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Abstract
Background No previous long-term studies have examined if workers with low physical fitness have an increased risk of cardiovascular mortality due to long work hours. The aim of this study was to test this hypothesis.
Methods The study comprised 30-year follow-up of a cohort of 5249 gainfully employed men aged 40–59 years in the Copenhagen Male Study. 274 men with cardiovascular disease were excluded from the follow-up. Physical fitness (maximal oxygen consumption, Vo2max) was estimated using the Åstrand bicycle ergometer test, and number of work hours was obtained from questionnaire items; 4943 men were eligible for the incidence study.
Results 587 men (11.9%) died because of ischaemic heart disease (IHD). Cox analyses adjusted for age, blood pressure, smoking, alcohol, body mass index, diabetes, hypertension, physical work demands, and social class, showed that working more than 45 h/week was associated with an increased risk of IHD mortality in the least fit (Vo2max range 15–26; HR 2.28, 95% CI 1.10 to 4.73), but not intermediate (Vo2max range 27–38; HR 0.94, 95% CI 0.59 to 1.51) and most fit men (Vo2max range 39–78; HR 0.91, 95% CI 0.41 to 2.02) referencing men working less than 40 h/week.
Conclusions The findings indicate that men with low physical fitness are at increased risk for IHD mortality from working long hours. Men working long hours should be physically fit.
- Community cardiology
- epidemiology
- public health
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Introduction
During the last decade, the negative consequence of long work hours on cardiovascular health has received considerable attention.1–5 Several studies have shown that employees working long hours are at increased risk for cardiovascular disease6–8 including cardiovascular mortality,9 10 hypertension11 and diabetes.12
Although a working hours directive has been implemented by the European Union,13 many countries do not have such regulations for long work hours (eg, the USA). Accordingly, more research on the effects of long work hours on hard cardiovascular outcomes is requested.1 2 4 Moreover, some occupational categories have exemptions for long work hours,14 15 illustrating the need for other initiatives besides regulation of work time for reducing potentially adverse health effects of long work hours.2
The main biological explanations for impaired cardiovascular health from long work hours encompass increased sympathetic nervous activity during work, causing a prolonged elevation of heart rate and mean blood pressure.16 17 Physical fitness is considered a relevant individual modifiable factor for tolerating high work demands.18 Because high physical fitness reduces the relative physiological stress during a specific task,19 and reduces the time required for recovery,20 it may be an effective individual modifying factor for reducing the risk of cardiovascular disease attributed to long work hours, corresponding to the notion that employees with high physical fitness are better able to handle shift-work; for example they become less fatigued or irritable, and have fewer sleep troubles than workers with low fitness.21 22 In the Copenhagen Male Study, shift work has not been shown to be a risk factor for cardiovascular mortality.23 However, the importance of physical fitness for tolerating long work hours without developing severe cardiovascular disease has not been investigated.
No previous long-term studies have examined if workers with low physical fitness have an increased risk of cardiovascular mortality due to long work hours. The aim of this study was to test this hypothesis.
Material and methods
The Copenhagen Male Study cohort was established in 1970–1971. All men aged 40–59 years from 14 companies in Copenhagen, covering the railway, public road construction, military, post, telephone, customs, national bank and the medical industry were invited to participate; 5249 men, 87% of potential participants, agreed to participate.24 25
The examination consisted of a questionnaire, a short interview and a clinical examination including measurements of height, weight and blood pressure, and measurement of physical fitness following a bicycle ergometer test. Indirect measurement of physical fitness was performed with a bicycle ergometer. Thirty-five men with orthopaedic problems unable to perform the bicycle test were excluded from the study.
Information was obtained from the questionnaire about working conditions, lifestyle and general health, including history of myocardial infarction, angina pectoris and intermittent claudication. The information given in the questionnaire was clarified in the ensuing interview with a physician (FG). A number of these factors are elaborated in more detail below.
Weekly work hours
Participants reported their weekly number of work hours in categories: 1. <30, 2. 30–35, 3. 36–40, 4. 41–45 and 5. >45. The distribution of answers in these groups was 0.2%, 0.6%, 12.1%, 68.6%, and 18.6%, respectively, among men eligible for study. Due to the small number of men working less than 36 h/week, the first three groups were pooled for the analyses.
Physical fitness
Heart rate was measured during bicycle work in a steady state with the aid of a stopwatch and stethoscope. The loads used were 100, 150 and 200 W. One, two or, in a few cases, three different loads were used. The load chosen in each case was determined from the weight and age of the person or heart rate during the first minute of the test, and the determination of physical fitness (Vo2max) was accomplished with the aid of Åstrand's nomogram.26 27 The correlation between directly and indirectly measured Vo2max is high. The method used has previously been described in detail.24
Physical activity at work, general description
Physical activity at work was estimated from the following question:
‘Which description most precisely covers your pattern of physical activity at work?’
You are mainly sedentary and do not walk much around at your workplace. For example desk work, work including assembling of minor parts.
You walk around quite a bit at your workplace but do not have to carry heavy items. For example light industrial work, non-sedentary office work, inspection and the like.
Most of the time you walk, and you often have to walk up stairs and lift various items. Examples include mail delivery and construction work.
You have heavy physical work. You carry heavy burdens and carry out physically strenuous work. For example work including digging and shovelling.
In the analyses, group 1 is referred to as Low and group 2 as Medium; as only 2.4% belonged to group 4, groups 3 and 4 were pooled and are referred to as High.
Strenuous work
Amount of strenuous work was estimated from the following question:
‘Do you perform strenuous work (work resulting in sweating)?’ Answer options were: ‘often’, ‘occasionally’ and ‘seldom or never’.
Physical work activity, combined variable
In order to further discriminate between men with presence or absence of physically demanding work, an additional variable was constructed based on the two described above. With respect to physical activity, general description, groups were coded: low=1, moderate=2 and high=3. With respect to strenuous work, groups were coded: often=3, occasionally=2, and seldom or never=1. Summing up the two gave values from 2 to 6. A low combined score of 2 was defined as low physical work demands, a score of 3 or 4 was defined as moderate physical work demands and a score of 5 or 6 was defined as high physical work demands.
Physical activity in leisure time
Leisure time physical activity was estimated from the following question:
‘Which description most precisely covers your pattern of physical activity in leisure time?’
You are mainly sedentary, for example you read, watch television, go to the pictures. In general you spend most of your leisure time performing sedentary tasks.
You go for a walk, use your bicycle a little or perform activity for at least 4 h/week. For example light gardening, leisure-time building activity, table tennis and bowling.
You are an active athlete, run, play tennis or badminton for at least 3 h/week. If you frequently perform heavy gardening, you also belong to this group.
You take part in competitive sports, swim, play European football, handball or run long distances regularly i.e. several times/week.
In the analyses, group 1 is referred to as Low and group 2 as Medium; as only 0.4% belonged to group 4, groups 3 and 4 were pooled and are referred to as High.
Lifestyle factors
Smoking
Participants reported if they smoked currently, previously or had never smoked.
Alcohol
Participants reported their daily average alcohol consumption as the number of alcoholic beverages consumed per day in categories: 0, 1–2, 3–5, 6–10 and >10.
Clinical and health-related factors
Body mass index (BMI)
Based on height and weight measurements, BMI was calculated as kg/m2.
Blood pressure
Measurements of blood pressure were carried out with the subject seated and after at least 5 min rest. A 12 cm wide, 26 cm-long cuff was firmly and evenly applied to the subjects right upper arm with the lower edge of the cuff placed 2 cm antecubitally. Diastolic blood pressure was recorded at the point where the Korotkoff sounds disappeared (phase 5).
Hypertension treatment
The participants were asked if they received treatment for hypertension from their physician or elsewhere. Answer options were ‘yes’ or ‘no’.
Diabetes treatment
The participants were asked if they received treatment for diabetes from their physician or elsewhere. Answer options were ‘yes’ or ‘no’.
Social class
The men were divided into five social classes according to a system originally elaborated by Svalastoga, later adjusted by Hansen.28 29 This classification system is based on education level, and job position in terms of number of subordinates. Typical jobs in the study cohort were, in social class I: officer, civil engineer, office executive, head of department; social class II: head clerk, engineer; social class III: engine driver, train guard; social class IV: machine fitter in a telephone company; social class V: unskilled labourer, mechanic, driver.
Eligibility criteria
In addition to the 35 men unable to carry out the bicycle test, men with a history of myocardial infarction (n=74), angina pectoris (n=165) or intermittent claudication (n=105) were excluded from this prospective study. In total, this latter group comprised 274 men and nine men with missing answers leaving 4943 men for the incidence study. With respect to all variables included in the analyses, missing values ranged from 0 to 2.7%.
End points
Information on death diagnoses within the period 1970–1971 to the end of 2001 was obtained from official national registers. The ICD ischaemic heart disease mortality diagnoses encompassed ICD-8: 410–14, and (from 1994) ICD-10: I20–I25.
Statistical analyses
Basic statistical analyses, χ2 analysis (likelihood ratio), analysis of variance and regression analyses, were performed. Relative risks were estimated by exp(β), where β is the hazard coefficient for the variable of interest in a Cox's proportional hazards regression model with the maximum likelihood ratio method. Assumptions regarding the use of Cox's proportional hazards were met by inspection of the log minus log function at the covariate mean. To obtain a statistical expression for the conspicuous difference in the predictive value of number of work hours per week and IHD mortality between men with high and men with low physical fitness, a final Cox's proportional hazards regression analyses with IHD mortality as outcome was performed. In addition to the main effects of the two variables, age and lifestyle factors, alcohol use and smoking habits, a multiplicative interaction term was included between physical fitness (lowest fifth/others) and the three group variables depicting increasing number of work hours. A two-sided probability value of p<0.05 was a priori taken as significant.
Results
In the eligible study population of male employees, n=4943, who completed the bicycle ergometer test, and were without history of myocardial infarction, angina pectoris or intermittent claudication, 587 died (11.9%) from IHD during the period 1970–1971 to 2001. During the same period, 2663 (54.0%) died in total.
Table 1 shows lifestyle and other characteristics of men belonging to groups with different numbers of weekly working hours; maximum of 40 h; 41–45 h; and >45 h. Working hour categories were associated with physical work demands, leisure-time physical activity, smoking, alcohol, social class, BMI and age. No significant relationship was found between working hour categories and systolic and diastolic blood pressure, diabetes and treatment due to hypertension.
Physical fitness, physical work demands, lifestyle and other characteristics according to weekly working hours among men without history of myocardial infarction, angina pectoris or intermittent claudication
Table 2 shows lifestyle and other characteristics of men belonging to groups with different levels of physical fitness; lowest fifth: range 15–26; three intermediate fifths: range 27–38; and highest fifth: 39–78. The level of physical fitness was inversely associated with systolic and diastolic blood pressure, and with alcohol consumption, treatment due to hypertension, BMI and age. No significant relationship was found between level of fitness and social class. As previously reported, smokers surprisingly had higher levels of physical fitness.24
Lifestyle and other characteristics according to measured physical fitness, Vo2max among men without history of myocardial infarction, angina pectoris or intermittent claudication
Table 3 shows the mortality rate, incidence and risk of IHD mortality and all-cause mortality among men belonging to groups with different numbers of weekly working hours. Men working between 41 and 45 h/week had a 59% higher risk for IHD mortality (age-adjusted HR 1.59, 95% CI 1.20 to 2.11), but no significant enhanced risk for all-cause mortality (age-adjusted HR 1.07, 95% CI 0.95 to 1.20) compared to men working less than 40 h/week. The particularly high risk for IHD mortality among this subgroup is largely influenced by the over-representation of lower social class in this subgroup (shown in table 1). Men working more than 45 h/week did not have a significantly higher risk for IHD mortality (age-adjusted HR 1.28, 95% CI 0.91 to 1.78) or all-cause mortality (age-adjusted HR 0.91, 95% CI 0.79 to 1.05) compared to men working less than 40 h/week. Moreover, table 3 illustrates that men with high physical fitness had a 45% lower risk of IHD mortality (age-adjusted HR 0.55, 95% CI 0.42 to 0.73) and a 38% lower risk of all-cause mortality (age-adjusted HR 0.62, 95% CI 0.55 to 0.71) compared to men with low physical fitness.
Working hours and physical fitness as predictors of ischaemic heart disease (IHD) mortality and all-cause mortality (ACM) during 1970–1971 to end of 2001 among men without history of myocardial infarction, angina pectoris or intermittent claudication at baseline
Table 4 shows the results of Cox's proportional hazard analyses including different potentially confounding variables. Among men with low physical fitness levels, those working more than 45 h/week had a more than twofold increased risk of IHD compared to men working less than 40 h/week. In contrast, among the fittest men, who overall had the lowest risk, no association was found between increasing working hours and risk of IHD mortality. Also, an age-adjusted Cox analysis was carried out including an interaction term between weekly working hours and physical fitness level with risk of ischaemic heart disease mortality as the end point. Also, including smoking habits and alcohol use, the interaction was statistically significant, p=0.005. Among men working less than 40 h/week (n=638), only 39 (6.2%) worked less than 35 h/week. Exclusion of the 39 men working less than 35 h/week from the analyses did not materially influence the results. Specifically, the fully adjusted hazard ratio for IHD mortality among the men with low physical fitness was 1.37 (95% CI 0.70 to 2.67) among those working 41–45 h/week, and 2.10 (95% CI 1.01 to 4.37) among those working more than 45 h referencing those working 36–40 h/week. The fully adjusted hazard ratio for IHD mortality among the fittest men was 0.74 (95% CI 0.38 to 1.45) among those working 41–45 h/week, and 0.84 (95% CI 0.38 to 1.87) among those working more than 45 h referencing those working 36–40 h/week.
Number of work hours and risk of ischaemic heart disease (IHD) mortality during 1970–1971 to end of 2001 according to level of physical fitness: lowest quintile, three intermediate quintiles and highest quintile
Table 5 shows the risk of all-cause mortality during follow-up among men with low, medium and high physical fitness with different numbers of weekly working hours. No significant interaction was found between increasing working hours and risk of all-cause mortality among men with low and high physical fitness. However, among men with mediate physical fitness, those working more than 45 h/week had a significantly lower risk for all-cause mortality compared to men working less than 40 h/week. Also, with respect to all-cause mortality, an age-adjusted Cox's analysis was carried out including an interaction term between weekly working hours and physical fitness. Including smoking habits and alcohol use, the interaction was not significant, p=0.11.
Number of work hours and risk of all-cause mortality during 1970–1971 to end of 2001 according to level of physical fitness: lowest quintile, three intermediate quintiles and highest quintile
Discussion
The hypothesis that level of physical fitness modifies the association between long work hours and risk of IHD mortality was supported. Working long hours (more than 45 h/week) was a significant risk factor for IHD mortality among men with low physical fitness, but not among men with mediate or high physical fitness.
Men working between 41 and 45 h/week were shown to be at 59% higher risk of IHD mortality compared with men working less than 40 h/week. This finding strengthens the present documentation of adverse health effects from long work hours,1–3 6 10 and supports that the lack of work time directives in some countries (eg, USA) may be a public health problem.
The finding that working more than 45 h/week is associated with a more than doubled risk of IHD mortality among men with low physical fitness, and not among men with mediate or high physical fitness is a new observation. If the relationship is causal, it obviously has major implications in the prevention of cardiovascular disease.
The cardiovascular health impairment attributed to long work hours is generally considered to be caused by a prolonged elevated heart rate and blood pressure when working long compared to normal work hours,16 irrespective of the work being physically demanding or not. Moreover, long work hours lead to reduced time for recovery,30 and incomplete or high need for recovery is associated with cardiovascular disease and mortality.31 32 Physically fit people have a lower relative cardiovascular stress during a specific task19 and improved recovery from physical exertion compared to persons with low physical fitness.20 Thus, it makes physiological sense that physically fit employees are less susceptible for IHD mortality from long work hours than employees with low physical fitness.
The traditional initiative for preventing adverse health effects from long work hours is work time regulations.2 However, because many countries do not have work time directives, and that occupational categories have exemptions for long work hours, other initiatives for reducing health impairments from long work hours are requested.2 This study indicates that long work hours can be tolerated among men with medium and/or high physical fitness.
Methodological considerations
A methodological aspect of this study is that information on long work hours was based on self-assessment. Another aspect is that only Caucasian men are included in this cohort. Further research is needed to investigate whether the findings of this study also applies to women and non-Caucasian populations. In this study, long work hours are mainly considered to be working hours spent doing little physical activity as only a minority of this cohort had physically demanding work tasks. In the multivariable analyses, occupational physical activity was taken into account suggesting that the duration of work per se was a risk factor. With respect to all-cause mortality, the association between number of work hours and the outcome was quite weak. This observation supports that the adverse effects of long work hours are related to cardiovascular health in particular. Furthermore, this observation suggests that men in the least fit group did not have a low fitness due to underlying disease. The particularly high risk for IHD mortality among the men working between 41 and 45 h is largely caused by the over-representation of lower social class in this subgroup, demonstrating the importance of appropriate control for social class in the study of long work hours and cardiovascular mortality. A limitation of the study is the lack of control for serum lipids at baseline.
In conclusion, the hypothesis that men working long hours who have a low physical fitness are at an increased risk of IHD mortality, was supported. These findings suggest that men working long hours should be physically fit.
What is already known on this subject
Long working hours is a well established risk factor for cardiovascular disease.
It is unknown if workers with low physical fitness have an increased risk of cardiovascular mortality due to long work hours.
What this study adds
Men with low physical fitness working long hours (more than 45 h/week) have a doubled risk for cardiovascular mortality compared to men working less than 40 h/week.
Men with intermediate and high physical fitness are not at increased risk for cardiovascular mortality from working long hours.
Men working long hours should be physically fit.
References
Footnotes
Competing interests None.
Ethical approval When the Copenhagen Male Study (CMS) was initiated as a closed cohort study in 1970–71 no ethics committee for medical research had been established in Denmark. However, in 1985–86, when survivors from the first baseline were re-examined, the study was approved by the ethics committee for medical research in the county of Copenhagen, and all participants in the study gave informed consent to participate, as stated in many previous publications from the CMS based on analyses using the 1985–86 baseline.
Provenance and peer review Not commissioned; externally peer reviewed.