The purpose of this study was to examine the relationship between shift work and BMI among nurses in Korea. The prevalence of overweight/obesity and obesity significantly increased with increasing shift work duration. Although there was no association between current shift work and BMI, we found that the subjects in the highest tertile of shift work duration were more likely to be overweight/obese after adjustment for potential confounders.
In this sample of 9,989 female nurses, 66.5% were of normal weight, suggesting that overweight or obesity is not common in the population of nurses in Korea. In addition, 18.6% of the participants were either overweight (11.2%) or obese (7.4%), which is lower than the age-standardized obesity prevalence of 25.7% in the general Korean population, as reported by the Korean National Health and Nutrition Examination Survey (KNHANES) [18]. These results might be explained either by a healthy-worker effect among nurses or the large proportion of participants who were 20–29 years of age (44.4%) and 30–39 years of age (33.8%). The BMI in non-shift workers was higher than that in current shift workers, which might have been influenced by the relatively older age of non-shift workers, as well as other potential confounding factors. When adjusted for potential confounders in multivariate logistic regression analysis, no significant association was found between current shift work and BMI. Because non-shift workers had not provided any information about past shift work experience, they were excluded from the main analyses.
In a further analysis, we also found that most of the subjects in the lowest tertile of shift work duration had never married (93.0%), whereas those in the highest tertile were more likely to be married (61.2%). As shown in Additional files 3 and 4, married women who were current shift workers had a higher risk of overweight/obesity and obesity compared to never-married women, which is consistent with an earlier study [16]. In the present study, the finding that nurses with lower levels of education and income had an increased risk of overweight/obesity could be explained by a decreased interest in health problems in this group.
Several studies report finding associations between shift work and weight gain, overweight or obesity [14–16, 19]. The female workers in electronic device factories in Malaysia who worked night shifts had significantly elevated ORs for being overweight, even after adjustments [16]. A study including 377 shift workers and non-shift workers found a positive relationship between duration of shift work and BMI [14], and a cross-sectional survey found that longer exposure to shift work was a highly significant predictor of increased BMI [15]. Our results were consistent with those obtained in earlier studies; multiple logistic regression revealed that Tertile 3 of shift work duration had a significantly increased risk for overweight/obesity (OR, 1.24; 95% CI, 1.07-1.43) and obesity (OR, 1.32; 95% CI, 1.08-1.62) compared with non-shift work group. When the shortest duration of shift work was used as a reference, the nurses with the longest duration of shift work had a 1.63 times greater risk of overweight/obesity. Similarly, a cross-sectional study from Australia found that nurses working shifts were 1.15 times more likely to be overweight or obese than day workers, and night-only shift work was associated with obesity only [20, 21]. A study in 85 hospital shift workers reported that mean weight gain was greater in those on evening and night shifts (4.3 kg) than in those on day shifts (0.9 kg) [13]. Although our findings showed no association between shift work duration and obesity in the multivariate model, statistically significant correlations was observed after adjustment for age, current smoking status, regular drinking habit, breakfast skipping and regular exercise. This might be explained by the relatively low proportion of obese nurses among current shift workers (5.8% total; 3.1% in Tertile 1, 4.9% in Tertile 2, and 9.6% in Tertile 3). Therefore, we combined overweight and obesity into one category as dependent variables. For further analysis, we selected the participants over the age of 30 years. Tertile 2 and Tertile 3 showed the significant association with overweight/obesity after adjustment for potential confounders (OR, 1.45; 95% CI, 1.07-1.98 and OR, 1.45; 95% CI, 1.04-2.02). There was a significant association between obesity and Tertile 3 in the unadjusted analysis (OR, 1.59; 95% CI, 1.07-2.36); however, it was no longer significant in the adjusted models.
It is well known that lifestyle factors are potential mediators linking shift work to BMI [3]. In a logistic regression model including confounding factors as covariates, the factors that remained significantly associated with overweight/obesity were age, current smoking status, marital status, family income, education, breakfast skipping and sleep problems. It is known that inadequate sleep and poor quality sleep are associated with obesity [7]. Consistent with those findings, our results showed that sleep problems were associated with overweight/obesity [see Additional file 3]. Approximately 27% of shift workers responded that they had difficulty with sleeping or excessive sleep for 7–13 days or every day during the past 2 weeks. Interestingly, sleep problems were reported most frequently in Tertile 1 (32.9%), followed by Tertile 2 (30.9%) and Tertile 3 (17.8%), showing a higher prevalence among subjects who started shift work recently. One previous study reported that nurses’ diet and exercise habits were negatively influenced by sleep disturbance by working at night [22], and that they also faced changes in eating habits and food selection. Previous studies found that midnight shift workers had the highest total energy intake and usually consumed more calories in the evening [4]. In another study, 40% of nurses who recognized that they were overweight or obese responded that they ate a healthy diet and exercise regularly, but could not lose excess weight [23]. In the present study, 43.1% of shift workers skipped breakfast. The percentage was highest in Tertile 2 (47.1%), followed by Tertile 1 (43.2%), and Tertile 3 (38.4%). In addition, there was an inverse relationship with breakfast skipping and the risk of overweight and obesity. In the current study, we did not investigate diet quality, types, or amount of food eaten and energy intake; this should be further examined. In general, physical activity is inversely associated with obesity; however, our results indicated that regular exercise had a positive relationship with overweight/obesity. The prevalence of shift workers who exercised regularly was 40.3% in those with BMI <23 kg/m2 and 46.2% in those with BMI ≥23 kg/m2. This might imply that nurses who recognized that they were overweight or obese were more likely to exercise regularly. Because this was a cross-sectional study, additional investigations are needed for better understanding of the relationship between shift work and physical activity. Interestingly, the participants with the longest duration of shift work had a significantly increased risk of hyperlipidemia after adjustment for age (OR, 1.68; 95% CI, 1.11-2.55).
This study included a large, representative sample of nurses in Korea and it confirmed an association between increasing duration of shift work and overweight/obesity after considering obesity-related lifestyle factors as confounders. However, there are some study limitations. First, as the study was cross-sectional in design, it is difficult to conclude that the relationship between shift work and BMI was causal. Further longitudinal study will be needed to achieve higher levels of evidence. Second, our data were collected from self-reported questionnaires. Additional metabolic markers should be included in future research for a better understanding of the relationship between shift work and body weight. Third, nurses not currently performing shift work might be misclassified because they had not provided any information about past shift work experience. Finally, shift work exposures at multiple aspects, such as frequency of night shifts, duration of each shifts, speed and direction of shift rotation, were not collected in the current study. Accordingly, further studies will be needed to assess the relationships of different kinds of schedules and duration of shift work to obesity.