“Walking cuts breast cancer risk,” the Daily Mail today reported. The Mail said that “walking for an hour-and-a-half every day could reduce a woman’s risk of breast cancer by 30%”.
The news is based on research into the known association between women’s physical activity and risk of breast cancer. Researchers recruited women who had been diagnosed with breast cancer and a control group with no history of breast cancer. The women were asked about their physical activity levels over the course of their lives, and for each activity level category, the women’s breast cancer risk was estimated.
Women who reported exercising regularly during their lives had similar risk of breast cancer history compared with women who reported no regular physical activity. However, the subgroup of post-menopausal women who reported at least 10 hours of physical activity each week had a decreased risk of having the illness. It is unclear whether this reduction represents a true difference in risk.
Overall, this study suggests that regular physical activity may reduce the risk of breast cancer in some women. There is of course plenty of evidence of the health benefits of getting enough exercise. While any steps towards reducing breast cancer risk are welcome, many women may well view the thought of walking for 90 minutes a day rather daunting. However, it is recommended that adults get at least 150 minutes of exercise each week.
Where did the story come from?
The study was carried out by researchers from the University of North Carolina at Chapel Hill, Mount Sinai School of Medicine and Columbia University. It was funded by the US Department of Defence and National Institutes of Health.
The study was published in the peer-reviewed medical journal Cancer.
The Mail reported the story appropriately, and included a summary of the study’s limitations, as did the Express.
What kind of research was this?
This was a case-control study examining the relationship between physical activity and breast cancer risk among women living in and around New York City. Case-control studies such as this are often used to estimate the risk associated with various activities or factors, but cannot tell us whether or not these factors directly cause the illness.
Case-control studies have several weaknesses that can influence the reliability of their results. Studies such as this identify participants according to their disease status, recruiting people with the disease of interest (the “cases”) as well as people without the disease (“controls”). They then ask the participants to report information on the factors thought to be associated with the disease (in this case, physical activity levels over the lifetime). Because they recruit participants after the development of a disease and ask participants to report on risk factors after the fact, case-control studies are prone to several types of bias, which may influence the results. These include:
- recall bias, which occurs when participants are unable to accurately recall the details of the risk factor
- reporting bias, which occurs when participants do not accurately report their exposure
- selection bias, which occurs when the manner in which cases or controls are identified results in their being dissimilar in important ways, or if the cases are not really representative of the people in the population who are diagnosed with the illness
It is important to keep these sources of bias in mind when interpreting the results of a case-control study.
What did the research involve?
The researchers recruited women with breast cancer from 31 hospitals in or near New York City. These cases were aged from 20 to 98 years old and were diagnosed with breast cancer between 1996 and 1997. The controls were women who had never been diagnosed with breast cancer, and were matched to cases based on age. This is important as age is a significant risk factor for breast cancer.
Approximately 82% of the identified cases and 63% of the identified controls agreed to participate in the study. Participants from both groups were interviewed to collect information on type, amount and intensity of lifetime physical activity. Data was also collected on when the women engaged in such activity (during adolescence, reproductive years or after menopause). Information on potential confounding factors was also collected, including data on demographic characteristics, medical histories and other breast cancer risk factors such as drinking, smoking, weight and hormone medications.
The researchers analysed the data and estimated the odds of having a breast cancer diagnosis based on physical activity levels. They conducted a subgroup analysis based on whether the women were currently pre-menopausal or post-menopausal, and the timing of physical activity. Generally, when multiple comparisons such as this are carried out, the researchers will be conservative in what they consider to be statistically significant. The current study did not report whether or not such a statistical correction was made, so it is difficult to determine whether the results represent true differences in risk.
What were the basic results?
In all, 1,508 cases and 1,556 controls participated in the study. The researchers found no significant difference in risk of breast cancer between women who reported having ever engaged in regular physical activity and those who reported never having done so.
When adjusting for age, the researchers found that:
- Regular physical activity during adolescence was not associated with a difference in risk of developing breast cancer.
- Women who reported engaging in 10 to 19 hours of physical activity during their reproductive (pre-menopausal) years had a 33% reduction in the odds of developing breast cancer after the menopause, compared with women who reported no regular activity during these years (odds ratio 0.67, 95% confidence interval 0.48 to 0.94). No significant differences were seen at other activity levels.
- Women who reported engaging in approximately 9 to 17 hours of physical activity during post-menopausal years had a 30% reduction in the odds of developing breast cancer after the menopause, compared with women who reported no regular physical activity during these years (odds ratio 0.70, 95% confidence interval 0.52 to 0.95). No significant differences were seen at other activity levels.
- No significant differences in odds of developing breast cancer were found in pre-menopausal or post-menopausal women regardless of reported activity levels over the lifetime.
How did the researchers interpret the results?
The researchers concluded that women can “reduce their breast cancer risk later in life by maintaining their weight and engaging in moderate amounts of physical activity”.
This study suggests that regular physical activity may be associated with a reduced risk of breast cancer for some women. However, weaknesses in the research design and statistical analysis make it difficult to be sure that this association represents a true difference in risk.
This study has several limitations, related to both the study design and statistical analysis, that make it difficult to be sure that the results seen are not simply due to chance:
Self-reporting of activity
Average activity levels and weight over the lifetime were based on self-reporting. Asking someone to recall how many hours a week they walked and how much they weighed 20 to 50 years earlier may not result in the most accurate of measurements.
Unclear statistical significance cut-offs
It is unclear from the published study whether or not the researchers used a more stringent cut-off for statistical significance based on the multiple comparisons made. The few comparisons that appear to reach a traditional level of statistical significance may not have met more stringent criteria. As such, it is difficult to say whether or not the approximately 30% reduction in the odds of being diagnosed with breast cancer after the menopause reflect a true difference in risk.
Low participation by controls
The proportion of invited control participants who were ultimately involved in the study was quite low (63%). If these controls differed in systematic ways from the cases, this may have influenced the results.
Ultimately, a study like this can add to the evidence surrounding the relationship between physical activity and risk of breast cancer. While it is not strong enough on its own to tell us much about the relationship, engaging in regular physical activity and avoiding substantial weight gain have proven health benefits. These include reducing the risk of diabetes, heart disease, stroke and other cancers. These more certain benefits, alongside this possible benefit in reducing risk of breast cancer, make getting enough physical activity a vital goal for all women. The recommended UK exercise target is a more realistic and achievable 150 minutes per week than the daily 90 minutes cited in the headlines.