Background: The incidence of obesity and other features of the metabolic syndrome is increased in shift workers. This may be due to a misalignment between the internal circadian rhythm and the behavioral rhythm. The stress hormone cortisol could play a role in this phenomenon because it is secreted in a circadian rhythm, and long-term elevated cortisol leads to components of the metabolic syndrome. We compared cortisol levels in scalp hair of shift and day workers to study changes in long-term cortisol due to shift work.
Methods: Hair samples were collected from 33 shift workers and 89 day workers. Cortisol was extracted from the hair samples with methanol, and cortisol levels were measured using ELISA. Height and weight were measured, and body mass index (BMI) was calculated.
Results: Shift workers had higher hair cortisol levels than day workers: 47.32 pg/mg hair [95% confidence interval (CI) = 38.37-58.21] vs. 29.72 pg/mg hair (95% CI = 26.18-33.73) (P < 0.001). When divided in age groups based on the median age, elevated cortisol levels were present only in younger shift workers: 48.53 pg/mg hair (95% CI = 36.56-64.29) vs. 26.42 pg/mg hair (95% CI = 22.91-30.55) (P < 0.001). BMI was increased in younger shift workers as well: 27.2 (95% CI = 25.5-28.8) vs. 23.7 (95% CI = 22.8-24.7) in young day workers (P = 0.001). Hair cortisol and BMI were positively correlated (β = 0.262; P = 0.005).
Conclusion: Shift work at a young adult age is associated with elevated long-term cortisol levels and increased BMI. Elevated cortisol levels and BMI may contribute to the increased cardiovascular risk found in shift workers.