healthy lifestyle choices such as eating a healthy diet.
Nevertheless, walking as a form of exercise is accessible, free and good for the heart and for weight control. So the finding that it may also protect against breast cancer is welcome news.
Read more about the benefits of walking.
Where did the story come from?
The study was carried out by researchers from the American Cancer Society and was also funded by the Society. The study was published in the peer-reviewed medical journal Cancer Epidemiology, Biomarkers & Prevention.
The findings of the study were reported accurately by the UK, with several papers including useful comments from breast cancer experts.
What kind of research was this?
This was a prospective cohort study that aimed to examine the association between all kinds of physical activity and leisure time sitting (for example time spent watching TV) and the risk of breast cancer. The cohort consisted of postmenopausal women aged 50 to 74.
Researchers also looked at whether this association differed according to whether the breast cancer was oestrogen receptor positive or negative (OR status). Oestrogen receptor positive cancers are where the cancer cells have receptors for the oestrogen, and therefore the hormone is stimulating the cancer to grow. These women may be candidates for the hormone therapies for breast cancer such as Tamoxifen.
Other factors that were taken into account were the women’s body mass index, weight gain and use of hormone replacement therapy (HRT).
The researchers point out that there is evidence for a lower risk of breast cancer in women who do vigorous physical activity such as swimming or aerobics.
However, it is said to be unclear whether moderate exercise such as walking has the same association. And if there is an association does it differ according to the factors described above?
Prolonged periods of sitting have been associated with some cancers but any link between sitting time and breast cancer is not well understood, they say.
What did the research involve?
The researchers drew on data from the American Cancer Society Cancer Prevention Study II Nutrition Cohort study. This was established in 1992 and looked at cancer incidence and nutrition in nearly 98,000 women.
Participants aged 50 to 74 years were enrolled in the study in 1992-93, when they completed a detailed questionnaire about factors such as income and background, reproductive and medical history, and lifestyle. From 1997 onwards, participants were sent follow-up questionnaires every two years to update this information and to find out about newly diagnosed cancers. The response rates among living cohort members was 88% or more.
Excluded from the analysis were 3,111 women who did not return follow-up questionnaires, 12,059 who reported a diagnosis of cancer (except non-melanoma skin cancer) at enrolment, and 4,712 who had not yet gone through the menopause. Sixty-six women who reported breast cancer on their first follow-up questionnaire, that was never verified, were also excluded.
The final group for analysis consisted of 73,615 postmenopausal women (defined as women whose periods had stopped permanently before enrolment, naturally or for surgical/medical reasons) with an average age of 62.7 years. They were followed up on average for 14.2 years between enrolment and 2009.
Information on their weekly recreational activities was collected when the women were first enrolled.
Researchers assessed the average number of hours per week spent on each of the following activities:
- lap swimming
- bicycling/stationary bike (exercise bike)
They calculated for each woman an estimate of total hours per week of the metabolic equivalent (MET). MET is the ratio of the energy expenditure during a specific activity, to the resting metabolic rate (this is the rate at which the body uses energy).
Women who reported no activity were categorized as "none," and the remaining women were categorised in five groups (quintiles) according to the number of MET-hours they expended per week:
- between nil and 7.0
- between 7.0 and 17.5
- between 17.5 and 31.5
- between 31.5 and 42.0
- more than 42.0
The hours spent each week walking was categorized as none, 3 or less, 4-6, or 7 or more.
Declining health or chronic conditions associated with ageing or low oestrogen (e.g. osteoporosis) may prevent some women from engaging in even light recreational physical activity. So women reporting the lowest level of walking activity (3 hours or less), as opposed to "none", were the reference group for all comparisons.
Leisure time sitting (time spent watching TV, reading, etc.) was categorised as 0-3 hours, 3-5 hours, or 6 hours or more, daily.
Questions about the women’s physical activity and the time they spent sitting were repeated and updated in 1999, 2001 and 2005.
Women were asked to self-report any diagnosis of breast cancer and this was then verified by medical records or by linking to state cancer registries. The study also had an automatic link to the National Death Index to identify cases where women had died.
The researchers analysed the results and adjusted their findings to take account of factors (confounders) that might influence breast cancer risk. These included race, education, BMI, weight change, alcohol intake, smoking status, age at menopause, number of live births/age at first live birth, family history of breast cancer and HRT use.
What were the basic results?
4,760 women (6% of the cohort) were diagnosed with breast cancer between 1992 and 2009. Of the 69% of breast cases where information about oestrogen status was available, 84% were OR positive and 15% were OR negative.
The main findings were as follows:
- The most active women (those reporting more than 42 MET-hours per week of physical activity) had a 25% lower risk of breast cancer compared to the least active (those reporting between none and seven MET-hours/week) – relative risk [RR], 0.75, 95% confidence interval [CI] 0.63 to 0.89).
- Forty-seven per cent of women reported walking as their only recreational activity. Among this group, women who walked for seven or more hours a week had a 14% lower risk of breast cancer compared to those walking three hours or less a week (RR 0.86, 95% CI, 0.75 to 0.98).
- Oestrogen receptor status, BMI, weight gain, or HRT use made no difference to these associations.
- Sitting time was not associated with an increased (or decreased) risk of breast cancer.
How did the researchers interpret the results?
The researchers say their results suggest that physical activity is associated with a lower risk of breast cancer that does not differ by OR status, BMI, weight gain or HRT use.
Walking on average at least one hour a day was modestly associated with a lower risk even in the absence of other physical activities, while more vigorous activity was associated with a 25% reduction in risk.
Physical activity, they suggest, has a beneficial effect on hormone levels, weight control, glucose metabolism, insulin sensitivity and inflammation – all factors implicated in the development of postmenopausal breast cancer.
They suggest that promotion of leisure time walking may be an effective strategy for increasing physical activity among postmenopausal women
The major strengths of this study are its large size, long follow-up period and the availability of detailed information provided by the women both at the start and during follow-up.
As the authors point out the assessment of recreational physical activity in this study may not reflect total physical activity in working women in manual occupations. But they also note that most women in the study were “homemakers”.
The women in the study population were predominantly white, middle-aged or elderly, and well educated, so as the authors also note, the results may not be generalisable to other groups.
Another limitation is that the study, it is based on self-reported exercise which may introduce inaccuracies.
Overall though, this type of study cannot prove that exercise directly helps to prevent breast cancer. It is possible that exercise may help to reduce risk, in addition to other healthy lifestyle factors. For example smoking, alcohol and a high-fat, low fruit and vegetable diet have all been associated with increased risk of breast cancer. Though the researchers adjusted for alcohol intake and smoking status, a more active lifestyle may be associated with a healthier diet and this may also be contributing to the decreased risk.
Also of course, despite the best efforts that may be made to modify risk of cancer, some risk factors, such as our biology and hereditary risk cannot be altered.
Nevertheless, walking as a form of exercise is accessible, free and good for the heart and for weight control. The finding that it may also protect against breast cancer is an additional benefit.
It is easier than you think to stick to a regular walking routine in order to boost your "MET score". For example you could:
- Walk part of your journey to work.
- Walk to the shops.
- Use the stairs instead of the lift.
- Leave the car behind for short journeys.
- Walk the kids to school.
- Do a regular walk with a friend.
- Go for a stroll with family or friends after dinner.
There are also walking groups that can also provide you with the opportunity to socialise with new people. For more information play the video on this page.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Guardian, 4 October 2013
BBC News, 4 October 2013
The Daily Telegraph, 4 October 2013
Daily Mail, 4 October 2013
Links to the science
Cancer Epidemiology. Published online October 3 2013