Low Bone Mineral Density in Rotating-Shift Workers

Journal of Clinical Densitometry: Assessment of Skeletal Health, vol. 13, no. 4, 467e469, 2010 Ó Copyright 2010 by The International Society for Clinical Densitometry 1094-6950/13:467e469/$36.00 DOI: 10.1016/j.jocd.2010.07.004

Original Article

Low Bone Mineral Density in Rotating-Shift Workers Ivan Quevedo*,1,2 and Ana M. Zuniga2 1

Division of Endocrinology, University of Concepcion, Concepcion, Chile; and 2Naval Hospital of Concepcion, Concepcion, Chile

Abstract Shift workers have been reported to have an increased bone resorption. However, no existing evidence indicates lower bone mineral density (BMD) in this group. The objective of this study was to test the hypothesis that a rotating-shift work schedule is associated with low BMD and osteoporosis. We evaluated 70 postmenopausal nurses from the Naval Hospital in Concepcion, Chile. The participants were categorized according to the type of work schedule: 39 had a rotating shift and 31 were daytime workers. Medical history, a health examination, a questionnaire on health-related behaviors and biochemical determinations, and BMD examination were obtained for all participants. When comparing the 2 groups, the rotating-shift workers had lower BMD in the lumbar spine (L1eL4: 0.957  0.15 vs 1.104  0.13; p ! 0.05) and lower bone density in both femoral neck bones (right: 0.936  0.17 vs 1.06  0.12; p ! 0.05 and left: 0.956  0.19 vs 1.05  0.12; p ! 0.05). Additionally, the T-scores for 10 (25.6%) of the rotatingshift workers indicated osteoporosis at lumbar spine (T-score O 2.5). No evidence of osteoporosis was found for daytime workers. When comparing the 2 groups, the rotating-shift workers had a higher prevalence of osteopenia (T-score 5 1.0 to 2.5) than the daytime workers: 46.2% vs 35.5%, respectively. We found significant evidence that rotating-shift workers have lower BMD in the trabecular and cortical bones, thus suggesting that this type of work may be a risk factor for osteoporosis. Because this is the first time that this osteoporosis risk factor has been reported, the association needs to be replicated and confirmed in other settings. Key Words: Bone mineral density; osteoporosis; rotating-shift work.

Circadian rhythms regulate hundreds of functions in the human body, including sleep and wake fullness, body temperature, blood pressure, digestive secretion, immune activity, and hormone production. Changes in circadian rhythms were proposed as a factor contributing to the observed increased in risk (3). A previous study has showed an accelerated bone resorption in female rotating-shift workers because of perturbations of the diurnal rhythms of reproductive hormones (6). The present study was conducted to test the hypothesis that the chronic disruption in circadian rhythms observed in rotating-shift workers can lead to developed precocious osteoporosis.

Introduction Although osteoporosis and subsequent increased fracture risk occur primarily in older persons, a predisposition for osteoporosis is partially established during adulthood. The number of women in the workforce has grown over the past 20 yr and continues to expand. Many of the women workers in industrialized countries work in rotating shifts (1,2). Rotating-shift workers are known to be a high-risk population for sleep disturbances; hypertension; gastrointestinal disturbances; obesity; and some cancers, including breast, colorectal, and prostate (3e5). Received 04/01/09; Revised 06/23/10; Accepted 07/14/10. Dr. Ivan Quevedo received a grant DIUC #205.085.030.1-0 from University of Concepcion and one from Proyecto SAVAL for Clinic Research. *Address correspondence to: Ivan Quevedo, MD, Pucara Poniente 2310, San Pedro de la Paz, Concepcion, Chile. E-mail: equevedo@ udec.cl

Materials and Methods Subjects The study considered a total of 70 subjects: 39 were nurses who had worked a rotating shift, alternating night 467

468

Quevedo and Zuniga Table 1 Clinical and Biochemical Characteristics of the Included Individuals According to the Job Schedule Type

Variables Number of subjects Age (yr) Body mass index (kg/m2) Menarche Number of children Menopause Smoking (yes) Alcohol drinking (yes) Coffee drinker (yes) Glucose (mg/dL) Serum calcium (mg/dL) Creatinine (mg/dL) Albumine (g/L) TSH AST ALT

Rotating-shift worker

Daytime worker

39 55.56  3.16 23.82  1.81 12.05  1.66 1.54  1.07 49.21  2.00 19 12 26 92.51  4.54 9.23  0.56 0.98  0.15 39.77  1.78 2.14  0.88 27.15  5.73 49.67  7.84

31 55.87  3.01 24.57  2.03 12.35  1.80 2.16  1.17 49.19  1.76 10 10 5 92.29  5.86 9.05  0.44 0.95  0.12 40.23  1.85 2.26  0.78 26.58  5.12 47.81  8.67

and day work during the last 10 yr (for each set of 3 day shifts and 1 night shift), and 31 were nurses who had worked only in the daytime in the last 10 yr. Eligible nurses were ambulatory postmenopausal older than 50 yr. All the activities performed in this study were conducted in accordance with the guidelines of The Declaration of Helsinki. All the participating individuals voluntarily signed written consent forms in accordance with the procedures approved by the Ethical Committee of our institution. All the subjects were workers in the Naval Hospital in Concepcion, Chile. All of the recruited women were in the first decade of natural menopause. A physical examination, bone mass measurement, biochemical measurement, detailed interviews regarding medical history, and medication that may affect bone mass were determined for the women who participated in the study. From the self-administered questionnaire, we acquired information on the following factors: age, height, weight, past

p-Value 0.68 0.45 0.42 0.27 0.45

0.22 0.17 0.28 0.30 0.21 0.66 0.35

medical history, family history of osteoporosis fracture, calcium intake (cup of milk); coffee drinking (more than 4 cups or none), alcohol drinking (habitual drinking or nondrinker), and smoking (current smoker or nonsmoker). Those with cancer disease and psychiatric or other significant medical illnesses were excluded from the study. Participants who had taken bisphosphonate, estrogen therapy, chronic glucocorticoid therapy, or alcohol at more than 12 oz/wk were also excluded. The bone mineral densities (BMD, g/cm2) of the lumbar spine (L1eL4) and both hips were measured by dual-energy X-ray absorptiometry (DXA) using a LUNAR DPX-L densitometer (GE Lunar Corporation, Madison, WI) in a posterioranterior projection. BMD was measured on the same machine and analyzed by the same operator. Measurements of BMD phantoms by each DXA detected no significant drifts during the 4-mo study and during the data acquisition period none major repairs received the densitometer.

Table 2 Bone Mineral Density Reported as Absolute Values and T-Scores, According to the Job Schedule Type

Number of women Absolute bone mineral density (g/cm2) Lumbar spine (L1eL4) Right femoral neck Left femoral neck Bone mineral density (T-score) Lumbar spine (L1eL4) Right femoral neck Left femoral neck

Rotating-shift worker

Daytime worker

39

31

p-Value

0.977  0.13 0.960  0.18 0.926  0.09

1.04  0.13 1.05  0.12 1.06  0.12

!0.05 !0.05 !0.05

1.68  1.07 0.37  0.68 0.45  0.81

0.66  1.07 0.61  1.05 0.68  1.05

!0.05 !0.05 !0.05

Journal of Clinical Densitometry: Assessment of Skeletal Health

Volume 13, 2010

Low BMD in Rotating-Shift Workers In the statistical analysis, normally distributed data were expressed as means  standard deviation and the categorical variables as the absolute number and as percentage of the total. Data were analyzed using the SPSS software 11.0 (Statistical Package for Social Sciences, Chicago, IL). Statistical analysis was accomplished by 2-tailed, paired Student’s t-test for continuous variables and by the chi-squared test for categorical variables. Significance was set at p  0.05.

Results Clinical features, anthropometric variables, and laboratory findings of the participants according to the work schedule are shown in the Table 1. When comparing the 2 groups, women who worked in rotating shifts reported a higher prevalence of current smoking (48.7% vs 32.2%), coffee drinking (66.6% vs 16.1%), and a smaller number of children. The prevalence of family history of osteoporosis fracture and current alcohol drinking revealed no differences in magnitude. The anthropometric and gynecological variables did not differ significantly between the 2 types of workers. As shown in Table 2, women who worked in a rotating-shift schedule had lower BMD values measured at the spine and femoral neck when compared with those women who worked in daytime schedule. Additionally, the T-scores from 10 participants (25.6%) of the rotating-shift group indicated osteoporosis at the lumbar spine (as defined by a T-score O 2.5). No evidence of osteoporosis was found in the group of daytime workers. Compared with those who had engaged in only daytime work, the rotating-shift worker presented higher prevalence of osteopenia (as defined by a T-score of 1.0 to 2.5), 46.2% vs 35.5%, respectively.

Discussion This study demonstrates that BMD is lower in the rotating-shift workers in comparison with daytime workers. A significant evidence was that rotating-shift work had an adverse effect on trabecular and cortical bone health. A previous study showed a great bone resorption (measured by urinary deoxypyridoline) in female rotating-shift workers; however, the BMD was not evaluated in this study (6). Several possible mechanisms could explain our results. Decreased exposure to daylight is known to reduce exposure to ultraviolet rays, resulting in a lower vitamin D production, a major calciotropic hormone (7). When working in rotating shifts, daylight exposure is shortened, thus explaining the elevated risk. Another possible explication is associated with sleep abnormalities, frequently associated with major depressive diseases, which have been found to result in a profound morning elevation of plasma interleukin (IL)-6 in absence of hypercortisolism. In women during their first decade of menopause, serum IL-6 is the single most important producer of femoral bone loss (8). Thus, IL-6 may be a pathogenic factor in shift worker’s bone loss. Some studies have indicated that stressors as subtle as a rotating-shift work can result in Journal of Clinical Densitometry: Assessment of Skeletal Health

469 irregular menstrual cycles because of perturbations of the diurnal rhythms of reproductive hormones (9). Diet is also known to play a role in the pathogenesis of osteoporosis. The nutrients known with certainty to be important in bone homeostasis are calcium, vitamin D, and protein. In our study, both groups have suboptimal calcium intake, confirming another report that Chilean women have low levels of calcium intake (10). Smoking and current caffeine intake can also contribute to bone loss (11). We found a higher prevalence of smokers and coffee drinkers in the rotating-shift workers. In contrast, alcohol drinking, family history of osteoporosis fracture, gynecological history, age, body mass index, and biochemical examination were similar in both groups. Still, our study presents important limitations. First, in terms of ascertaining the risk of osteoporosis, the cohort was relatively small, although this study had enough power to show the effect of a rotating shift on bone density. Second, our study is limited by the post hoc nature of our data collection and consequent inability to capture potential covariates of interest, for example, measures of biochemical markers of bone turnover. In conclusion, we have shown a significant association between rotating-shift work and precocious bone loss. Because this is the first time that this risk factor is reported, the association needs to be replicated and confirmed in other settings.

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