Stress of finding link to heart disease

Work stress can “dramatically increase the risk of a heart attack”, reported The Independent . Various other news sources report that stress at work can be “a killer”, can “change your body” and “push up heart disease by 68 per cent”. “Scientists have uncovered the biological mechanism that shows how work stress causes ill health, providing the strongest evidence yet of its link with heart disease”, The Independent said

The news story is based on a large study of more than 10,000 civil servants that found that the risk of heart disease was greater in under-50s who had high levels of work stress compared with those who were not stressed. Stress has frequently been linked to an increased risk of heart disease, but proving or quantifying the size of any link is very difficult. The measure of stress used in this study does appear to be linked to some increased risk of heart disease. However, as the researchers have demonstrated, heart disease is not caused by one risk factor; instead, it is made up of a collection of risk factors, with metabolic syndrome and health behaviours playing a very large part, along with stress.

Where did the story come from?

Tarani Chandola and colleagues at the Department of Epidemiology and Public Health, University College London, and the Department of Cardiac and Vascular Science, St George’s University of London carried out this research. The study was funded by various grants from the Medical Research Council, the Economic and Social Research Council and the British Heart Foundation. It was published in the peer-reviewed: European Heart Journal .

What kind of scientific study was this?

This was a cohort study designed to investigate the biological and behavioural factors that link work stress with coronary heart disease. The Whitehall Study recruited 10,308 participants (aged 35 to 55) from 20 civil service departments in London between 1985 and 1988. Until 2004, researchers collected information from the participants through postal questionnaires or clinical examinations. 

A job-strain questionnaire was used at two points in the study to provide a measure of “cumulative work stress”. Job strain was defined as a high job demand with low personal control over the job and decision making. People who have job strain and who are socially isolated at work (without supportive co-workers) were said to have work stress (also called “iso-strain”).

Researchers recorded the number of non-fatal heart attacks, or angina, as well as deaths due to heart disease, that occurred during the study. They also collected information on biological risk factors for heart disease such as cholesterol, blood pressure, blood sugar levels, waist circumference, cortisol levels, and heart rate variability; and behavioural risk factors such as alcohol, smoking, diet and exercise. Statistical methods were used to determine the risk of heart disease related to work stress, taking into account other biological and behavioural factors.

What were the results of the study?

At the end of the study, six per cent of the participants had died. The researchers found that cumulative work stress (recorded at the start of the study and at the next five-year assessment) was associated with an increased risk of heart disease, cardiovascular death or angina. 

When the researchers divided the group by age into those with cumulative work stress aged 37–49 at start of the study, there was a 68% increased risk of heart disease in this group. Those in the 50–60 year age group at the start of the study did not have a significantly increased risk.

Cumulative work stress was linked with an increased risk of having metabolic syndrome (a collection of certain conditions such as high blood pressure, high cholesterol and obesity that are associated with increased risk of heart disease). It was also linked with other health behaviours, including eating less fruit and vegetables, less physical activity and not drinking alcohol.

When the researchers examined the relationship between cumulative work stress and heart disease with adjustment for both health behaviours and the metabolic syndrome, the increased risk in the under 50s age group was no longer significant. Likewise, while work stress was independently associated with an increased risk of heart disease, the size of risk from known heart risks such as having a high waist circumference, high triglyceride levels, low HDL ("good") cholesterol, high blood pressure, less than 5-a-day daily fruit and vegetable consumption and no physical activity, were much higher.

What interpretations did the researchers draw from these results?

The authors conclude that cumulative work stress may be an important risk factor for heart disease amongst working age people, with its affects partly mediated by the effect of stress on health behaviours and the metabolic syndrome.

What does the NHS Knowledge Service make of this study?

This was a large and well-conducted study, which has attempted to try and unravel the complex relationship between risk factors for heart disease. It does have several limitations, which are worth noting when interpreting the news reports:

• Although this study found an increased risk of heart disease with cumulative work stress in the under 50s group, the risk was no longer significant when adjustment was made for the metabolic syndrome and health behaviours.
• The actual size of risk from stress was less than the risk from other, well-established risk factors, such as blood pressure and cholesterol levels.
• Some measures in the study, including stress, smoking and alcohol consumption, diet and exercise, were self-reported. This may have meant that they were not recorded accurately. In particular, those who were suffering from angina may have been more likely to over-report their levels of stress at work, possibly in an attempt to try to find a cause for their condition.
• The scales that were used to determine whether the individual was “stressed” or not, are quite subjective, and reports from this group of London civil servants may not be transferable to other professions or population groups. 
• The “cumulative work stress” measure was taken from a measurement at two separate time points, five years apart. The person may not have been continuously stressed for the duration of that time. 
• Although many adjustments for biological and behavioural factors were made, certain factors that may are known to have an influence on heart disease risk could not be examined in the analysis. For example, heart rate variability and cortisol levels could not be reliably examined as potential factors, as data on these variables was not collected in the early phases of data collection. 
• Finally, some people from the initial study failed to respond to the follow-up questionnaires or complete clinical examination and their data might have effected the results.