Friday September 14 2012
Is your boss making you sick?
'Being in a stressful job where you are bossed around could raise the risk of a heart attack by a quarter' the Daily Mail has warned.
This story follows news published earlier this week that long working hours may increase heart disease risk.
In the study that this latest news is based on, researchers wanted to know what effect ‘job strain’ had on the risk of heart disease for workers. Job strain was defined as having a combination of a demanding job with little freedom to decide how the job should be done. Previous research has found that this combination can increase psychological stress.
The researchers pooled the results of 13 large observational studies (both published and unpublished) conducted across Europe over the past 25 years. The studies included almost 200,000 people and used validated questionnaires to assess their job strain. Fifteen percent reported job strain, and over an average 7.5 years of follow-up, they had a 23% increased risk of suffering, or dying from, a heart attack. This risk increase remained significant after taking into account conventional risk factors for coronary heart disease (CHD), including lifestyle.
Given past research observing links between stress, workload and CHD, the link is plausible. It is possible that measures to prevent stress in the workplace might lower levels of heart disease.
If you are not in a position to improve your lot, the researchers also point out other effective methods of reducing your heart disease risk, including avoiding smoking or drinking too much alcohol, eating a healthy diet and taking regular exercise.
Read more about reducing your heart disease risk.
Where did the story come from?
The study was carried out by researchers from University College London and many other institutions in the UK and Europe. It was funded by the Finnish Work Environment Fund, the Academy of Finland, the Swedish Research Council for Working Life and Social Research, the German Social Accident Insurance, the Danish National Research Centre for the Working Environment, the BUPA Foundation, the Ministry of Social Affairs and Employment, the Medical Research Council, the Wellcome Trust, and the US National Institutes of Health.
The study was published in the peer-reviewed medical journal The Lancet.
The reporting on the study was generally an accurate representation of the findings of this research. The Mail should also be praised for including an important piece of advice from one of the lead researchers, noting that ‘stress reduction would have a much smaller impact (on heart disease) than tackling either lack of exercise or smoking’.
What kind of research was this?
This research included the findings of 13 prospective cohort studies commenced between 1985 and 2006 in Finland, Sweden, Denmark, the Netherlands, Belgium, France, and the UK. All studies were reported to be part of the individual-participant-data meta-analysis in working populations (IPD-Work) consortium, which was established at a meeting in London in 2008. Therefore the current study represents the pooled results of these observational studies.
Prospective cohort studies which assess the exposure of interest (in this case job strain) at the start of the study and then follow-up over time to look at the development of disease outcomes (in this case the development of coronary heart disease) are the best way of looking at whether exposure increases risk of the outcomes.
Because the job strain was examined before a heart attack had happened, this reduces the possibility of recall bias. Recall bias is where participants may recall having higher job strain in an attempt to find possible reasons for their heart attack. Cohort studies, though, cannot prove causation (direct cause and effect), and the individual studies would need to take account of many other potential confounding factors that could be involved in the relationship (especially other health and lifestyle factors). In this case, the researchers did take these into account.
The researchers also made a point to include previously unpublished studies in their research as they were concerned that publication bias may have distorted previous findings. That is studies that found a strong link between stress and heart disease were more likely to get published in a medical journal than studies that found a very weak link, or none at all.
What did the research involve?
This research included the 13 independent studies in the IPD-Work consortium. Participants with no data on job strain, or who had coronary heart disease events such as heart attack, or a diagnosis of CHD, at the time of enrolment were excluded from the analysis. Overall, information from 197,473 people was analysed.
In all studies, job strain was assessed using questions from validated job-content and demand-control questionnaires. For each study participant, average scores were calculated for job-demand questions (such as whether they believed they had to work very hard, had excessive amounts of work, conflicting demands, or insufficient time) and job-control questions (such as those about decision freedom). High-demand jobs were considered as those where the job-demand score was above the average in the study. Low-control jobs were those where the job-control score was lower than the study average.
The researchers used national hospital admission records and death registries to find out about CHD events of fatal or non-fatal heart attack.
When looking at the relationship between job strain and CHD events, the researchers took into account potential confounders such as:
- socioeconomic status
- body mass index (BMI)
- physical activity
Four of the studies had data on other conventional risk factors; cholesterol, blood pressure and diabetes.
What were the basic results?
The average age of participants at the time of enrolment to the studies was 42 years, and the studies included an even proportion of men and women. Overall, 15% of participants (30,214 people) were defined as having job strain at study start, varying from 13% to 22% across the cohorts.
Over an average 7.5 years of follow-up across the 13 studies there were 2,358 new events of fatal or non-fatal heart attack. The pooled results of all studies found that people with job strain had a 23% increased risk of a CHD event compared to those with no job strain (hazard ratio 1.23, 95% confidence interval 1.10 to 1.37). These studies had all adjusted for age and gender, with some studies adjusting for other lifestyle and medical risk factors, where this data had been collected.
All of the studies gave broadly similar results (they had low statistical heterogeneity).
When all studies were adjusted for socioeconomic status, the risk increase for job strain reduced to 17% (hazard ratio 1.17, 95% confidence interval 1.05 to 1.31).
The researchers also observed that three of the studies had been published while 10 had not. Those that were published tended to find higher risk associations between job strain and CHD events than those which had not been published (suggesting publication bias).
The researchers calculated the population attributable risk factor for job strain as 3.4%; this means they estimate that 3.4% of all CHD events could be avoided if all people were free from job strain.
Seeing that heart disease is responsible for around 94,000 deaths in the UK each year, this suggests that reducing job strain could save a few thousand lives each year.
How did the researchers interpret the results?
The researchers conclude that their findings suggest that prevention of workplace stress might decrease the incidence (new cases) of coronary heart disease. However, they acknowledge that this strategy would have a much smaller effect than tackling other well known risk factors, such as smoking.
This is valuable research that has pooled the results of 13 European cohort studies, all looking at whether job strain influences the risk of developing a fatal or non-fatal heart attack. As the researchers say, they find that ‘job strain is associated with a small, but consistent, increased risk of an incident of cardiovascular heart disease’. Furthermore, adjustment for age, gender, socioeconomic status and other conventional risk factors did not significantly alter the association.
The findings are plausible, and follow many previous research studies suggesting links between stress, strain and workload, and risk of coronary heart disease. This study benefits from combining:
- the results of 13 studies
- very similarly designed studies
- studies which gave broadly similar results
- studies that assessed job strain before the participants had a heart attack (removing a potential bias)
The resulting pooled analysis being based on a large number of almost 200,000 middle-aged adults, with an equal balance between men and women.
However, there are some important limitations to consider, such as:
- The difficulty of ensuring that all cases of CHD were taken into account. By only looking at records of fatal or non-fatal heart attack, cases could have been missed. For example, some participants could have had CHD by the end of the follow-up period, but it may not have developed into a heart attack or been diagnosed yet.
- Although the study excluded people who had already suffered a heart attack or been diagnosed with CHD at the start, some people could have been included who (unknown to them) already had CHD development at the time their job strain was assessed. To try to partially account for this the researchers also conducted analyses only looking at people who developed CHD events after five years of follow-up (believing the longer time frame may exclude people who already had disease development at the start of the study). These results still gave similar, and significant, increased risk figures when compared to those people who developed disease events in the first three years of follow up.
- Job strain was only assessed at one baseline assessment. As the researchers say, some previous studies have suggested that cumulative exposure to job stress over time could be a better predictor of CHD risk.
- The researchers also acknowledge, that despite all included studies being part of a European consortium, this was not a systematic review of the literature, and there may be other relevant studies assessing links between job stress and strain and CHD that have not been included.
Overall, the researchers’ conclusions are apt: it is possible that measures to prevent stress in the workplace might decrease CHD disease incidence, but such a strategy ‘would have a much smaller effect than tackling standard risk factors, such as smoking’.
Analysis by Bazian. Edited by NHS Choices. Follow Behind the Headlines on twitter.