Occupational heavy lifting and risk of ischemic heart disease and all-cause mortality

A positive association between occupational physical activity, cardiovascular disease and mortality has been found in several cohorts [1‐7]. However, the specific types of occupational physical activity conferring the increased risk for cardiovascular disease remain unsettled [8]. Identification of the types of occupational physical activity imposing the increased risk for cardiovascular disease is essential for designing effective preventive initiatives.

One previous retrospective case–control study has indicated that occupational heavy lifting increases the risk for acute myocardial infarction while occupational walking and leisure time physical activity decrease the risk [9]. Heavy lifting is known to impose an acute static cardiovascular strain excessively raising the blood pressure. In contrast, occupational walking and leisure time physical activities are characterized by dynamic movements of large muscle groups potentially promoting cardiovascular fitness and health.

However, no prospective studies have investigated if occupational heavy lifting is an independent risk factor for cardiovascular disease and mortality. Moreover, the role of general occupational and leisure time physical activity for the association between occupational heavy lifting and cardiovascular disease and mortality is unknown.

Therefore, we investigated if occupational heavy lifting was associated with IHD and mortality, and if general occupational and leisure time physical activity modify these associations.

During the study period 538 (8.0%) men and 215 (3.6%) women were diagnosed with IHD and 521 (7.8%) men and 260 (4.4%) women died.

Table 1, 2, and 3 show selected characteristics of the study participants stratified by occupational heavy lifting, occupational physical activity, and physical activity in leisure time, respectively. Men as well as women with occupational heavy lifting were more likely to have a low educational level, have a high occupational physical activity level, smoke heavily but less likely to drink alcohol compared to those with no occupational heavy lifting (Table 1). Furthermore, women with occupational heavy lifting were more likely to be obese (BMI > 30) compared to women with no heavy lifting. Men exposed to occupational heavy lifting were less likely to be hypertensive and stressed compared to men unexposed to heavy lifting. Occupational heavy lifting was not associated to physical activity in leisure time.

Overall, the association between occupational physical activity and selected socio-demographic and lifestyle related factors were similar to the associations for occupational heavy lifting (Table 2).

Study participants, men as well as women, with a high compared to a low level of physical activity in leisure time were more likely to have a high educational level, be never-smokers, to have a BMI below 25 kg/m², and to have a normal blood pressure (Table 3). Additionally, men with a high compared to low physical activity level in leisure time were more likely to drink less alcohol.

The associations between various forms of physical activity and IHD incidence as well as all-cause mortality are shown in Table 4. Men exposed to occupational heavy lifting had a higher risk of IHD compared to those who were unexposed (HR = 1.52, 95% CI: 1.15, 2.02). No associations between occupational heavy lifting and either IHD or all-cause mortality were found in women.

Men reporting a high compared to a low occupational physical activity level had a reduced risk of IHD even after adjusting for socio-demographic and lifestyle related factors including physical activity in leisure time and occupational heavy lifting (HR = 0.50, 95% CI: 0.37, 0.68) (Table 4). Among women, a high occupational physical activity level was not associated with risk for IHD compared to women with a low occupational physical activity level (HR = 1.55, 95% CI: 0.98, 2.44). No association between occupational physical activity and all-cause mortality was observed for either men or women. Physical activity level in leisure time was associated to the risk of IHD and all-cause mortality among men but not among women. Highly physically active men had a 23% reduced risk of IHD and a 26% reduced risk of all-cause mortality (HR = 0.73, 95% CI: 0.56, 0.95 and HR = 0.72, 95% CI: 0.54, 0.96). Additional analysis, showed that among men with low levels of both physical activity in occupation and in leisure time, the risk of IHD was significantly increased compared to high levels of both occupational and leisure-time physical activity (HR = 3.04, 95% CI: 1.77-5.23).

Table 5 shows the a combined measure of occupational heavy lifting and occupational physical activity level in relation to IHD and all-cause mortality. Compared to men with high level of occupational physical activity level and no lifting activities, those with low occupational physical activity who were exposed to occupational heavy lifting had a higher risk of IHD (HR = 2.56, 95% CI: 1.52, 4.32). In contrast, among men with a high level of occupational physical activity level exposure to occupational heavy lifting made no difference in the risk of IHD. In women, there seemed to be no interplay between occupational physical activity and occupational heavy lifting.

Physical activity in leisure time also seemed to modify the association between occupational heavy lifting and the risk of IHD among men (Table 5). Occupational heavy lifting in combination with low physical activity in leisure time was associated with increased risk of IHD compared to men with a high leisure-time physical activity level and no occupational heavy lifting (HR = 1.97, 95% CI: 1.33, 2.91). In contrast, among men who were highly physically active in leisure-time, exposure to heavy lifting did not change the risk of IHD (HR = 1.37, 95% CI: 0.81, 2.33).

Men exposed to heavy lifting in combination with both low levels of leisure-time physical activity and occupational physical activity had almost three times higher risk of IHD (HR = 2.92) compared to men with no heavy lifting in combination with high levels of occupational and leisure-time physical activity (p = 0.04). No significant associations were seen among women or for all-cause mortality (data not shown).

The main finding of this study was that occupational heavy lifting was independently associated with the risk of IHD among male workers. Moreover, the risk of IHD from occupational heavy lifting was modified by both occupational and leisure time physical activity.

Male workers reporting lifting or carrying of heavy burdens of at least 10 Kg at work more than 2 days a week had 52% higher risk for IHD compared to males workers not reporting lifting or carrying of heavy burdens at work after adjusting for demographic, social and lifestyle-related factors as well as occupational and leisure time physical activity. This finding indicates that occupational lifting of heavy burdens is an independent risk factor for IHD among males. We are not aware of previous prospective cohort studies showing an independent risk for cardiovascular disease from occupational heavy lifting. However, our finding is in accordance with a previous retrospective case–control study of Fransson and colleagues, finding a tendency for an increased risk for acute myocardial infarction from occupational heavy lifting [9]. Occupational heavy lifting is well known to impose an acute cardiovascular strain excessively raising blood pressure [11]. Therefore, it is a plausible biological causation that the cardiovascular strain from heavy lifting more than two days per week over several years increases the risk for IHD.

Moreover, the association between occupational heavy lifting and IHD was shown to be modified by both occupational and leisure time physical activity among men. Men with high occupational physical activity and not performing heavy lifting did not have an increased risk for IHD, while men with low occupational physical activity and performing heavy lifting had a more than doubled risk for IHD, referencing men with high occupational physical activity not performing occupational heavy lifting. The same tendency was found for leisure time physical activity among men. Men who had a low occupational physical activity level and performed heavy lifting had an almost doubled risk of IHD, while men who had a high occupational physical activity level and also performed heavy lifting did not have an increased risk of IHD, referencing men with high occupational physical activity not performing heavy lifting. These findings indicate that occupational heavy lifting is a particularly strong hazard for IHD among men who are not being generally physically active at work or in leisure time. The positive effect of both occupational and leisure time physical activity in this study is supported by the 50% reduced risk for IHD among men with high versus low occupational physical activity, and the 27% reduced risk among men with high versus low leisure time physical activity. The observation of a modifying effect of leisure time physical activity is in agreement with our previous observation that physical activity in leisure time and physical fitness counteracts the adverse effects of occupational physical activity (including heavy lifting) on health [6, 12]. However, these studies did not investigate occupational heavy lifting separately from occupational physical activity. Findings from the present study indicate that the type of activity (isometric vs. dynamic movements) is an important aspect that may determine the beneficial or adverse effects of occupational physical activity. The positive cardiovascular effects of occupational walking and leisure time physical activity are previously well documented [13‐15], and can be explained by the dynamic movements of large muscle groups promoting cardiovascular fitness and health. Therefore, it is biologically plausible that high occupational and leisure time physical activity offers a protective effect against the cardiovascular strain imposed by occupational heavy lifting.

Among men, no association between occupational heavy lifting and all-cause mortality was found. Because IHD only constitutes about 25% of all deaths among men in Denmark, and that the biological causation is mediated by the negative effects of occupational heavy lifting on the cardiovascular system, we consider this finding to support the independent relation between heavy lifting and IHD among men.

Although not statistically significant, the risk of IHD by heavy lifting among women showed opposite associations than among men. Most likely, the sex differences can be explained by different work tasks for males and females imposing different physiological impacts. Factors such as work load and frequency of lifted burdens may play an important role. Unfortunately, detailed information on duration of occupational lifting as well as exact weight and frequency of the lifted burdens was not available.

The results of this study indicate that occupational heavy lifting is an independent risk factor for IHD among men, while this was not shown among women. Occupational heavy lifting seems to be a particularly strong risk factor among men with low occupational and leisure time physical activity. Preventive initiatives for IHD should be targeting men with occupational heavy lifting.