Job stress was not directly associated with the risks of lung, breast, colorectal, or prostate cancer in a large retrospective study.
"Work-related psychosocial stress is unlikely to be an important risk factor for these cancers," wrote Katriina Heikkilä, Ph.D., and her coauthors in the Feb. 7 issue of the British Medical Journal (BMJ 2013;346b:f165 [doi: 10.1136/bmj.f165]).
"Thus, though reducing work stress would undoubtedly improve the psychological and physical well-being of the working individuals as well as the working population, it is unlikely to have an important impact on cancer burden at a population level."
The study does not completely exonerate the role of stress in predisposing to cancer, said Dr. Heikkilä of the Finnish Institute of Occupational Health, Helsinki.
"For example, in a meta-analysis of different types of stress and risk of breast cancer, stress from adverse life events was consistently associated with an increased risk (Breast Cancer Res. 2011;13:208). ... In a French study, people with brain cancer were more likely to report adverse life events than controls without cancer, but there was no clear evidence for a difference in terms of stress at work between these groups (J. Neurooncol. 2011;103:307-16)."
The review included 116,056 subjects who had participated in 10 different trials conducted in Denmark, Finland, and Sweden. The mean follow-up in these studies was 12 years, but follow-up ranged from 5-23 years.
This long span is a particular strength of the review, the authors noted, "because most cancers have a latent period of years or even decades. If a true association between job strain and incident cancer existed because the physiological stress response has a role in cancer promotion or progression (for example, via the regulation of the inflammatory pathways), the follow-up periods in our analyses should have been long enough to detect such an association."
At baseline, the subjects were a mean of 38 years old. Most (62%) were of normal weight. A quarter of the subjects smoked tobacco at baseline, while 10% were classified as heavy drinkers, defined as consuming at least 15 drinks per week for women and 22 drinks per week for men.
Psychological stress at work was defined as a combination of high work demand and low control at the workplace. The results were examined as stress or no stress, and in quartiles of high-stress job (high demand/low control); active job (high demand/high control); passive job (low demand/low control); and low-strain job (low demand/high control). This was measured with a Likert-type scale and ascertained by questions taken from two validated stress questionnaires. The investigators also controlled for age, gender, socioeconomic position, body mass index, smoking, and alcohol intake.
The overall rate of cancer was 5% (5,765). The largest proportion of these cancers was breast cancer (0.9% of the study cohort; 1,010), followed by prostate cancer (0.7%; 865), colorectal cancer (0.5%; 522), and lung cancer (0.3%; 374).
In the binary analysis of job stress/no job stress, there was no significant association with overall cancer risk (hazard ratio, 0.97).
Nor were there significant associations between job stress and colorectal cancer (HR, 1.16), breast cancer (HR, 0.97), lung cancer (HR, 1.17), or prostate cancer (HR, 0.86). "There was also no clear evidence for an association between the categories of job strain and the risk of cancer," the authors wrote.
The study "suggests that many of the previously reported associations ... between work-related stress and risk of cancer could have been influenced by chance, low power in some studies, different covariate adjustment, or residual confounding from possible unmeasured common causes of work stress and cancer. Such common causes could include shift work (for which there is some evidence of an association with risk of breast cancer) or other sources of stress, perhaps combined with one another."
The study was sponsored by several research consortiums and federal agencies in Scandinavia. None of the authors had any financial disclosures.