Past Shift Work and Incident Coronary Heart Disease in Retired Workers

A Prospective Cohort Study

Wending Li; Kuai Yu; Ningning Jia; Xuedan Xu; Yu Yuan; Rong Peng; Rundong Niu; Xiaomin You; Handong Yang; Gaokun Qiu; Huan Guo; Meian He; Xiaomin Zhang; An Pan; Lap Ah. Tse; Tangchun Wu

Disclosures

Am J Epidemiol. 2021;190(9):1821-1829. 

In This Article

Abstract and Introduction

Abstract

Present shift work has been associated with coronary heart disease (CHD) among employed workers, but it remains unclear whether shift work performed in the past is still associated with CHD in retired workers. We recruited 21,802 retired workers in Shiyan, China, in 2008–2010 and 2013 and followed them for CHD events occurring up to December 31, 2018. Retired workers with longer durations of past shift work (rounded to 0.25 years) had higher CHD risks (for those with ≤5.00, 5.25–10.00, 10.50–20.00, and >20.00 years of past shift work, hazard ratios were 1.05 (95% confidence interval (CI): 0.94, 1.16), 1.08 (95% CI: 0.94, 1.25), 1.23 (95% CI: 1.07, 1.42), and 1.28 (95% CI: 1.08, 1.51), respectively). The association was substantially higher among service or sales workers than among manufacturing or manual-labor workers (for every 5-year increase in past shift work, hazard ratio = 1.11 (95% CI: 1.05, 1.16) vs. hazard ratio = 1.02 (95% CI: 0.98, 1.06)). Moreover, the risk was lower among those who were physically active than among their inactive counterparts (P for interaction = 0.019). Longer duration of past shift work was associated with higher risk of incident CHD among these retired workers, especially those from the service or sales sectors. Physical exercise might be beneficial in reducing the excess risk.

Introduction

The rapid industrialization of developing countries has witnessed an increasing demand for 24-hour availability and its accompanying shift-work schedules in the labor force, especially in the manufacturing and service sectors.[1] This has raised new public health concerns, since an increasing number of prospective studies have suggested that shift work is associated with a higher risk of coronary heart disease (CHD),[2,3] a leading cause of morbidity and mortality worldwide.[4] However, these studies have focused solely on present shift work; whether the association persists among retired workers who have worked shifts in the past remains largely unclear. In the Nurses' Health Study (NHS), a prospective study of 116,430 US nurses, Vetter et al.[5] found that past shift work was associated with an elevated risk of CHD, even among those who had not performed shift work for 12–24 years, indicating that the excess risk could persist into later life after cessation. However, this finding was confined to females of working age, and further evidence in other populations, particularly men and older age groups, is still needed.

On the other hand, although an association between present shift work and CHD was consistently reported in 2 meta-analyses,[2,3] the authors of both noted substantial heterogeneity across studies. In our review of the literature, significant associations between shift work and CHD have been consistently reported among nurses in 2 prospective studies (hazard ratio (HR) = 1.27 (95% confidence interval (CI): 1.13, 1.42) and HR = 1.71 (95% CI: 1.09, 2.69))[5,6] but not among workers from other occupations. For example, although significant associations between shift work and incident CHD have been identified among pulp and paper manufacturing workers in Sweden (relative risks were 1.24 and 2.80, respectively)[7,8] and among fertilizer workers in the Middle East (relative risk = 1.65),[9] other prospective studies have reported nonsignificant results among manufacturing workers, with relative risks ranging from 0.77 to 1.04.[10–15] This inconsistent evidence among manufacturing workers could be due to differences in study designs or populations or to differing definitions and exposure levels of shift work. Large prospective studies covering a diversity of occupations with consistent assessment of shift work would help elucidate how the association varies across different job categories.

Therefore, in the present study, we prospectively examined the association between duration of past shift work and risk of incident CHD in a retired population. We also explored potential effect modification by job category and important CHD risk factors.

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