The contribution of health behaviours to occupational class inequalities in cardiovascular disease: a longitudinal study of Finnish municipal employees

This longitudinal study of Finnish municipal employees found that while health behaviors such as smoking, diet, and physical activity explain approximately 40% of the excess cardiovascular disease risk associated with lower occupational class, the majority of this inequality remains unexplained by these factors, pointing to the significant role of broader social determinants.

The Gist

Imagine your health as a garden. In this garden, some people have rich, fertile soil (high occupational class), while others have rocky, nutrient-poor soil (low occupational class). Naturally, you'd expect the plants in the rocky soil to struggle more. This study looked at why that happens, specifically focusing on Cardiovascular Disease (CVD)—which is like a storm that can damage the heart and blood vessels.

The researchers wanted to know: Is the damage caused because the soil is bad, or because the gardeners in the rocky soil are making different choices?

Here is the breakdown of their findings using simple analogies:

1. The Setup: The "Gardeners"

The study followed a group of city workers in Helsinki for over 20 years. They split them into two groups:

• The High-Class Gardeners: Those with higher-paying, more secure jobs.
• The Low-Class Gardeners: Those with lower-paying, more stressful jobs.

They tracked who got hit by the "heart storm" (CVD events like heart attacks or needing long-term sick leave).

2. The Suspects: The "Bad Habits"

The researchers looked at four specific habits that could damage the garden:

• Smoking (like pouring poison on the roots).
• Unhealthy Diet (like feeding the plants junk food instead of nutrients).
• Not Moving Enough (like letting the soil get compacted and hard).
• Drinking Too Much Alcohol (like flooding the garden).

3. The Findings: What the Data Showed

• The Gap: The "Low-Class Gardeners" were indeed hit by the heart storm more often (50% of them) compared to the "High-Class Gardeners" (46%).
• The Habits: The Low-Class group was more likely to smoke, eat poorly, and not exercise. However, surprisingly, they weren't necessarily drinking more alcohol than the High-Class group.
• The "Missing Link" (Mediation): The researchers tried to calculate how much of the extra risk was caused just by these bad habits.
  • The Result: Bad habits explained about 40% of the difference.
  • The Analogy: Imagine the "Heart Storm" is a 100% flood. About 40% of that flood was caused by the gardeners' bad choices (smoking, diet, etc.). But 60% of the flood happened for other reasons that had nothing to do with their daily habits.

4. The Twist: Who is Most Vulnerable?

The study found something interesting about how these habits affect different people:

• Moving is Key: Not exercising was the single biggest danger for everyone. It was like leaving the garden gate wide open to a hurricane.
• Diet is Tricky: Surprisingly, eating poorly was actually more dangerous for the High-Class Gardeners than the Low-Class ones.
  • Why? Maybe because the High-Class group had other safety nets (like better healthcare or less stress) that protected them from other things, so when they ate badly, it stood out as the main problem. The Low-Class group was already under so much other pressure (financial stress, poor housing, etc.) that their diet wasn't the only thing hurting them.

The Big Takeaway

Think of the "Occupational Class" as the foundation of a house.

• The study found that while the people living in the "rocky foundation" houses were making more mistakes (smoking, not exercising), those mistakes only explained a little less than half of why their houses were falling down.
• The other 60% of the damage was caused by the foundation itself being shaky. This means things like stress, money, job security, and the environment play a massive role.

In short: You can't fix the health gap just by telling people to "eat better and exercise more." While those habits matter, the real solution requires fixing the "soil" (the social and economic conditions) so that everyone has a fair chance to grow a healthy garden, regardless of their job title.

Technical Summary

Technical Summary: The Contribution of Health Behaviours to Occupational Class Inequalities in Cardiovascular Disease

1. Problem Statement

Cardiovascular diseases (CVD) exhibit a well-documented socioeconomic gradient, with significantly higher prevalence and incidence among individuals in lower occupational classes compared to those in higher classes. While this disparity is established, the specific mechanisms driving it remain partially understood. A critical gap in existing literature is the quantification of the mediating role of health behaviours (such as diet, physical activity, smoking, and alcohol consumption) in explaining these inequalities. Furthermore, it is unclear whether the impact of these behaviours on CVD risk varies across different occupational strata. This study seeks to address these gaps by quantifying the extent to which health behaviours mediate the association between occupational class and CVD, evaluating their relative contributions, and assessing potential effect modification by occupational class.

2. Methodology

The study employed a longitudinal cohort design utilizing data from the Helsinki Health Study.

• Population: Municipal employees from Helsinki, Finland, aged 40–60 at baseline (2000–2002). The baseline response rate was 67%.
• Follow-up: Participants were tracked from baseline until 2022.
• Outcome Measurement: CVD events were identified through comprehensive national registers, encompassing hospitalizations, long-term sickness absence, disability pensions, and mortality.
• Exposure & Mediators:
  • Exposure: Occupational class (categorized into high and low).
  • Mediators: Four specific health behaviours were analyzed: excessive alcohol consumption, smoking, unhealthy diet, and insufficient physical activity.
• Statistical Analysis:
  • Mediation Analysis: The study utilized counterfactual mediation analysis within an additive survival regression framework. This advanced statistical approach allowed for the decomposition of the total effect of occupational class on CVD into direct and indirect (mediated) effects, specifically quantifying the proportion of excess risk explained by health behaviours.
  • Effect Modification: Cox proportional hazards regression was used to assess whether the association between specific health behaviours and CVD risk differed significantly between high and low occupational classes.

3. Key Contributions

• Quantification of Mediation: The study provides a precise statistical estimate of how much of the socioeconomic inequality in CVD is attributable to modifiable lifestyle factors versus other unmeasured social determinants.
• Differentiation of Behaviours: It distinguishes the relative impact of four distinct health behaviours, identifying which specific factors drive the inequality most significantly.
• Assessment of Heterogeneity: By testing for occupational class differences in the effects of health behaviours, the study investigates whether the same risk factor (e.g., poor diet) carries different weights for CVD depending on one's socioeconomic status.

4. Results

• Incidence Rates: Over the follow-up period, 50% of participants in the low occupational class experienced a CVD event, compared to 46% in the high occupational class.
• Prevalence of Behaviours: All unhealthy behaviours, with the exception of heavy alcohol use, were more prevalent in the low occupational class.
• Mediation Effect: Health behaviours collectively explained approximately 40% of the excess risk of CVD associated with moving from a high to a low occupational class. This implies that 60% of the inequality is driven by factors other than the four measured health behaviours.
• Predictive Strength: Insufficient physical activity emerged as the strongest predictor of CVD across the cohort (Hazard Ratio [HR] 1.44, 95% CI 1.35–1.54).
• Effect Modification: A significant interaction was found regarding diet; an unhealthy diet was more strongly associated with CVD risk in the high occupational class compared to the low occupational class.

5. Significance and Conclusion

The study concludes that while health behaviours are significant contributors to occupational class inequalities in CVD, they account for less than half of the disparity. The finding that 40% of the inequality is mediated by lifestyle factors suggests that interventions targeting smoking, diet, and physical activity could substantially reduce CVD gaps. However, the fact that 60% of the inequality remains unexplained underscores the critical importance of broader social determinants of health (e.g., working conditions, psychosocial stress, access to healthcare, and environmental factors) that operate independently of individual lifestyle choices.

The observation that an unhealthy diet poses a higher relative risk for the high occupational class is a novel and counter-intuitive finding, suggesting that the pathophysiological impact of diet may differ by socioeconomic context or that other protective factors in the low class mitigate dietary risks. Overall, the research advocates for a dual approach: promoting individual health behaviour changes while simultaneously addressing the structural social determinants that perpetuate health inequities.