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Original research
Temporal trends in the healthy soldier effect in a cohort of Royal Norwegian Navy servicemen followed for 67 years
  1. Leif Aage Strand1,
  2. Jan Ivar Martinsen2,
  3. Elin Anita Fadum1,
  4. Einar Kristian Borud1,3
  1. 1Norwegian Armed Forces Joint Medical Services, Institute of Military Medicine and Epidemiology, Norwegian Armed Forces, Ullensaker, Norway
  2. 2Institute of Population-based Research, Cancer Registry of Norway, Oslo, Norway
  3. 3Department of Community Medicine, UiT The Arctic University of Norway, Tromso, Norway
  1. Correspondence to Dr Leif Aage Strand, Norwegian Armed Forces Joint Medical Services, Institute of Military Medicine and Epidemiology, Norwegian Armed Forces, Ullensaker, Norway; laastrand{at}forsvaretshelseregister.no

Abstract

Objectives To investigate temporal trends in the ‘healthy soldier effect’ (HSE) among 28 300 Royal Norwegian Navy servicemen who served during 1950–2004.

Methods Standardised mortality ratios (SMRs) for all causes, diseases and external causes were calculated from national rates for the entire study period (1951–2017), and for seven successive follow-up periods after the first recorded day of Naval service, for the overall cohort and for two subgroups: land-based personnel and vessel crews. Poisson regression, expressed as rate ratios, was used to compare all-cause mortality between the subgroups.

Results In the overall cohort, SMRs for all-cause mortality increased steadily during the first six 10-year follow-up periods, from 0.52 to 0.94, which was still lower than national rates. After 60 years, the lower mortality compared with national rates was no longer statistically significant (SMR=0.93). Low non-neoplastic disease mortality contributed most to the longevity of the HSE. For neoplastic diseases, there was a mortality deficit only for the first and third 10-year follow-up periods. External-cause mortality rose to national rates after 40 years. An HSE was present among vessel crews, but their total mortality rate was 24% higher than that among land-based personnel, who also showed a longer-lasting HSE.

Conclusions The HSE eroded gradually over time but was still present at 60 years of follow-up for all-cause mortality. The effect was strongest and most long-lived for non-neoplastic disease, lasted up to 40 years for external causes, and was relatively short for cancers. Land-based personnel showed stronger and longer-lasting HSE than vessel crews.

  • epidemiology
  • public health
  • longitudinal studies
  • mortality studies
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Footnotes

  • Contributors LAS and EKB designed the study. JIM performed the statistical analyses, and quality control of data and algorithms was performed by JIM and LAS. LAS wrote the first draft of the manuscript and is the guarantor of the manuscript. The Figures were created by EAF. All authors interpreted the data and revised the manuscript carefully. All authors approved the final version of the manuscript, and LAS had the final responsibility to submit the manuscript.

  • Funding This study was funded by the Norwegian Armed Forces Joint Medical Services.

  • Competing interests None declared.

  • Patient consent for publication Not required.

  • Ethics approval The present study was approved by the Regional Committees for Medical Health Research Ethics of Southern Norway. Data used in our study are retrieved from national health registries in Norway. The registries were established by the Norwegian government and contains personal identifiable information that are not based on consent. The registries are regulated according to the Health Register Act.

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

  • Data availability statement Data are available on reasonable request. The data that support the findings of this study are available from The Norwegian Armed Forces Health Registry but restrictions apply to the availability of these data, which were used under license for the current study, and so are not publicly available. Data are however available from the authors on reasonable request and with permission of The Norwegian Armed Forces Health Registry.

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