1. A stronger association between working long hours and risk of developing type 2 diabetes mellitus (T2DM) was found in low socioeconomic status (SES) groups as compared to high SES groups.
2. An association between long working hours and T2DM was found according to the grade of SES in the intermediate SES group.
Evidence Rating Level: 1 (Excellent)
Study Rundown: More than 285 million people worldwide suffer from type 2 diabetes mellitus (T2DM), a disease characterized by inability to regulate blood glucose due to insulin insufficiency or resistance. Working long hours was found to be one of the contributing factors for developing T2DM, but studies demonstrating a direct association between working long hours and the incidence of T2DM have been scarce. This meta-analysis examined associations between working hours and T2DM among populations of difference socioeconomic status (SES).
A stronger association was found between working long hours and risk of developing T2DM in low SES groups than in high SES groups. Among individuals of low SES groups, those worked 55 hours or more per week had almost 30% increase in risk of developing T2DM than those who worked 35-40 hours per week. This association was not observed in individuals of high SES groups. The study adjusted for confounding variables such as sex, age, BMI, and lifestyle factors. One limitation was that the observational nature of data precluded establishment of causal links. Additionally, the use of occupational titles (i.e., managers vs manual workers) to classify SES may fail to correlate with income levels.
This study was funded by Medical Research Council, European Union New and Emerging Risks in Occupational Safety and Health research programme, Finnish Work Environment Fund, Swedish Research Council for Working Life and Social Research, German Social Accident Insurance, Danish National Research Centre for the Working Environment, Academy of Finland, Ministry of Social Affairs and Employment (Netherlands), Economic and Social Research Council, US National Institutes of Health, and British Heart Foundation.
In-Depth [meta-analysis]: This meta-analysis was performed by systematically searching Embase and PubMed and by analyzing individual-level data from unpublished studies. Overall, 23 cohort studies met inclusion criteria. This study included 4,963 incident cases of type 2 diabetes mellitus (T2DM) in 222,120 men and women over 1.7 million person-years of follow-up. Long working hours were defined as 55 hours or more per week. The reference category was 35-40 hours per week. Socioeconomic status (SES) was categorized by occupational titles into three groups – low (e.g., manual workers), intermediate (e.g., skilled non-manual or clerical), and high (e.g., directors or managers). Incident T2DM was established through records from health registers, blood testing, and self-reports.
Long working hours and T2DM were strongly associated in the low SES group (risk ratio [RR] 1.29, 95% confidence interval [CI] 1.06-1.57, difference in incidence 13 cases per 10,000 person-years). This association was not observed in high SES groups (RR 1.00, 95% CI 0.80-1.25, difference in incidence 0 per 10,000 person-years). An association between long working hours and T2DM was found according to the grade of SES in the intermediate SES group (RR 1.13, 95% CI 0.88-1.44, difference in incidence 4 per 10,000 person-years). Failure to consider this SES-specific pattern provides an explanation for discrepancies in previous published studies.
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