To explore the relationship between shiftwork duration and metabolic risk factors on cardiovascular disease in shiftworkers.

The study subjects comprised of 226 nurses, from a hospital, and 130 male workers, from a diaper and feminine hygienic material manufacturing firm. The mean ages of the male workers and nurses were 29 and 28.5 years, respectively. The fasting blood sugar, serum cholesterol, blood pressure, height and weight, waist and hip circumferences (only in nurses), and number of step for a shift as indices of physical activity were measured. Using the Korean version of Karasek's job contents questionnaire, the job stress was assessed. Information about the number of years worked, duration of shiftwork, and past medical and behavioral histories, including smoking, were obtained by self-administrated questionnaires. Linear regression analyses were performed, to show the relationships between shiftwork duration and metabolic risk factors, using simple and multivariate models, adjusted for age, smoking, job strain and physical activity. The following criteria were defined: hypertension as a SBP>or160 or a DBP>or=90 mmHg at least once, hypercholesterolemia, as a serum total cholesterol >or=240 mg/dl, obesity as BMI (Body Mass Index) >or=25kg/m2 and central obesity as a WHR (Waist to Hip Ratio) >or=0.85; and the logistic regression analyses, according to years of shiftwork, were performed using simple and adjusted models.

The cholesterol and fasting blood sugar showed increasing trends, but without statistical significances, according to the increase in shiftwork duration of the male workers, although, the increases in the blood pressure and BMI were statistically significant. In the nurses, only the WHR showed a significant increase in relation to the shiftwork duration. In the logistic regression analyses, hypercholesterolemia and obesity showed significant increasing risks according to the number of years of shiftwork (OR=3.32 95%CI 1.27-8.72 and OR=3.21 95%CI 1.24-8.32 respectively) in the male workers, but hypertension showed no significance. In the nurses, only an increased central obesity was significant as a risk factor (OR=1.30 95%CI 1.05-1.62).

These results provide evidence of the associations between shiftwork and metabolic risk factors for cardiovascular disease, although healthy shiftworker effects might exist in our cross sectional study design.