Summary

There were clear links between job stress and coronary heart disease in a group of UK government workers. A third of these links were clearly related to low physical activity, poor diet, or the features of the metabolic syndrome; the remainder were apparently more direct, mediated through the body's self-regulating systems.

Introduction

While it's accepted that work stress can increase the risk of coronary heart disease (CHD), the mechanism of this cause-and-effect remains unknown. British scientists, writing in the European Heart Journal, have tried to solve this problem. Here's a summary of their findings.

What was done

The subjects of this study were over 1,000 civil servants in London - we'd call them government workers in the USA. They were enrolled in the Whitehall II study, in 1985 to 1988, and followed for about 12 years. At various stages throughout the study work stress was assessed, along with behavioral factors, the metabolic syndrome, heart rate variability, and the morning rise in serum cortisol, as well as the actual occurrence of CHD events (cardiac death, non-fatal heart attack, and angina).

Work stress was self-reported using questionnaires about job strain. Strain was considered high when the job demands were high and the 'decision latitude' (i.e. the subject's control over the job) was low. Strain was also scored as high (iso-strain) when the worker was isolated (i.e. without supportive co-workers or supervisors).

The behavioral risk factors analyzed included: alcohol use, smoking, physical activity, and diet (chiefly fruit and vegetable consumption); these are known risk factors for CHD.

The metabolic syndrome was diagnosed using the components: waist more than 102 cm = 40 inches (men) or more than 88 cm = 34½ inches (women); triglycerides over 150 mg/dL; HDL below 40 mg/dL (men) or 50 mg/dL (women); blood pressure over 130/85 mmHg or on hypertensive medication; fasting glucose over 110 mg/dL.

Heart rate variability (HRV) was calculated by analyzing a time series of beat-to-beat intervals from the ECG. It's regarded as an indicator of the activity of nervous sytem regulation of circulatory function. Reduced HRV predicts sudden death in patients with a heart attack.

Saliva samples taken on waking and 30 minute after waking were used to estimate cortisol levels; the rise of cortisol immediately after awakening may indicate an increased risk of developing cardiovascular diseases via the metabolic syndrome.

What was found

Chronic work stress was clearly linked to CHD, with the strongest link seen in workers less than 50 years of age; their relative risk was 1.68 (i.e. they were 1.68 times more likely to develop CHD than non-stressed workers). Similar links between work stress and low physical activity, poor diet, and the metabolic syndrome were seen, with 32% of work stress on CHD explained by these factors.

Lower HRV and, to a lesser extent, higher morning cortisol increases were also linked with work stress. When adjustments were made to take into account various health behaviors (diet, smoking, etc), low HRV retained its link with work stress, suggesting a direct effect of stress on the nervous system.

What the findings mean

This study shows there were clear links between stress at work and coronary heart disease, with similar links seen between work stress and low physical activity, poor diet, and the features of the metabolic syndrome. Adjusting for these behavioral changes and the presence of the metabolic syndrome reduced the work stress-CHD link by about one third. This indicates that two-thirds of the effect of work stress is a direct one on the body, via nervous system (the autonomic system¹) and/or endocrine (adrenal cortisol²) effects.

Based on these findings, the authors of the study say that job stress should not be ignored, especially by people under 50. Simply adjusting lifestyle factors (diet, smoking, exercise, etc) may not be sufficient to prevent the deleterious effects of stress on the body, in particular the cardiovascular system.