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Do pre-employment influences explain the association between psychosocial factors at work and coronary heart disease? The Whitehall II study
  1. Taina Hintsa1,
  2. Martin J Shipley2,
  3. David Gimeno3,
  4. Marko Elovainio4,
  5. Tarani Chandola2,
  6. Markus Jokela1,2,
  7. Liisa Keltikangas-Järvinen1,
  8. Jussi Vahtera5,6,7,
  9. Michael G Marmot2,
  10. Mika Kivimäki2,7
  1. 1Department of Psychology, University of Helsinki, Finland
  2. 2Department of Epidemiology and Public Health, University College London, London, UK
  3. 3Division of Environmental and Occupational Health Sciences, The University of Texas Health Science Center, School of Public Health, San Antonio Regional Campus, San Antonio, Texas, USA
  4. 4National Institute for Health and Welfare, Helsinki, Finland
  5. 5Department of Public Health, University of Turku, Turku, Finland
  6. 6Turku University Hospital, Turku, Finland
  7. 7Finnish Institute of Occupational Health, Helsinki, Finland
  1. Correspondence to Taina Hintsa, University of Helsinki, PO Box 9, Helsinki, FIN-00014, Finland; taina.hintsa{at}helsinkifigi

Abstract

Objectives To examine whether the association between psychosocial factors at work and incident coronary heart disease (CHD) is explained by pre-employment factors, such as family history of CHD, education, paternal education and social class, number of siblings and height.

Methods A prospective cohort study of 6435 British men aged 35–55 years at phase 1 (1985–1988) and free from prevalent CHD at phase 2 (1989–1990) was conducted. Psychosocial factors at work were assessed at phases 1 and 2 and mean scores across the two phases were used to determine long-term exposure. Selected pre-employment factors were assessed at phase 1. Follow-up for coronary death, first non-fatal myocardial infarction or definite angina between phase 2 and 1999 was based on clinical records (250 events, follow-up 8.7 years).

Results The selected pre-employment factors were associated with risk for CHD: HRs (95% CI) were 1.33 (1.03 to 1.73) for family history of CHD, 1.18 (1.05 to 1.32) for each quartile decrease in height and 1.16 (0.99 to 1.35) for each category increase in number of siblings. Psychosocial work factors also predicted CHD: 1.72 (1.08 to 2.74) for low job control and 1.72 (1.10 to 2.67) for low organisational justice. Adjustment for pre-employment factors changed these associations by 4.1% or less.

Conclusions In this occupational cohort of British men, the association between psychosocial factors at work and CHD was largely independent of family history of CHD, education, paternal educational attainment and social class, number of siblings and height.

  • Coronary heart disease
  • job control
  • organisational justice
  • pre-employment factors
  • epidemiology
  • psychology
  • public health
  • health promotion
  • workload

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Introduction

A recent meta-analysis of observational cohort studies suggests an average 50% excess risk for coronary heart disease (CHD) among employees reporting stressful psychosocial factors at work, such as high demands, low control and low organisational justice.1 The extent to which these associations reflect causal effects arising from the workplace or are spurious due to bias and residual confounding remains a matter of controversy. One largely neglected source of bias is the fact that people are not randomly allocated to stressful jobs. For example, socioeconomic disadvantage in childhood, a risk factor for CHD, has been linked to lower socioeconomic position in adulthood2–4 and hazardous exposure to psychosocial work factors.5–13 Several other pre-employment factors are also related to increased risk of CHD and could potentially underlie the association between psychosocial factors at work and CHD. These include family history of CHD (a predictor of offsprings' CHD),11 large number of siblings (a predictor of unfavourable developmental endpoints, CHD and mortality)14 and short height (a proxy for unfavourable infancy and childhood circumstances).15

The Whitehall II study of British civil servants has been one of the leading investigations on psychosocial factors at work and CHD.8 16–20 In this secondary analysis, we examined the extent to which the previously reported associations between psychosocial factors at work and CHD are in fact explained by pre-employment factors.

Methods

Participants

The Whitehall II study is a prospective cohort study of London office workers aged 35–55 years in 20 civil service departments at study inception. The baseline cohort included 6895 men and 3413 women, the response rate being 73%.21 Of these, 6435 men, 93% of all Whitehall II study male participants, responded to questionnaires on job demands, job control and organisational justice at phase 1 (1985–1988) or phase 2 (1989–1990), and had no history of CHD at phase 2. The incidence of CHD was followed up from phase 2 to phase 5 (1999) as in previous Whitehall II studies establishing the association between psychosocial factors at work and CHD.8 18 20 The analyses were restricted to men only because there were insufficient incident CHD events during this follow-up period among women. The Whitehall II study has received ethics approval from the research ethics committee of University College London Hospitals, and all participants gave written informed consent.

Assessment of psychosocial factors at work

We measured job demands (4 items, Cronbach's α=0.67) and job control (15 items, Cronbach's α=0.84) with the Karasek Job Content Questionnaire22 and organisational justice with the same proxy measure of five items (Cronbach's α=0.72) as in all previous studies from Whitehall II.20 23 24 The organisational justice scale includes the following items: (1) ‘Do you ever get criticised unfairly?’, (2) ‘Do you get consistent information from line management (your superior)?’, (3) ‘Do you get sufficient information from line management (your superior)?’, (4) ‘How often is your superior willing to listen to your problems?’ and (5) ‘Do you ever get praised for your work?’. Participants rated their response to each of these items on a 4-point scale (1 indicates never, 2 seldom, 3 sometimes, and 4 often). For each scale, we calculated mean score across phases 1 and 2 to assess long-term exposure.

Assessment of incident coronary heart disease

The incidence of CHD was defined as a CHD death, a first non-fatal myocardial infarction (MI) or definite angina. Coronary deaths were defined by International Classification of Diseases, Ninth Revision (ICD-9) codes from 410 to 414. New cases of non-fatal MI were ascertained both by a questionnaire on chest pain25 and the physician's diagnosis of heart attack. Confirmation of MI was obtained according to the MONICA criteria.26 Assessment of angina was based on either the participant's reports with corroboration in medical records or abnormalities on a resting electrocardiogram, an exercise electrocardiogram or a coronary angiogram.

Assessment of pre-employment factors

The participants were asked whether either of their parents or both had experienced a stroke, a heart attack or angina. Family history of CHD was considered positive if either of the parents had suffered from any of these outcomes and negative otherwise. Father's education was defined as the age when he left full time education. Father's social class was coded according to the Registrar General's classification based on the question, ‘What is/was your father's main job?’ and additional questions about training, employment status and supervisory responsibility.27 A three level variable for father's social class was formed by combining managerial and professional occupations into a category of high social class, clerical and skilled manual occupations into a category of intermediate social class, and semi-skilled and unskilled manual occupations into a category of low social class. The number of siblings was divided into five categories (0, 1–2, 3–4, 5–6, 7+).14 Height was clinically measured in centimetres following standard guidelines, and expressed in quartiles (<172.9, 173.0–175.9, 176.0–180.9, ≥181.0 cm).

Data analysis

We used the maximum number of participants in all analyses, the only exception being the testing of the contribution of pre-employment factors to the association between psychosocial factors at work and CHD. To retain comparability between models, this was based on the same cohort of 3412 men (53% of the eligible participants) with no missing data in any variables included in the models. These men did not differ from the 3023 excluded men in terms of age (p=0.22), education (p=0.10), job demands (p=0.98), organisational justice (p=0.90) or incidence of CHD (p=0.34). The differences in employment grade (administrative grade 39.8% vs 37.6%, p<0.001) and job control (69.8 vs 67.9, p<0.001) between these groups, although small in absolute terms, reached conventional statistical significance due large sample size. Concerning the separate pre-employment factors, the included participants did not differ from those excluded as regards family history of CHD (p=0.35) and differences in father's education (included=3412 vs excluded=972: 1.3 vs 1.4, p=0.009), number of siblings (included=3412 vs excluded=1930: 1.6 vs 1.7, p=0.26) and height (included=3412 vs excluded=2997: 176.7 vs 176.2, p=0.001) were also small or non-existing.

The associations between pre-employment factors and psychosocial work factors (job demands, job control and organisational justice) were examined by calculating mean scores of psychosocial factors at work for each category of pre-employment factor. For further analyses, participants' scores for each scale were divided into three groups: the lowest third representing low level, the middle third intermediate level, and the highest third high level of job demands, job control and organisational justice. The associations of pre-employment and psychosocial work factors with incident CHD were computed by using Cox proportional-hazard regression analysis. Hazard ratios (HR) and their 95% CI are reported. The time-dependent interaction terms between each predictor and the logarithm (follow-up period) were all non-significant. Thus, the proportional hazards assumption was justified. The contribution of pre-employment factors to the association between psychosocial factors at work and incident CHD was determined by comparing models with and without these variables as covariates. All the analyses were performed using SPSS 14.0.

Results

Table 1 presents the descriptive statistics for the participants. As shown in table 2, family history of CHD was associated with high job demands and high job control. Lower educational level, father's low education, father's low social class, greater number of siblings and short height were related to low job control. Lower educational level, father's low social class, greater number of siblings and short height were related to low job demands. Lower educational level was related to low organisational justice.

Table 1

Descriptive statistics for the participants

Table 2

Associations between pre-employment factors and psychosocial factors at work

Several pre-employment factors predicted the development of CHD (table 3). The HR for incident CHD was 1.33 (95% CI 1.03 to 1.73) for individuals with a family history of CHD compared to those with no such history, 1.16 (95% CI 0.99 to 1.35) for each category increase in the number of siblings and 1.18 (95% CI 1.05 to 1.32) for each quartile decrease in height. Years of participant's education, years of education and social class of the father were not associated with incident CHD.

Table 3

Age-, grade- and ethnicity-adjusted associations between pre-employment factors and coronary heart disease

For the analyses testing the contribution of pre-employment factors to the association between psychosocial factors at work and CHD, 3412 men (53% of the eligible participants) with no missing data in any of the study variables were included. The extent to which pre-employment factors explained the association between psychosocial work factors and CHD is shown in table 4. Job demands were not related to incident CHD in this sample, so the results are shown only for job control and organisational justice. The baseline model in the third column shows the age-, grade- and ethnicity-adjusted association between psychosocial factors at work and incidence CHD. In subsequent models (columns 4–10), change in this association is shown after adjustments for each pre-employment factor in addition to age, grade and ethnicity. The last column shows the fully adjusted results. The age-, ethnicity- and employment grade-adjusted hazard ratios for incident CHD were 1.72 (95% CI 1.08 to 2.74) for those with low job control, and 1.72 (95% CI 1.10 to 2.67) for those with low organisational justice at work. Adjustment for the separate pre-employment factors attenuated these associations by less than 0.9%. Adjustment for all pre-employment factors simultaneously increased these hazard ratios by 4.1% and 0.9%, respectively, and left the associations statistically significant.

Table 4

Age-, grade- and ethnicity-adjusted associations of job control and organisational justice with coronary heart disease with before and after adjustments for pre-employment factors (HRs (and 95% CI) of Cox regression models)

Discussion

Among men participating in the Whitehall II study, the association between psychosocial factors at work and the incidence of CHD was not explained by pre-employment factors such as family history of CHD, education, father's education and social class, number of siblings and height. Those who reported low job control and low organisational justice during a period of 3 years had approximately a 1.7-fold increased risk for incident CHD. Adjustment for pre-employment factors changed these associations by 4.1% and 0.9%, respectively.

To our knowledge, this is the first large-scale study to examine a wide range of pre-employment exposures in relation to psychosocial factors at work and CHD. Our findings are in agreement with a smaller scale Finnish study of industrial employees that reported that father's occupation and height had a modest effect on the association between psychosocial work stress and cardiovascular mortality.28 However, a register study of Swedish men aged 40–53 years found that increased risk of CHD among employees with low job control was reduced substantially after controlling for pre-employment risk factors.9 Methodological differences between the studies may have contributed to these contradictory findings. In the Whitehall II study and Finnish studies, psychosocial factors at work were assessed individually by a questionnaire, whereas the Swedish study imputed scores based on occupational title. As such scores strongly reflect socioeconomic position and fail to capture any variation in psychosocial work factors between employees who belong to the same occupational group, the role of socially patterned pre-employment factors might have been overestimated.

A modest contribution of pre-employment factors to the association between psychosocial factors at work may result from imprecise measurement of pre-employment factors. Although these factors were measured retrospectively in the present study, we believe measurement imprecision is an unlikely explanation for our findings. First, imprecision would have affected all associations, but in this study pre-employment factors were associated with CHD, thus replicating findings from previous studies.11 14 15 Second, the measurement of height was precise but still adjustment for height had only little effect on the association between psychosocial factors at work and CHD. Third, our findings are consistent with previous evidence suggesting only a modest contribution of prospectively-assessed pre-employment factors to the association between psychosocial factors at work and carotid intima-media thickness, a valid indicator of atherosclerosis and pre-clinical CHD.11 29 We cannot rule out the possibility of selection bias in our results as only 53% of the participants had complete data in all pre-employment measurements. However, the differences in pre-employment factors between the included participants and the excluded were relatively small in absolute terms and thus the likelihood of a major bias due to selective sample retention seems small, although such bias cannot be fully ruled out given the relatively high proportion of participants excluded from the analysis.

In conclusion, data from the Whitehall II study provide no evidence for the hypothesis that the association between psychosocial work factors and CHD would be largely explained by influences of common pre-employment factors, such as family history of CHD, education, paternal education and social class, number of siblings and height. Further research is needed to examine whether this association is causal. Our findings should motivate the development of systematic intervention strategies for large-scale intervention studies to test whether giving employees a stronger say in decisions about their work and treating them in a righteous manner might reduce CHD.

What this paper adds

  • The contribution of pre-employment exposures to the association between psychosocial factors at work and coronary heart disease (CHD) is unclear; the few previous studies available on this topic reported contradictory findings.

  • To our knowledge, this is the first large-scale study to examine a wide range of pre-employment exposures in relation to psychosocial factors at work and CHD.

  • The association between psychosocial factors at work and CHD was largely independent of family history of CHD, education, paternal educational attainment and social class, number of siblings and height.

Acknowledgments

We thank all participating civil service departments and their welfare personnel, and establishment officers; the Occupational Health and Safety Agency; the Council of Civil Service Unions; all participating civil servants in the Whitehall II study; and all members of the Whitehall II study team. The Whitehall II Study team comprises research scientists, statisticians, study coordinators, nurses, data managers, administrative assistants and data entry staff, who make the study possible.

References

Footnotes

  • Funding TH was supported by the Finnish Cultural Foundation, JV and MK by the Academy of Finland (projects 117604, 124332, 124327 and 129262), LKJ by the Academy of Finland (Work Consortium project 124399) and MJS by the British Heart Foundation. MGM is supported by an MRC research professorship. The Whitehall II study has been supported by grants from the British Medical Research Council (MRC), the British Heart Foundation, the British Health and Safety Executive, the British Department of Health, the National Heart, Lung, and Blood Institute (grant HL36310), the National Institute on Aging (grant AG13196), the Agency for Health Care Policy and Research (grant S06516) and the John D. and Catherine T. MacArthur Foundation Research Networks on Successful Midlife Development and Socioeconomic Status and Health.

  • Competing interests None.

  • Ethics approval This study was conducted with the approval of the University College London Hospitals.

  • Provenance and peer review Not commissioned; externally peer reviewed.

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