“Women who walk for an hour a day can more than halve the risk of developing breast cancer,” reports The Daily Telegraph. The newspaper bases its story on a Japanese study of over 30,000 women.
In the study, researchers took results from a survey on physical activity and looked at which women went on to develop breast cancer over the next 12 years. The newspaper says that walking, jogging, swimming or working out at the gym once a week were linked to a 55% reduced chance of developing breast cancer.
The Japanese researchers said: “We recommend walking for one hour a day, along with additional weekly exercise, to protect against breast cancer, regardless of menopausal status or body mass index.”
This is a reliable, large study which confirms another benefit of physical activity for women, regardless of their age or weight.
Where did the story come from?
This research was carried out by Dr Sadao Suzuki from the Department of Public Health at Nagoya City University and colleagues from elsewhere in Japan. It was funded by a grant from the Ministry of Education, Culture, Sports, Science and Technology of Japan. The study was published in the peer-reviewed medical journal Cancer Epidemiology, Biomarkers and Prevention.
What kind of scientific study was this?
This was a prospective cohort study which looked for links between physical activity and breast cancer by following over 30,000 women. The study looked at data on Japanese women aged 40 to 69 years over a 12-year period.
The researchers used data from the Japan Collaborative Cohort (JACC) Study, which has collected a large amount of information on over 110,000 Japanese men and women over a number of years. The JACC study features data from 45 geographic areas. The researchers analysed data from 22 of these areas with data on both rates of cancer and physical activity.
When women had enrolled in the study they had completed a self-administered questionnaire on physical activity, covering the amount of time spent walking, performing specific exercise, and being physical active at the work place.
They grouped the answers to ‘time spent walking’ into three sets: less than 30 min per day, 30 to 59 minutes per day and one hour or longer per day. The answers to ‘time spent exercising’ were grouped into three sets: never or seldom, one to two hours per week, and three hours or longer per week. They did not ask about the type or intensity of the physical activity beyond these questions.
The authors also collected data at the start of the study on other potential breast cancer risk factors, such as family history, body mass index (BMI), tobacco use, alcohol use, age when periods started and finished, marital status, number of children, age at birth of first child, and hormone use.
They then followed the women using the local breast cancer registry to identify new cases of breast cancer. They used the National Registry of Deaths to identify those who had died and what they had died from.
Standard statistical techniques were used to look for the strength of any associations, and these were adjusted for the known breast cancer risk factors.
What were the results of the study?
The average age at entry into the study was approximately 57 years and the average (median) follow-up time was 12.4 years. During the study there were 207 new cases of breast cancer.
The most physically active group, those women who walked for one hour or more per day and exercised for one hour or more per week, had a lower risk of breast cancer compared with the least active group after the adjustments (HR, 0.45; 95% confidence interval, 0.25 to 0.78).
The researchers say that their results were not significantly modified by menopausal status or body mass index (BMI), meaning that there were no statistical differences when they analysed the results with or without applying these known breast cancer risk factors.
What interpretations did the researchers draw from these results?
The researchers conclude that their analysis of Japanese women has “revealed a significant inverse association between physical activity and breast cancer”, meaning that the women who exercised more, had less risk of developing breast cancer.
The authors note that the combined effect of walking and exercise was stronger than that expected, based on their individual effects. They also concluded that walking for one hour per day and undertaking additional weekly exercise seemed to be protective against breast cancer, regardless of menopausal status or obesity measures.
What does the NHS Knowledge Service make of this study?
This study confirms an important public health message on the protective benefits of physical activity and regular exercise. Although the study was conducted in Japan, where rates of exercise and obesity may be different from the UK, the same biological protection found in this study is likely to apply to British women too.
The prospective nature of the study is a strength as it avoids some of the problems of recall bias seen in other study designs, where women already diagnosed with breast cancer may answer questions about physical activity differently from healthy women.
There are a few limitations that the authors acknowledge. The researchers used a simple questionnaire and grouped the answers into broad categories. They did not ask about the intensity or type of exercise, which are both important in estimating overall physical activity.
There is a 95% confidence interval in the estimated 55% reduction in risk in the highest risk group. This suggests that the 55% reduction in risk in the highest risk group may not be precise and could be as low as 22%, which would make the estimate more in line with other studies.
Given that a protective effect was found independent of menopausal status and measures of obesity it supports the researchers’ recommendation that activity is good for you, whatever your age or weight.
Exercise is widely seen as a good way of protecting against breast cancer, just as obesity increases the risk of the disease developing.
Sir Muir Gray adds...
Walking gives lots of other benefits too.
Analysis by Bazian
Edited by NHS Website
Links to the headlines
The Daily Telegraph, 11 December 2008
Daily Mail, 11 December 2008
Marie Claire, 11 December 2008
Links to the science
Cancer Epidemiology Biomarkers & Prevention 17, 3396-3401, December 1 2008