Tuesday September 7 2010
A number of studies suggest long hours increase heart risk
“Men who work more than 45 hours a week are more than twice as likely to die from heart disease if they are unfit,” The Daily Telegraph has reported.
The news comes from a 30-year Danish study of 5,000 men, which looked at how their working hours and physical fitness related to their risk of dying of a heart attack. The research has a number of strengths, such as its unusually long length and its assessment of participants’ fitness and working hours at the start of the study, rather than estimating them during the past. Its limitations include the fact that the participants’ physical fitness and working hours were only measured once and may not have been representative of the men’s lives as a whole.
Further studies will be needed to confirm whether long working hours only affect risk of death from heart disease in the least fit individuals or if groups with higher levels of fitness are also affected. However, we already know that keeping physically fit reduces the risk of cardiovascular disease, and people should aim to stay physically active.
Where did the story come from?
The study was carried out by researchers from Denmark’s National Research Centre for the Working Environment and other Danish research centres. No sources of funding for the study were reported. The study was published in the peer-reviewed medical journal Heart.
Both The Daily Telegraph and The Guardian reported the story in a balanced way.
What kind of research was this?
This prospective cohort study looked at whether men who are less physically fit are at greater risk of death from cardiovascular causes as a result of working long hours. The researchers reported that long working hours are an established risk factor for cardiovascular disease. However, it is not known whether the long-term effect of working long hours differs depending on how physically fit a person is. The researchers say that physical fitness may be able to counteract some of the effects of long working hours.
This study used an appropriate design for investigating the relationship between fitness, working hours and risk of death. As a prospective study, it selected a group of people, assessed their risk factors (work hours and fitness) and then followed them over time to assess any future outcomes. This means that the data collected on physical fitness and work hours should be more reliable than if men or their families were asked to recall what they had done in the past.
What did the research involve?
The researchers enrolled working men aged 40 to 59 years old. They performed physical fitness tests and reported how many hours a week they worked. The men were then followed up over 30 years to determine which of them died and the causes of their deaths. The researchers then looked at whether the risk of death was increased in men with different fitness levels and longer working hours.
From 1970 to 1971, researchers enrolled men from 14 companies in Copenhagen, covering a range of industries including the railway, public road construction, the military, postal service, telephone companies, customs, national banking and the medical industry. Men who agreed to participate filled in questionnaires about themselves, detailing their working hours and physical activity at work and in their leisure time. They were also given a clinical examination, including a physical fitness test. Men who already had cardiovascular disease at the start of the study were excluded, as were men who could not complete the fitness test or who provided incomplete information. The final analyses included 4,943 men.
The researchers identified any deaths among the participants during the 30-year follow-up period that ended in 2001 using national registers. They also used these records to identify the causes of any deaths. The researchers were particularly interested in deaths from ischaemic heart disease (deaths from heart attack) as these are known to be related to a lack of physical activity.
Physical fitness was assessed based on estimates of the participants’ maximum volume of oxygen usage (VO2 max) during a standard stationary bicycling test. Based on their results in this test, men were classified into three fitness groups: the least fit (VO2 max range 15 to 26), those with intermediate fitness (VO2 max range 27 to 38) and the most fit (VO2 max range 39 to 78).
Within each fitness level, the researchers compared the risk of death in those men working more than 45 hours a week, 41-45 hours a week and less than 40 hours a week. In their analyses, they took into account other factors which could affect results (called confounders), including age, smoking, alcohol consumption, blood pressure at the start of the study, body mass index, treatment for high blood pressure or diabetes, physical demands of their work, and social class.
What were the basic results?
During the 30 years of follow-up, 2,663 out of the 4,943 men (54%) died. Of these, 587 (11.9%) died from ischaemic heart disease. The researchers then looked at how rates of death were related to length of working hours and physical fitness at the start of the study. Among the groups of men who worked different numbers of hours a week, ischaemic heart disease killed:
- 10.4% of men who worked more than 45 hours a week
- 13.0% of those who worked 41 to 45 hours a week
- 8.5% of those who worked up to 40 hours a week
Among the groups of men with different levels of physical fitness, ischaemic heart disease killed:
- 16.6% of those who were least physically fit
- 11.7% of those with intermediate fitness
- 8.4% of the most fit men
The researchers then carried out analyses with adjustments to account for potential confounders, such as smoking and social class. They found that:
- Men in the least fit group who worked more than 45 hours a week were over twice as likely to die from heart disease as those working 40 hours a week or less (hazard ratio [HR] 2.28, 95% confidence interval [CI] 1.10 to 4.73).
- Working more than 45 hours a week was not associated with an increased risk of death from heart disease in men of intermediate or high fitness.
- Men who worked 41 to 45 hours a week had no greater risk of death from heart disease than men who worked 40 hours a week or less at any of the fitness levels.
How did the researchers interpret the results?
The researchers concluded that men with low physical fitness are at increased risk of death due to ischaemic heart disease from working long hours. They say that “men working long hours should be physically fit”.
These findings suggest that long working hours may have the greatest effect on mortality risk in men who are physically unfit. There are some points to note:
- It is possible that the results were influenced by factors other than the hours men worked and their fitness levels. Although the researchers took into account some factors which could affect results, these or other unknown or unmeasured factors could still have an effect. For example, while the researchers made adjustments to account for the influence of smoking, smoking was recorded as current, previous or never, rather than by the number of cigarettes smoked. Smoking rates were also unusually high at 65-70% in some analysis groups.
- Physical fitness and working hours were only assessed at the start of the study. These measurements might not be representative of physical fitness and working hours in the men’s earlier lives or during the 30-year follow-up.
- These results were obtained in Caucasian men aged over 40 years old. Results may differ for different population groups, such as younger men, women or people from different ethnic groups.
- Some of the information was based only on the men’s reports, for example their working hours and whether they had been treated for diabetes or high blood pressure. There could be some inaccuracy in their reports.
- There were relatively small numbers of men in some groups. For example, there were only 110 men in the group who were the least fit and worked 40 hours a week or less. Only 150 men in the least fit group worked more than 45 hours a week. These small numbers mean that the analyses involving these groups may be less reliable.
Further studies are needed to prove conclusively whether long working hours only affect the risk of death from heart disease in the least fit individuals. However, we know that keeping physically fit reduces the risk of cardiovascular disease, and people should make time to keep fit whenever possible.