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Original article
Occupational physical activity, metabolic syndrome and risk of death from all causes and cardiovascular disease in the HUNT 2 cohort study
  1. Børge Moe1,
  2. Paul Jarle Mork1,
  3. Andreas Holtermann2,
  4. Tom Ivar Lund Nilsen1
  1. 1Department of Human Movement Science, Norwegian University of Science and Technology, Trondheim, Norway
  2. 2National Research Centre for the Working Environment, Copenhagen, Denmark
  1. Correspondence to Børge Moe, Department of Human Movement Science, Norwegian University of Science and Technology, Trondheim 7491, Norway; borge.moe{at}svt.ntnu.no

Abstract

Objectives To prospectively examine the independent and combined effect of occupational physical activity and metabolic syndrome on all-cause and cardiovascular mortality in a large population-based cohort.

Methods Data on 37 300 men and women participating in the Norwegian HUNT Study (1995–1997) were linked with the Cause of Death Registry at Statistics Norway. Cox proportional HR with 95% CI were estimated.

Results During a median follow-up of 12.4 years, a total of 1168 persons died. Of these, 278 died from cardiovascular disease. Persons with metabolic syndrome and much walking/lifting at work had a HR of 1.79 (95% CI 1.20 to 2.66) for cardiovascular death referencing persons without metabolic syndrome and much walking/lifting. Using the same reference, persons with metabolic syndrome and sedentary work had a HR of 2.74 (95% CI 1.82 to 4.12) while persons with metabolic syndrome and heavy physical work had a HR of 3.02 (95% CI 1.93 to 4.75). Associations with all-cause mortality were somewhat weaker, and were largely due to deaths from cardiovascular disease.

Conclusions The association between metabolic syndrome and cardiovascular mortality is stronger for persons with sedentary work and with physically heavy work than for persons with much walking/lifting at work.

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What this paper adds

  • Metabolic syndrome is associated with an increased risk of cardiovascular mortality.

  • Leisure-time physical activity is associated with favourable effects on cardiovascular mortality among persons with metabolic syndrome, but the effect of occupational physical activity is not known.

  • This study shows that the association between metabolic syndrome and risk of death from cardiovascular disease is stronger among persons with sedentary work and physically heavy work compared with persons with much walking/lifting at work.

  • Occupational physical activity can influence cardiovascular mortality among people with metabolic syndrome.

Introduction

Metabolic syndrome is a cluster of metabolic risk factors associated with increased risk of cardiovascular disease and mortality.1–5 In accordance with recommendations for a healthy population, people with metabolic syndrome are also advised to be physically active.6 Two recent studies, including one from the present cohort, found that leisure-time physical activity was associated with a substantially lower cardiovascular mortality among persons with metabolic syndrome.7 ,8 It is not known, however, whether occupational physical activity could have equally favourable effects, and current recommendations do not consider the possible importance of occupational physical activity.9 Studies of occupational physical activity and cardiovascular mortality have shown conflicting results. Some have reported high levels of physical activity during work to be associated with a reduced risk of cardiovascular death,10 ,11 while others have found an opposite association.12 ,13

The aim of this study was to prospectively examine the independent and combined effects of occupational physical activity and metabolic syndrome on all-cause and cardiovascular mortality in a large population-based cohort.

Methods

Study population

The HUNT Study is a large population-based health survey in Nord-Trøndelag County, Norway. Between 1995 and 1997, all inhabitants aged 20 years or older were invited to participate in the second wave of the study (HUNT 2). Among 94 194 eligible participants, a total of 65 215 (70%) accepted the invitation, completed the questionnaires and attended a clinical examination (34 786women and 30 575 men). For the purpose of this study, we selected 40 247 participants who reported to be in paid work or self-employed. Of these, a total of 2947 participants were excluded at baseline; 912 who reported prevalent cardiovascular disease (ie, angina, myocardial infarction, and/or stroke), 474 reporting to have diabetes, 692 with missing information on one or more of the variables constituting metabolic syndrome and 869 without information on occupational physical activity. After these exclusions, 37 300 participants (5672 with metabolic syndrome and 31 628 without metabolic syndrome) were available for follow-up on cause of death.

A detailed description of selection procedures, questionnaires and measurements can be found at http://www.ntnu.edu/hunt and in a report by Holmen and colleagues.14 Briefly, information was collected on a range of lifestyle and health-related factors, including medical history, occupational and leisure-time physical activity, smoking status, alcohol consumption and educational attainment. At the clinical examination, standardised anthropometric measures were obtained; height was measured to the nearest centimetre, weight to the nearest half kilogram, and waist and hip circumference to the nearest centimetre. Blood pressure was measured three times using a Dinamap 845XT (Critikon), and the mean of the second and third measures was calculated. A random blood sample (non-fasting) was drawn from all participants, and serum samples were analysed for glucose and lipid levels.

The study was approved by the regional committee for ethics in medical research, and all participants signed a written consent upon participation at the HUNT 2 Study.

Metabolic syndrome

Metabolic syndrome was defined according to the National Cholesterol Education Programme (NCEP/ATP III),15 where three or more of the following five factors had to be present: waist circumference >102 cm for men and >88 cm for women, serum triglyceride level >1.7 mmol/l, high-density lipoprotein <1.03 mmol/l in men and <1.29 mmol/l in women, blood pressure (systolic >130 mm Hg and/or diastolic >85 mm Hg). Since fasting glucose levels were not measured, a non-fasting (random) glucose level of >11.1 mmol/l replaced the fasting glucose criteria of >5.6 mmol/l. The same cut-off for non-fasting glucose has been used in previous studies,16 ,17 but is likely to be more strict since it is intended to identify undiagnosed diabetes.18

Occupational physical activity

Information on occupational and leisure-time physical activity was obtained from the questionnaire. The participants were asked ‘How would you describe your work?’, with four mutually exclusive response options: (1) mostly sedentary (eg, at a desk, on an assembly line), (2) much walking (eg, delivery work, light industrial work, teaching), (3) much walking and lifting (eg, postman, nurse, construction work) or (4) heavy physical work (eg, forestry work, heavy agricultural work, heavy construction work). To obtain a stable reference group of people who reported to walk a lot we collapsed the second and third category into one group of ‘much walking/lifting at work’.

Follow-up

Individual person-time at risk of death was calculated from the date of participation in the HUNT 2 study (1995–1997) until the date of death or until the end of follow-up 31 December 2008, whichever occurred first. The mandatory reporting of death to Cause of Death Registry in Norway constitutes the basis for the coding of underlying cause of death. Deaths were classified according to the International Classification of Disease (ICD-9 and ICD-10). Cardiovascular disease was defined by ICD-9: 390-459 and ICD-10: I00-I99.

Statistical analysis

A Cox proportional hazard model was used to estimate adjusted HRs of death from all causes, from cardiovascular disease, and from non-cardiovascular disease associated with metabolic syndrome and number of metabolic risk factors. A likelihood ratio test of interaction between metabolic syndrome and sex was conducted to assess if further analysis should be conducted sex-specific. We also used Cox regression for the main analyses of the combined effect of occupational physical activity and metabolic syndrome on risk of death from all causes and from cardiovascular disease, using participants without metabolic syndrome, and with much walking/lifting at work as reference. We tested for statistical interaction between metabolic syndrome and occupational physical activity in a likelihood ratio test, and also conducted analysis of occupational physical activity only among people with metabolic syndrome. The estimated associations were adjusted for potential confounding by age (as the time scale), sex (men, women), leisure-time physical activity (no light or hard activity, <3 h light and no hard activity, ≥3 h light and/or <1 h hard activity, any light and ≥1 h hard activity, unknown), smoking status (never, former, current, unknown), alcohol consumption (never, not the last 4 weeks, 1–3 units the last 4 weeks, more than 4 units the last 4 weeks, unknown) and education (<10 years, 10–12 years, >13 years, unknown).

Departure from the proportional hazards assumption was evaluated by Schoenfeld residuals and graphical procedures (log–log plots). All statistical tests were two-sided, and all analyses were conducted using Stata for Windows, V.11.2 (StataCorp LP, Texas, USA).

Results

Table 1 shows baseline characteristics of the study population. During a median follow-up of 12.4 years (460 032person-years), a total of 1168 persons died, and of these, 278 persons died from a cardiovascular disease. There was no evidence of departure from the proportional hazards assumption for any of the exposure variables under study.

Table 1

Baseline characteristics of the study population

People with metabolic syndrome had a higher risk of death than people without metabolic syndrome. The adjusted HR was 1.36 (95% CI 1.19 to 1.56) for death from all causes, and 2.13 (95% CI 1.66 to 2.73) for death from cardiovascular disease (table 2). The HR for death from causes other than cardiovascular disease was 1.16 (95% CI 0.99 to 1.36), indicating that the association with overall mortality is largely due to increased risk of cardiovascular disease (data not shown). In analysis stratified by sex, the HR for cardiovascular death was 1.99 (95% CI 1.51 to 2.64) among men, and 2.59 (95% CI 1.48 to 4.52) among women (data not shown). There was no statistical evidence of interaction with sex (p value, 0.34), and to maintain sufficient statistical power the remaining analyses were conducted in a pooled sample.

Table 2

HR for death from all causes and cardiovascular disease associated with metabolic syndrome and number of metabolic risk factors

There was statistical evidence of a positive dose-response association between number of metabolic risk factors and risk of all-cause and cardiovascular mortality (p trend <0.001 for both associations; table 2). Compared with people without any metabolic risk factors, the adjusted HR for death from cardiovascular disease associated with the presence of one, two, three and four risk factor were 1.40 (95% CI 0.87 to 2.26), 1.70 (95% CI 1.05 to 2.75), 2.84 (95% CI 1.75 to 4.62) and 4.01 (95% CI 2.25 to 7.16), respectively.

Table 3 shows the combined effect of metabolic syndrome and occupational physical activity on risk of death from all causes and cardiovascular disease. People without metabolic syndrome and ‘much walking/lifting’ at work were used as the reference group for all comparisons. Although there was no statistical evidence of interaction between metabolic syndrome and occupational physical activity level (p=0.51), the point estimates indicate a differential association between persons with and without metabolic syndrome. For persons without metabolic syndrome, there were no clear association between occupational physical activity and mortality. However, among people with metabolic syndrome who reported much walking/lifting, the adjusted HR associated with cardiovascular death was 1.79 (95% CI 1.20 to 2.66). For people with metabolic syndrome who reported ‘mostly sedentary work’ the HR was 2.74 (95% CI 1.82 to 4.12), and on the other hand, for those who reported ‘heavy physical work’ the HR was 3.02 (95% CI 1.93 to 4.75). The corresponding associations for death from all causes were weaker but remained statistically significant.

Table 3

The combined effect of metabolic syndrome and occupational physical activity on risk of death from all causes and cardiovascular disease

A supplementary analysis only among persons with metabolic syndrome gave a HR of 1.53 (95% CI 0.95 to 2.47) in persons with sedentary work and a HR of 1.69 (95% CI 1.02 to 2.82) in those with heavy physical work, compared with the reference group of people who walked/lifted much (data not shown). Excluding those who reported ‘much walking and lifting’ (ie, keeping only those who walked much in the reference group) gave largely similar, although less precise HRs of 1.50 (95% CI 0.85 to 2.65) and 1.53 (95% CI 0.84 to 2.80), respectively.

In the current study, persons were defined as having metabolic syndrome when having at least three out of five risk factors. However, the increased mortality found in persons with two risk factors (table 2) would inflate the risk in the reference group, and thus, underestimate the associations. To explore this, we conducted a sensitivity analysis excluding people with two risk factors, leaving people with zero and one risk factor as reference, and the associations with cardiovascular mortality became somewhat stronger than in the main analyses; metabolic syndrome was associated with a HR of 2.09 (95% CI 1.34 to 3.27) among those with much walking/lifting, and the HR was 3.30 (95% CI 2.09 to 5.20) and 3.56 (95% CI 2.16 to 5.88) for persons with mostly sedentary work and heavy physical work, respectively.

Discussion

In this large population-based cohort study, the association of metabolic syndrome with risk of all-cause and cardiovascular death was higher in persons with sedentary work or heavy physical work than in those with much walking/lifting at work. The association with all-cause mortality was largely due to death from cardiovascular disease. Occupational physical activity showed no clear association with mortality among people without metabolic syndrome. To the best of our knowledge, this is the first study to assess the effect of occupational physical activity on risk of mortality among persons with metabolic syndrome. The findings of the present study are in agreement with previous prospective studies showing that sedentary occupations are associated with an increased risk of cardiovascular mortality.10 ,11 On the other hand, our findings are also in line with studies reporting positive associations between physically heavy work and risk of all-cause and cardiovascular mortality.12 ,13

The present study highlights the effects of occupational physical activity on risk of all-cause and cardiovascular mortality among persons with and without metabolic syndrome. Occupational physical activity is often characterised by static activities like prolonged standing and heavy lifting, carrying, pushing or pulling, that could increase blood pressure and subsequent risk of atherosclerosis,13 ,19 ,20 without elevating the heart rate to levels required for improved cardiovascular fitness and health.21 By contrast, leisure-time physical activity involves dynamic contractions of large muscle groups that induce improvements in cardiovascular fitness, reduce blood pressure, and improve blood lipid profile and body composition.22–24 The current study may indicate that walking at work could induce some of the favourable effects associated with leisure-time physical activity.

Strengths of the study include the prospective study design, the large populations-based sample, the long follow-up period, the ascertainment of causes of death through the Cause of Death Registry at Statistics Norway, as well as the large number of potential confounding factors that were available. However, relatively small numbers of persons in some of the categories calls for cautious interpretations of the results. Limitations of the study include the assessment of occupational physical activity with a questionnaire at baseline, enabling subjective interpretation of the questions, and individual perception of the activity. A validation study including the occupational physical activity questions, found the questions to have good repeatability, and correlate well with other measures of physical activity in a subsample of men.25 However, because there was no follow-up information on the participants, we cannot exclude the possibility that individual changes in occupational physical activity could influence our results. As in all observational studies, the possibility of biased estimates due to confounding by unmeasured or unknown factors cannot be excluded. Unfortunately, we did not have information on fasting glucose. The cut-off used for random glucose in the present study is probably stricter than criteria based on fasting glucose. Thus, fewer people could have been classified as having metabolic syndrome, and the association between metabolic syndrome and mortality could be somewhat underestimated. This was also indicated by a somewhat stronger association in a sensitivity analysis excluding persons with two risk factors from the reference group.

In conclusion, the association between metabolic syndrome and risk of death from cardiovascular disease were stronger among persons with sedentary work and physically heavy work compared with those with much walking/lifting at work. This suggests that occupational physical activity can influence cardiovascular mortality among people with metabolic syndrome.

Acknowledgments

The HUNT Study is a collaboration between the HUNT Research Centre, Faculty of Medicine, Norwegian University of Science and Technology, Verdal, Norway, The National Institute of Public Health, the National Health Screening Service of Norway, and the Nord-Trøndelag County Council.

References

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Footnotes

  • Contributors All authors were involved in drafting the article and revising it critically, and all authors approved the final version to be submitted for publication.

  • Competing interest None.

  • Patient consent Obtained.

  • Ethics approval The regional committee for ethics in medical research.

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