Shift work and insulin resistance (HOMA-IR) among professional drivers

1 Introduction

Circadian rhythm is a biologic and internal clock that regulates the sleep-wake cycle. Our physiology and behavior are influenced by this cycle [1]. Circadian rhythms misalignment can be the result of lifestyle, environmental and behavioral factors such as sleep disturbance, shift work, eating habits, and jet lag [2]. Sleep deprivation is related to misalignment between the timing of wake-sleep schedule due to shift work and circadian rhythm, which in turn compromises the subject’s health [3].

The modern 24-hour society due to the rapid technological development, economic competition and progressive, productive industries has been doing everything possible at any time of the day. Shift work is a working outside the most common full-time work schedule (7 a.m. to 7 p.m.). Shift work occurs in many public services such as transportation, healthcare industry, firefighting, security, etc. Data from the European Union in 2017 showed that the prevalence of shift work and night work 2017, is 21% and 19% of the workforce, respectively [4]. It is estimated that the prevalence of shift work in the European countries is approximately 20% [5]. Shift work has been associated with increased risk of several health problems such as altered nutritional habits, cardiovascular, gastrointestinal and neurological disorders, dyslipidemia, diabetes, and metabolic syndrome [6–10].

Insulin resistance (IR) is a pathologic condition in which insulin-tissue targets fail to respond normally to the action of insulin. IR typically develops with obesity and extends to the onset of type 2 diabetes and metabolic syndrome, which are risk factors of cardiovascular disease [11].

Some studies propose that shift work may be related to insulin resistance [12, 13]. The estimation of IR and prevention strategies could be very important. Homeostatic model assessment of insulin resistance (HOMA-IR) has proven to be a useful tool in the assessment of insulin resistance [14]. Therefore, the present study aimed to investigate the association between shift work and HOMA-IR index in professional drivers.

2 Materials and methods

This cross-sectional study was carried out from October 2018 to September 2019 in the occupational health clinic of a university hospital in the province of Tehran (Iran). The survey has been done on professional drivers as a part of their periodic medical examination. For the purpose of this study, night shift workers were considered as drivers who work from 7 p.m. till 7 a.m., while daytime drivers work from 7 a.m. till 7 p.m. and early morning subjects work from 4 a.m. to 4 p.m. One hundred seventy seven day workers were compared with 175 night time and 101 early morning drivers. To meet the inclusion criteria, subjects had to be healthy driver ≥20 years with at least one year of working history. Exclusion criteria included the presence of any chronic disease such as cardiovascular, diabetes and drug intake. Participation was voluntary and all of the drivers signed an informed consent. This study had been approved by the Tehran University of Medical Sciences Ethical committee.

All drivers were interviewed by a trained occupational medicine specialist. Information regarding demographic and occupational characteristics consists of age, weight, height, smoking, marital status, educational degree, physical activity and working history was obtained from the participants. Driving location divided to two groups urban and suburban according to driving license (group1, group2). Physical activity was considered if the participants would engage in regular and active exercise at least one day per week. Also, all drivers were asked to complete the STOP-Bang questionnaire, including four subjective (STOP: Snoring, Tiredness, Observed apnea and high blood Pressure) and four objective items (Bang: BMI, age, neck circumference, gender). The cutoff score was ≥3 to predict obstructive sleep apnea [15]. The subjects were asked to fast for at least 9 hours before blood sample collection in the morning. Serum insulin, fasting blood glucose, triglyceride, total cholesterol, HDL and LDL cholesterol were measured by using the standard auto analyzer. Immunoassay test was used to measure insulin, and enzymatic assay were performed for other analysis. All of the laboratory instruments had the quality certificate. HOMA-IR index has been calculated as fasting insulin (microU/L) x fasting glucose (nmol/L)/22.5. The HOMA-IR cutoff points of > 2.5 were considered in our study population [16].

Systolic and diastolic blood pressure was measured with a mercury barometric device at sitting position in a quiet room after 15 minutes rest. Anthropometric measurements, including height and weight, waist circumferences were performed. The subject’s height was measured on a scale with minimum coverage without footwear. The waist circumference (between 12 ribs and iliac crest) was measured in standing position, using a non-elastic meter tape measure of 1 cm accuracy. All measurements were done by a trained person.

Metabolic syndrome (Mets) was assessed using ATP III criteria with the presence of ≥3 of the following criteria: waist circumstances (WC)≥90cm, triglycerides (TG)≥150 mg/dL, HDL-cholesterol (HDL-C) < 40 mg/dL, fasting plasma glucose (FPG)≥110 mg/dL, systolic blood pressure (SBP)≥130 mmHg and/or diastolic blood pressure (DBP)≥85 [17].

We used the SPSS version 21 to perform the analysis (IBM Corp., Armonk, NY, USA). Quantitative and qualitative data were expressed as mean (SD) and frequency (percent); respectively. In univariate analysis, one-way ANOVA and Chi-square were used to determine whether there are any statistically significant differences between the means and frequency of variables and different types of shift work, respectively. Linear regression analysis was carried out to investigate the association between HOMA-IR index (as dependent variable) with demographic, occupational and metabolic syndrome components (as independent variables). A P value less than 0.05 was considered statistically significant.

3 Results

In this study 453 male professional drivers were examined during the periodic medical examination. The frequency of day and night shifts was around 39%, while the frequency of early morning shifts was about 22%. The mean age and working history of our study population was 45.8 and 18.6 years, respectively.

Daytime, night and early morning shift work drivers were comparable in terms of their demographic, occupational and clinical characteristics (Tables 1 and 2). The three groups have no statistically difference regarding the variables investigated: age, marital status, body mass index (BMI), educational degree, working history, smoking behavior, physical activity, lipid profile and Stop bang score.

BMI average values and mean HOMA-IR index levels were above the normal range (25) in all groups. However, mean SBP was significantly higher in night shift drivers. Mean levels of fasting blood glucose, insulin and HOMA-IR were significantly lower in daytime shift workers as compared to night and early morning shift workers.

Linear regression analysis showed a significant positive association between increased values of HOMA-IR and shift work, metabolic syndrome and driving location (Table 3).

The purpose of this study was to provide information regarding HOMA-IR of 453 male professional drivers and to look into the relationship between HOMA-IR and shift work. There are many occupational hazards in professional drivers including: shift work, stress, air pollution and ergonomic hazards which could predispose them to cardiovascular diseases and musculoskeletal disorder [18].

Driving performance reduction in the early morning and night shifts could be associated with increased risks of work-related injuries [19]. Also, there are some evidences that shift work predisposes the subject to develop metabolic syndrome, insulin resistance and diabetes [12, 21].

In our study, regardless of the shift type, more than 70% of the drivers were overweight. Similar to our result a Korean study, which had been conducted with 9,989 nurses found that there was no association between shift work and BMI [22], but some studies found that shift workers are more likely to be overweight [23]. It seems that the irregular working schedule has a major role in having an unhealthy lifestyle (lack of exercise, poor diet, etc.) [24]. In a US study, most of the truck drivers reported unhealthy eating habits due to insufficient time [25]. Therefore, the important role of some confounding factors (poor diet, drinking) in professional drivers makes it more difficult to establish causal associations between shift type and obesity.

The high frequency of blood pressure (≥140/90) was 16% and 22% in day and night shift drivers, respectively. Although the mean SBP for all drivers were within the normal range, but there was a statistical significant difference between night and daytime drivers, and this difference had an increasing tendency with shift work duration (mean SBP difference between night and day workers was 6 mmHg and 16 mmHg in drivers with less than 5 years and more than 5 years history of shift work, respectively). However, no study has reported the specific working history for shift workers after which the risk of high blood pressure clearly increases.

Our research indicates that participants who were night shift workers were more likely to have metabolic syndrome (62.9%) than daytime drivers (44.6%). Many studies have shown the different rate of MetS, while, according to a review study in Iranian adults, the rate of MetS varied from 10% to 60% in men [26]. The reasons for approximately 2 times higher risk of MetS in night shift drivers in comparison with the general population need to be studied. Some studies had been suggested several mechanisms regarding the relationship between shift work and metabolic syndrome [27, 28]. However, sleep deprivation is likely due to circadian rhythm disruption, but it is still unclear whether the circadian rhythm disorder or sleep deprivation has the main role in developing metabolic syndrome.

Usually, these cut-off values are different according to the ethnicity of individuals. The mean value for HOMA-IR was 5.6 in all drivers, which was above the proposed cut-off values (ranging from 1.7 to 3.8) in different studies [29]. This study investigated the positive association between HOMA-IR and MetS in professional drivers. Insulin resistance is the characteristics of obesity which can develop into diabetes and cardiovascular disease by inducing inflammatory process on the vascular system, leading to increased morbidity and mortality [30].

Meanwhile, we did not find a statistically significant effect of demographic factors on HOMA-IR. Similar to our results Gayoso-Diz found a slight decrease of HOMA-IR levels with age in both genders [31]. Some studies found that age has a high correlation with insulin resistance. This could be due to the role of confounding factors such as hypertension, cardiovascular disease, diabetes and hyperlipidemia that increases with age [32]. Previous studies reported an association between BMI and the HOMA-IR index [33].

Our study found a significantly increased level of insulin and HOMA-IR index, with a decreasing trend in the night, early morning shifts and daytime drivers. Sleep deprivation is expected due to the reduction of night’s sleep before an early morning shift [34]. Similar to our result Sookion et al. showed that rotating shift works lead to higher levels of insulin and HOMA-IR [12].

Appetite regulating hormones have been influenced by circadian rhythm disruption throughout nervous system activity. Shift work by inducing sleep deprivation may have a negative effect on regulating these hormones, which in turn contributes to elevated total energy uptake [35].

This is the first study to investigate the association between shift work and marker of insulin resistance (HOMA-IR) in professional drivers. But it is important to mention that we have some limitations in our study as well. The cross-sectional nature of our study does not infer a causal relationship between insulin resistance and shift work, and limits its generalizability. Furthermore, we didn’t have information regarding dietary habits. Also, because of the male gender of our sample, we couldn’t justify this difference by gender.

5 Conclusion

This study found an association between HOMA-IR and MetS in professional drivers. Insulin resistance can develop into cardiovascular disease. Although, we did not evaluate dietary intake and physical activity etc but this high prevalence may be explained as a result of long working hours, unhealthy lifestyles, poor diet, high-calorie intake, lack of physical activity and sleep deprivation and lack of proper attention to general health [36].

The improvement of public transport performance mandates comprehensive and applicable strategies such as educational programs and appropriate interventions to overcome the problem. Therefore, some interventions such as structured lifestyle counseling in occupational health settings were effective in improvement of cardiometabolic risk factors. More interventional and prospective studies are needed to investigate these concerns.

Conflict of interest

The authors do not have any conflicts of interest.