Monday October 1 2007
The association between job strain and breast cancer is weak
Stress at work increases breast cancer risk by a third, reported the Daily Mail. The newspaper said that “women in demanding jobs are 30 per cent more likely to develop the disease than those who feel on top of their work”.
The story is based on research carried out in women aged between 30 and 50. Although the study did find a weak association between breast cancer risk and “job strain”, there are several shortcomings which should be considered when interpreting the results. The situation is not as clear cut as the newspaper suggested.
Where did the story come from?
Drs Hannah Kuper and colleagues from the Clinical Research Unit at the London School of Hygiene and Tropical Medicine in London conducted this research. The study was funded by the Swedish Council for Planning and Co-ordination of Research, Swedish Cancer Society, The Swedish Foundation for International cooperation in Research and Higher Education, Pharmacia, Medical Products Agency and Schering-Plough. It was published as a brief report in the peer-reviewed medical journal Epidemiology.
What kind of scientific study was this?
The study was part of a large prospective cohort study involving 96,000 women aged 30 to 50 years from the Swedish Central Population Registry – the Women’s Lifestyle and Health Cohort study. The women were asked to complete a lifestyle questionnaire in the early Nineties and this study used the information from a particular group of women - working women (part time or full time), without cancer who returned their questionnaires and had information available about job strain were included. This meant that data from 36,332 women was analysed to determine if they had a breast cancer diagnosis, had emigrated or died over the next 13 years.
The researchers analysed whether the level of job stress (a measure of the balance between demand and control at work), measured through the questionnaire at the start of the study, affected the risk of women developing breast cancer during a follow up.
Stress was classified into one of four categories: “active work” where women felt their jobs were high demand but they had high levels of control; “job strain” where demand was high but control was low; “low strain” where demand was low but control was high; and “passive work” where neither demand nor control were considered high. Women in the “job strain” category were considered to be suffering from stress at work.
What were the results of the study?
The researchers found that women in full-time employment who were in the “job strain” category, were 1.4 times more likely to be diagnosed with breast cancer during the 13 year follow up.
This means that for every 1,000 women followed for 10 years, about 20 (2 per cent) developed breast cancer in the “job strain” group compared with 17 who developed breast cancer in the group considered to have no stress at work. There was no association between “job strain” and breast cancer risk in women who worked part time.
What interpretations did the researchers draw from these results?
The researchers concluded that there was a small increased risk of breast cancer among women in full-time employment who experienced stress at work.
What does the NHS Knowledge Service make of this study?
This large cohort study shows a weak association between “job strain” and the risk of breast cancer. The study’s shortcomings lead us to a more moderate view of the results than the newspaper story suggests:
- Only 51% of the women originally selected for the large study returned their questionnaires. Subsequently, others were excluded for various reasons (e.g. no information available on job strain, women did not work full or part time). There is no way of knowing how the women who didn’t respond differ from those who did. For example, if none of the non-responders had breast cancer, then the results of the study would change.
- Another weakness of this study is that “job strain” was measured at one time point only; at the time of enrolment in the Women’s Lifestyle and Health Cohort, up to 13 years before the study ended. Job strain is likely to have changed as the study progressed and this was not taken into account.
- Though the researchers tried to consider other factors that may have been responsible for the increased risk of breast cancer, they may not have included all possible factors. The researchers report that women with “job strain” in this study were more likely to report other high-risk characteristics for breast cancer (e.g. obesity, smoking, lack of exercise) than women with low strain or active work conditions.
- The findings of this study are not consistent with many other studies that have looked at the link between stress and risk of cancer. Other studies have found that stress does not increase cancer risk. Cancer Research UK states that “even in the event that stress and cancer are linked, the effects would be very small compared to other factors such as lifestyle, age or family history”.
Keeping in mind the shortcomings of this study and its modest results, the association here should not be over interpreted. As the researchers conclude, “At present, the lack of consistent epidemiologic data or biologic rationale limits interpretation of these findings”.
Sir Muir Gray adds...
Breast cancer and strain at work are important problems for women in their own right and each deserves to be tackled seriously, even if there is no relationship between the two.