Exercise and breast cancer

Friday October 31 2008

The Daily Express stated today that “scrubbing floors cuts risk of breast cancer”. It said a study of over 32,000 women found that doing heavy household chores can slash the chance of breast cancer by a third. Apparently, scrubbing floors, washing windows and digging the garden are just as effective as running, cycling and playing tennis. However, lighter tasks such as vacuuming and painting, or pastimes such as bowls and walking do not have the same effect.

In this 11-year study, the active women who were protected also tended to be slimmer non-smokers. The study also only looked at cancers developing after the menopause, that is in older women. However, it reliably confirms advice that participating in vigorous physical activity for at least 30 minutes a day has benefits. Surprisingly, the less vigorous activities referred to by the researchers as “moderate activities” were not associated with any benefits. Therefore any suggestion that less vigorous activities are beneficial, such as washing clothes and mowing the lawn, is not supported by the results of this study.

Where did the story come from?

Dr Michael Leitzmann and colleagues from the Division of Cancer Epidemiology and Genetics at the National Cancer Institute in the US carried out this research. It is not clear from the publication whether the study had any external funding support. The study was published in the peer-reviewed medical journal, Breast Cancer Research.

What kind of scientific study was this?

The researchers say that two recent meta-analyses concluded that there is a relationship between breast cancer and physical activity, but the type, frequency, duration and intensity of activity is not known. They aimed to shed more light on this by analysing data from a prospective cohort (group) study in which the researchers studied the relationship of total, vigorous and non-vigorous physical activity to the risk of developing postmenopausal breast cancer.

The current research used data from a previous study called the Breast Cancer Detection Demonstration Project (BCDDP) Follow-up study. The original BCDDP study aimed to demonstrate the value of mammography in screening for breast cancer, and enrolled over 280,000 women between 1973 and 1980.

The BCDDP follow-up study began in 1979 and used over 64,000 women from the original study who had developed breast cancer, non-cancerous breast lumps, or who had been referred for a surgical opinion, and a sample of people who had not had surgery or been referred. These women were followed-up until 1998 with annual telephone calls or mailed questionnaires that asked them for basic information, and whether or not they had developed breast cancer.

This current study was only interested in the 32,269 women in the follow-up study who had been followed-up between 1987 and 1998. During this period, these women were sent a questionnaire asking about the women’s “usual physical activity”, including household, occupational and leisure activities in the previous year. Participants were asked the number of hours during the week and weekend that they typically spent in moderate and vigorous physical activities. The answers to the questionnaire were then converted to a weekly average.

The researchers defined moderate (or non-vigorous activity) as light housework, vacuuming, washing clothes, painting, home repairs, lawn mowing, general gardening, raking, light sports or exercise, walking, hiking, light jogging, recreational tennis, bowling, golf and bicycling on a level ground. Examples of vigorous activity included heavy housework, such as scrubbing floors or washing windows, heavy garden-work, digging in the garden, chopping wood, strenuous sports or exercise, running, fast jogging, competitive tennis, aerobics, bicycling on hills and fast dancing.

The postmenopausal breast cancer cases were identified through self-reports, death certificates and linkage to state cancer registries. Standard statistical methods were used to estimate the relative risk of postmenopausal breast cancer in groups associated with physical activity.

What were the results of the study?

Between 1987 and 1998, 1,506 new cases of postmenopausal breast cancer were found. After adjusting for (taking into account) other risk factors for breast cancer, there was a tentative link between total physical activity and postmenopausal breast cancer (confirmed by the confidence interval in the results spanning one). Comparing the most active with the least active groups the relative risk was 0.87 (95% CI, 0.74 to 1.02).

The relationship between activity and risk of breast cancer was limited to women who were lean (defined as body mass index less than 25. In these women there was a significant difference, with a relative risk of 0.68 (95% CI, 0.54 to 0.85). This is almost a third reduction and the result quoted by the newspaper.

In contrast, there was no association with vigorous activity in women who were overweight or obese (BMI more than 25). Non-vigorous activity also had no relationship to risk of breast cancer.

What interpretations did the researchers draw from these results?

The researchers say that the results support the hypothesis of an inverse association between physical activity and postmenopausal breast cancer, and that the risk reduction appeared to be limited to vigorous forms of activity.

They say that the potential protective effect of vigorous activity is most apparent among lean or normal-weight women rather than overweight women, and the researchers call for more studies to further evaluate the relationship and potential biological mechanisms underlying it.

What does the NHS Knowledge Service make of this study?

This is a large study, which has the benefit of being prospective. This means that the outcome, breast cancer, was recorded after the questionnaires asked for details on the women’s physical activity. This is important as it means we can be more certain of a causal relationship and that other things are not responsible, for example that women with breast cancer did not simply become less active due to their diagnosis.

Other strengths to the study included the careful assessment of the diagnosis of breast cancer, which ensures that the chance of misdiagnosis is reduced.

The authors note a few limitations:

  • The questionnaire format may have led to some degree of over-reporting of activity. It is known that people often over-estimate their time spent in physical activity when answering on paper compared to their answers given in interviewer-administered assessments. The authors also suggest that this may explain why the overweight women appeared to derive no benefit from activity, in that they may be more prone to over-reporting their activity levels.
  • Most of the women in the study were white, therefore the results may not be strictly relevant to all women.
  • Physical activity details were gathered once, at the start of the study, and as people may have changed their activity levels over the 10 years of follow-up, this is a further source of bias.

The fact that the analysis has shown that vigorous, but not gentle, exercise cuts the risk of breast cancer, and only in women who were not overweight, is of interest. Although this study has not resolved the issue about how often a women needs to exercise (or work physically) to reduce their risk of breast cancer after the menopause, it does confirm the theory that the more vigorous the activity the better.

Sir Muir Gray adds...

The evidence of a reduction of cancer risk as a result of exercise is getting stronger and stronger.

Analysis by Bazian
Edited by NHS Choices