According to the 2005–2008 National Nutrition Survey conducted in Taiwan, the average 25 (OH)D concentration of 2,596 adults aged 19 years and older was 18.1. In this study, the vitamin D levels of the three groups of shift nurses were as follows: 21.1 ± 6.2 ng/ml for day shifts, 21.3 ± 6.9 ng/ml for evening shifts, and 20.1 ± 4.7 ng/ml for night shifts; these results were like those of the National Health Survey. In Iran, Thailand, and India, studies conducted in nursing staff showed that 89%, 95.4%, and 94% of nurses had vitamin D deficiency (< 30 ng/ml) (21–23). These findings are consistent with those of the present study. In another study conducted in female nurses in Iran, the serum vitamin D levels in young nurses (< 50 years) was lower than that in older nurses (> 50 years) (24). The participants in this study were younger female nurses, and age possibly contributed to the lower vitamin D levels in this group.
The other study in China, the incidence of depression among nurses was 61.7%, and 74.9% of them had mild depression (25). Our results showed that all three groups of nurses had mild depression. This study found no significant differences in the vitamin D levels and depression incidence, although many studies have shown an inverse relationship between vitamin D and depression, probably due to the small sample size. However, results of previous studies are consistent with the findings of this study. In the 2005–2006 United States National Health and Nutrition Survey, the results did not show a significant correlation between vitamin D levels and mild depression, moderate-to-severe depression, and severe depression in American adults (26). The study conducted by Pan et al. also found that vitamin D level was not associated with depression incidence. They also pointed out that due to poor eating habits and lack of outdoor activities, the relationship between vitamin D and depression incidence could not be verified (27). This factor can also be used to explain the results of this study.
Our study results show that nurses experience a high degree of fatigue (average score: ≥3.3), but it is not related to level of vitamin D. Other research in Taiwan also pointed out that 37.1% of nurses experience fatigue (28). In our study, the nurses with a vitamin D level of ≥ 30 ng/ml had trend lower fatigue score than the other two groups, but the difference was not significant. The results of our study could be due to the small sample size. In a double-blind placebo-controlled clinical trial (29), 120 patients with fatigue and vitamin D deficiency (serum 25(OH)D < 20 µg/L) were recruited and randomly assigned to receive an oral dose of 100,000 units of vitamin D or placebo. After 4 weeks, the fatigue of patients with vitamin D deficiency who received vitamin D supplementation significantly improved. However, the frequency of sun exposure and physical activity in each study population were not taken into account.
A previous study (30) showed no difference in the degree of fatigue between night-shift and day-shift nurses; however, night-shift nurses had worse sleep quality than day-shift nurses, which is consistent with our study results; despite the presence of fatigue and sleep disturbance, the scores of nursing staff in the night shift were significantly higher than those in the evening shift and day shift, but the level of fatigue of the three groups of nurses did not show a significant difference. Another study has pointed out that young nursing staff with 1–2 years of experience are more likely to have a shift work disorder (31). This result is consistent with that of our study. When comparing the night-shift and day-shift nurses, results showed that night-shift nurses are 20 years younger and have higher incidence of sleep disorders than day-shift nurses. A previous study examined the relationship between the daytime sleepiness (assessed using the Epworth sleepiness scale) and vitamin D concentration in white and black patients. No significant correlation was observed between daytime sleepiness and vitamin D in the white group (32). In our study, no significant difference was found between vitamin D levels and sleep disturbance after grouping the nurses according to vitamin D levels.
In our study, none of the inflammatory factors any significant differences. In other studies (33), only the concentration of TNF-α in shift nurses was significantly higher than that in day-shift nurses, while IL-6 and IFN-γ were not significantly different. A possible reason is that the number of samples was too small, and the blood samples were not obtained at the end of night shift, which may have affected the results.
This study has several limitations. In the future, it may be possible to use objective testing methods, such as cortisol, to detect depression symptoms, and an exercise meter to record the quality of sleep. In addition, future studies should include participants of different ages and a larger sample size. Moreover, the number of samples with sufficient amounts of vitamin D (≥ 30 ng/dl) was relatively small; hence, it was difficult to judge a significant difference in the results. In addition, further research is needed to examine the relationship between sunlight and production of vitamin D in the body, as well as explore other factors that can effectively improve the depression symptoms including eating habits and lifestyle.