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 Table of Contents  
ORIGINAL ARTICLE
Year : 2021  |  Volume : 41  |  Issue : 4  |  Page : 179-185

Exploring the relationship between serum Vitamin D and shift work


1 Department of General Medicine, National Defense Medical Center, School of Medicine; Department of Family and Community Medicine, Division of Family Medicine, National Defense Medical Center, School of Medicine, Tri-Service General Hospital, Taipei, Taiwan
2 Department of Family and Community Medicine, Division of Family Medicine, National Defense Medical Center, School of Medicine; Department of Family and Community Medicine, Division of Geriatric Medicine, School of Medicine, National Defense Medical Center, Tri-Service General Hospital, Taipei, Taiwan
3 Department of Ophthalmology, National Defense Medical Center, School of Medicine, Tri-Service General Hospital, Taipei, Taiwan
4 Department of Neurology, National Defense Medical Center, Tri-Service General Hospital, Taipei, Taiwan
5 Department of Internal Medicine, Division of Endocrinology and Metabolism, National Defense Medical School, Tri-Service General Hospital, Taipei, Taiwan
6 Department of Family and Community Medicine, Division of Family Medicine, National Defense Medical Center, School of Medicine; Department of Family and Community Medicine, Division of Geriatric Medicine, School of Medicine, National Defense Medical Center, Tri-Service General Hospital; Department of Biochemistry, National Defense Medical Center, Taipei, Taiwan

Date of Submission16-Jul-2020
Date of Decision07-Aug-2020
Date of Acceptance23-Sep-2020
Date of Web Publication03-Feb-2021

Correspondence Address:
Dr. Wei-Liang Chen
Number 325, Section 2, Chang-gong Rd, Nei-Hu District, 114, Taipei
Taiwan
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Source of Support: None, Conflict of Interest: None


DOI: 10.4103/jmedsci.jmedsci_223_20

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  Abstract 


Background: Vitamin D deficiency became increasingly common worldwide and was related to various disorders. However, there was little information concerning the relationship between serum 25-hydroxyvitamin D (25(OH) D) level and shift work. The aim of this paper was to investigate the relation between serum 25(OH) D level and shift work in the American citizens. Methods: From the 2005–2006 NHANES, a total of 3025 participants were included. The relation between serum 25(OH) D level and shift work was investigated through the regression models. Results: There was an inverse relationship between serum 25(OH) D level and female shift worker in the fully adjusted models (β coefficient = −8.774, 95% confidence intervals [CI] = −16.83–−0.71, P = 0.03). Additionally, a negative correlation was presented between serum 25OHD level and Mexican American (β coefficient = −9.825, 95% CI = −19.31–−0.33, P = 0.04). No significant differences in serum 25(OH) D level were presented among indoor/outdoor works or manual/nonmanual works. However, long working hours (>40 h/week) were remarkably correlated with low serum 25(OH) D level (β coefficient = −8.007, 95% CI = −15.32–−0.68, P = 0.03). Conclusions: Our study concluded there was an inverse correlation between serum 25(OH) D level and shift work, especially in female population.

Keywords: Shift work, 25-hydroxyvitamin D, long working hours


How to cite this article:
Yang ZY, Wang CC, Chen YJ, Tsai CK, Li PF, Peng TC, Sun YS, Chen WL. Exploring the relationship between serum Vitamin D and shift work. J Med Sci 2021;41:179-85

How to cite this URL:
Yang ZY, Wang CC, Chen YJ, Tsai CK, Li PF, Peng TC, Sun YS, Chen WL. Exploring the relationship between serum Vitamin D and shift work. J Med Sci [serial online] 2021 [cited 2021 Oct 25];41:179-85. Available from: https://www.jmedscindmc.com/text.asp?2021/41/4/179/308686




  Introduction Top


Vitamin D is a fat-soluble vitamin which is derived from synthesis in the skin by sunlight and some fish.[1] Vitamin D absorbed from dietary intake and skin is converted in the liver to 25-hydroxyvitamin D (25(OH) D), which is commonly used as an indicator of the Vitamin D status of an individual.[2] 25(OH) D is converted in the kidney by 1-alpha hydroxylase enzyme, to 1,25-dihydroxy Vitamin D (1,25-(OH)2D3) which is the active form of Vitamin D.[2] Vitamin D controls serum calcium and phosphorus levels and plays an critical role in bone metabolism and mediating normal function of immune systems.[2],[3]

Vitamin D deficiency is widespread and is developing as a major health problem globally.[4] High prevalence of Vitamin D deficiency is noted in U. S. and European elderly population.[2] Vitamin D deficiency is related to various diseases, such as cardiovascular disease, the metabolic syndrome, Type 2 diabetes mellitus, infectious/inflammatory disease, autoimmune disorders, and cancer.[5],[6],[7],[8],[9] Some factors are considered to be associated with Vitamin D deficiency, such as inadequate sun exposure, less physical activity and limited oral intake, or impaired intestinal absorption.[10],[11] However, the association between Vitamin D deficiency and working condition is less discussed.[12],[13]

Working condition, such as shift work and long working hours, is a public issue worldwide.[14],[15] Approximately 20% workers in Europe are employed on shift work involving night work, and due to the increasing demand for services, the percentage of shift worker may increase over time.[15] Shift work and long working hours may both cause various health disorders (i.e., psychological, metabolic, and cardiovascular disorders).[14],[16] Knutsson. stated that shift workers had a 40% excess risk for cardiovascular disease compared with day workers.[17] A prospective study in the UK revealed that long working hours is associated with the development of depressive and anxiety symptoms in women.[18] Only a few studies demonstrated epidemiological relationship between these two: serum 25(OH) D level and shift work.[12],[13],[19],[20],[21] The purpose of our study was to explore the relation between serum 25(OH) D level and shift work.


  Methods Top


Ethics statement

The NHANES study protocol was approved by the National Center for Health Statistics (NCHS) Institutional Review Board (NHANES 2005-2006, Protocol #2005-06). All the eligible participants had signed the informed consents.

Study populations

The NHANES is a cross-sectional survey of noninstitutionalized US adults, which was executed by the Centers for Disease Control and Prevention, NCHS. The survey consisted of an extensive household interview (demographic material, past medical records, and laboratory statistics) and a subsequent physical examination performed by well-trained examiner at a specially equipped Mobile Examination Center. These data were obtained from the NHANES dataset from 2005 to 2006, which included serum 25OHD level and information on working condition. Initially, we included 6139 participants in the NHANES dataset from 2005 to 2006. The exclusion criteria included missing information (n = 2782), another schedule (n = 330), refused to answer (n = 1), and don't know the answer (n = 1). Finally, 3025 participants were included in the present study.

Covariates

Participants were asked the question “Which best describes hours worked?” Response answer included: (1) a rotating shift, (2) a regular daytime schedule, (3) a regular evening schedule, (4) a regular night shift, and another schedule. We excluded another schedule for no further information and included a regular daytime schedule, regular evening schedule, a regular night shift, and a rotating shift.

Ward et al. divided occupations into four classifications: professional/managerial, nonmanual, manual, and unskilled manual.[13] Sowah et al. divided occupations into six classifications: (1) shift workers, (2) indoor workers (3) outdoor workers, (4) coalminers, (5) healthcare professionals, and (6) lead/smelter workers.[22] Based on these two previous study,[13],[22] we modified the classification of working condition. We used two classifications to divide occupations: (1) indoor or outdoor (2) manual or nonmanual. Based on occupation groups of NHANES, we first excluded 3 groups (“Armed Forces,” “Text present but uncodable,” and “Blank but applicable”). The remaining 21 occupation groups were divided into two classifications and detail contents of two classifications are present in [Table 1]. There were various definitions of long working hours.[23] We used the definition (>40 h/week) from a systemic review.[14]
Table 1: Two classifications of National Center for Health Statistics Institutional Review Board occupation groups

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We collected demographic variables that are age, sex, race/ethnicity, smoking history, and medical status from computer-assisted personal interviewing system. Smoking status was determined by asking the question “Do you now smoke cigarettes?” Other medical history diagnosed or told by a doctor including coronary artery disease, congestive heart failure, and angina, was determined by self-reported data. The level of creatinine, alanine aminotransferase, glucose, albumin, and total protein was measured with Beckman Synchron LX20.

Serum 25-hydroxyvitamin D measurement

The level of serum 25OHD was determined by the Diasorin (formerly Incstar) 25-OH-Vitamin D assay using an equilibrium radioimmunoassay procedure. More detailed information about the Diasorin radioimmunoassay method can be obtained from the NHANES network.

Statistical analyses

We utilized Predictive Analytics Suite Workstation Statistics, version 18.0 for Windows (SPSS Inc., Chicago, IL, USA) to analysis our data. Continuous data were listed as means and standard deviations; categorical data were listed as numbers and percentages. The Student's t-test was used for continuous variables and the Chi-square test was used for categorical variables. Two-sided P < 0.05 was indicated to be statistically significant. We examined the relation between serum 25(OH) D level and shift work or work condition using multivariate regression models. We used 3 models for adjustments: Model 1 were adjusted for age, gender, and race/ethnicity; Model 2 were Model 1 plus serum albumin, serum ALT, serum Cr, fasting glucose, and total protein; Model 3 were Model 2 plus smoking history, coronary heart disease, congestive heart failure, and angina/angina pectoris.


  Results Top


Characteristics of the study population

The characteristics of the study population stratified by shift work are listed in [Table 2]. In the study, the mean age of participants with regular daytime workers was 40.38 ± 15.02 years for males and 38.87 ± 13.85 years for females. In shift worker group, the mean age was 32.09 ± 14.86 years for males and 27.98 ± 13.12 years for females. A lower serum 25(OH) D level and younger age were significantly prominent in shift workers compared with regular daytime workers.
Table 2: Characteristics of study participants

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Association of shift work and serum 25-hydroxyvitamin D level in different gender

[Table 3] presents the regression coefficients for the association between males and females of shift work and serum 25(OH) D level. After adjusting covariates in model 1-3, a negative association was found between shift work and serum 25(OH) D level in female group (P < 0.05), but not in male. The correlation between different shift work pattern and serum 25(OH) D level is shown in [Table 4].
Table 3: The association between shift work and serum 25-hydroxyvitamin D level in total population and in different gender

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Table 4: The association between different shift work pattern and serum 25-hydroxyvitamin D level in different gender

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Association of shift work and serum 25-hydroxyvitamin D level in race/ethnicity difference

[Table 5] presents the regression coefficients for the relationship of shift work and serum 25(OH) D level in different race/ethnicity. After adjusting covariates in model 1-3, a negative association was found between shift work and serum 25(OH) D level only in Mexican American (P < 0.05).
Table 5: The association between shift work and serum 25-hydroxyvitamin D level in different race

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Association of working condition and serum 25-hydroxyvitamin D level

[Table 6] demonstrates the regression coefficients for the association of different working condition and serum 25(OH) D level. Long working hours (>40 h/week) were remarkably correlated with low serum 25(OH) D level (P < 0.05).
Table 6: The association between shift work and serum 25-hydroxyvitamin D level in different working condition

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  Discussion Top


In this study of the American citizens, younger age and lower serum 25(OH) D level were observed in shift worker. Notably, shift work was negatively associated with serum 25(OH) D level in female participants. In addition, long working hours had a significant inverse correlation with serum 25(OH) D level. We also noted an adverse relationship between shift work and serum 25(OH) D level only in Mexican American.

Few previous studies have focused on relationship between shift work and serum 25(OH) D level. We listed five articles exploring the relationship between shift work and serum 25(OH) D level [Table 7]. Two studies, one proposed by Ward et al. and one by Alefishat E et al., included 6154 and 140 participants, respectively, and both showed that female night shift workers have a low serum 25(OH) D levels than female daytime workers, while there was no correlation between male night shift workers and serum 25(OH) D level. In the 2011 study of a Japanese population, no significant differences in serum 25(OH) D levels were observed between daytime workers and shift workers. This result was consistent with our study.
Table 7: Comparison of different cross sectional studies investigating the association between shift work and 25-hydroxyvitamin D

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However, the relationship between serum 25(OH) D level and night shift work in gender difference is controversial. Jeong et al. had found a significant correlation between Vitamin D deficiency and shift workers among male participants. Romano et al. had also reported a remarkably correlation between shift work and serum 25(OH) D level in male factory workers. However, the study population was only derived from male participants. The possible mechanisms of the association between shift work and serum 25(OH) D level in gender difference might be caused by different lifestyles.[13] Prior studies showed women tended to engage more housework and child care and had less recreational time.[24],[25] Women preferred to wear clothes for sunlight protection could be another explanation.[4]

There were many determinants related to Vitamin D deficiency such as age, gender, season, skin color, sunlight exposure, and dietary habits.[10],[26],[27],[28],[29],[30] Roomi et al. demonstrated that Vitamin D deficiency was associated with female gender, less physical activity, indoor jobs, and less sun exposure.[11] Sowah et al. reported in a systemic review that compared to other occupations, shift worker had increased risk of Vitamin D deficiency.[22] Our study stated long working hours was negatively correlated with serum 25(OH)D level. Working condition factors including shift work, indoor work, and long working hours might be potential risks of low serum 25(OH) D level.[22],[31] Previous studies had speculated that higher risk of Vitamin D deficiency in shift work and long working hours might due to less sunlight exposure.[12],[19],[22]

There are several limitations of our study. First, the study was a cross-sectional survey of the NHANES database. Hence, the chronological correlation between shift work and low serum 25(OH) D level was not clear. Second, other determinants related to serum 25(OH) D level (i.e., sunlight exposure, working hours, Vitamin D supplement, and seasonal variation) was not included, because there was no available information. For these unadjusted confounding variables, there may had been some potential impacts on the relationship between shift work and serum 25(OH) D level. Third, the generalization of the results to different racial populations might be limited because all enrolled participants in our study were Caucasian.


  Conclusions Top


The present study emphasized a negative correlation between serum 25(OH) D level and shift work in females. The results suggest that periodic monitoring for serum 25(OH) D level and adequate supplements of Vitamin D can be beneficial for shift worker. Further research might focus on different factors associated with shift work and serum 25(OH) D level. Our study demonstrated an epidemiologic result to support further studies on relationship between shift work and serum 25(OH) D level.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.



 
  References Top

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    Tables

  [Table 1], [Table 2], [Table 3], [Table 4], [Table 5], [Table 6], [Table 7]



 

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