Objectives Over 20% of employees in Europe work in shifts. Shift work increases the risk for chronic diseases, but a healthy lifestyle may attenuate the adverse effect of shift work. The aim of this study was to explore food and nutrient intake differences between working time groups.
Methods The participants were 1478 employees (55% of men) of an airline divided into three working time groups: day work (n=608), shift work without in-flight work (n=541) and in-flight work (n=329). Measures included laboratory tests, physical measurements, a questionnaire, and food and nutrient intake estimations by a validated 16-item food intake questionnaire.
Results Shift working men were less likely to consume vegetables (p<0.001) and fruits (p=0.049) daily than male day and in-flight workers. In women, energy intake from saturated fat was higher among shift workers compared with day workers (12.6 vs 12.2 E%, p=0.023). In older female participants, energy intake from fat and saturated fat was higher in the shift work and in-flight work groups than in the day work group (p<0.001).
Conclusions In this study, shift work and working environment were associated with dietary habits, and this association was not explained by other characteristics such as workers’ educational level. Shift workers’ increased risk for chronic diseases should be taken into account and lifestyle counselling including advice in nutrition should be incorporated in routine occupational healthcare of shift workers.
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What this paper adds
Shift work is known to increase the risk for coronary heart disease, type 2 diabetes, metabolic syndrome and overweight.
A healthy lifestyle including wholesome nutrition, sufficient sleep and regular physical exercise may prevent metabolic disorders of shift work.
This study showed that shift workers have more unfavourable dietary habits compared with day workers.
Nutrition counselling should be part of shift workers’ occupational healthcare check-ups.
Shift work refers to work at changing hours of the day (morning, afternoon and night shift) or work at constant hours of the day outside the normal nine-to-five (eg, permanent night shift). Night shift work especially results in sleep loss and can cause circadian disruption with adverse metabolic and cardiovascular consequences.1 Many studies have indicated that shift work increases the risk for cardiovascular diseases,2 metabolic syndrome3–5 and, recently, type 2 diabetes also.6 Follow-up studies suggest an increasing trend in weight with increasing duration of shift work.7–9 In addition, former male shift workers’ risk for metabolic syndrome can remain even after they have changed from shift work to day work.10
Shift work may also alter employees’ lifestyle in many ways. Shift work can change food timing and a healthy diet can be difficult to maintain, especially if the facilities for eating outside the normal working hours are not well organised and healthy food choices are not available.
Studies on shift workers’ nutrition have mainly focused on the determination of food and nutrient intake in different shifts and shift work systems with findings indicating some special characteristics of dietary habits. The results show that shift workers had more eating events per day than day workers.4 ,11 Night shift workers ate more snacks compared with day workers.11 ,12 Night working men's energy intake from sodas, added fats and fruits and vegetables was higher and energy intake from bread and meats was lower than that for day workers.13 Night shift workers consumed more sweet drinks than day workers.14 Generally, it seems that shift workers make more unfavourable food choices than day workers do.
In relation to nutrient intakes, only minor differences between day workers and shift working people have been identified. Day and shift workers’ dietary energy intake seems to be similar.4 ,11 ,14 ,15 The exception was older male shift workers who reported higher energy intake than older day workers.7 Differences in nutrient intake between day and shift workers in saturated and polyunsaturated fatty acids, dietary fibre, vitamin B1, vitamin A, potassium and carbohydrates have been suggested in some studies.11 ,16 In a recent paper, shift workers’ diet was shown to be more proinflammatory than the diet of day workers when 30 different nutrients were used to count the dietary inflammatory index score.17
Only a few studies have explored changes in nutrient intake after the transition from day work to shift work or differences in nutrient intake between different specific shifts or shift systems. Knutson analysed food diary data and discovered that after workers had changed from day work to shift work, they increased the consumption of soft drinks and lowered intake of vegetables and potatoes and consequently increased sucrose intake and decreased fibre intake.18
A review by Lowden et al19 evaluated the results of shift workers’ dietary intake studies since 1967. They highlighted several shortcomings of dietary studies among shift workers, including unreliable methods used for nutrition research, insufficient number of participants and the under-representation of women in the study populations. In addition, the effects of shift work on nutrition have often been explored among workers who work permanent evening or night shifts and the effect of very irregular working hours on nutrition is not evident. Permanent shifts used to be common in earlier years but not anymore. Transportation, including aviation, is a typical example of a sector with daily, weekly and annual variation of peak hours and the related irregularity in working hours.
Our aim was to address these gaps in knowledge by exploring how irregular working hours and different occupations influence male and female workers’ food and nutrient intakes in an airline company where the majority of employees are shift workers.
Study design and participants
Occupational healthcare of the airline implemented the screening and prevention programme of chronic diseases in 2006.20 During the programme period 2006–2009, altogether 4169 employees were invited and 2312 participated in the health check-up. People with known diabetes were excluded from the study. The baseline health check-up consisted of diabetes risk screening, laboratory tests, physical measurements, and a questionnaire on lifestyle, work and sleeping habits. Increased risk of diabetes was identified with the Finnish Diabetes Risk Score (points over 9)21 or with fasting plasma glucose between 6.1 and 11.0 mmol/L. Those employees who had an elevated risk for diabetes were asked to complete a 16-item food intake questionnaire and were offered lifestyle intervention comprising one to three face-to-face counselling sessions. Participants were invited on average 2.5 years later to a follow-up health check-up, which was almost similar to the baseline study, except that all participants were asked to fill in the 16-item food intake questionnaire. Altogether, 1523 employees completed the questionnaire. We excluded 175 employees who had retired or stopped working in the company and 7 employees who had reported unfeasible amounts of food or had too many missing answers. Those employees who had completed the 16-item food intake questionnaire at the baseline but did not take part in the follow-up (n=137) were included in this study, and for them the baseline results were used. The final study population consisted of 1478 employees, of which 815 (55%) were men.
The participants’ characteristics are shown in table 1. All participants gave their written informed consent.
Food and nutrient intake
We estimated the participants’ food and nutrient intake using a validated 16-item food intake questionnaire.22 The questionnaire contained 16 questions addressing the number and type of meals per day, the type of fat or cream used in cooking, as well as the type of fat used on bread and type of salad dressings. Vegetables, fruits, sugar rich foods and sweets consumption was rated on a five-point scale: at least 2 portions per day, 1 portion per day, 4–6 portions per week, 1–3 portions per week, and less than a portion per week or none. Fast food consumption was assessed using a five-point scale: at least 1 portion per day, 4–6 portions per week, 1–3 portions per week, 1–3 portions per month, and less than 1 portion per month or none. Amount of milk products, bread and breakfast cereals, cheese, cold cuts and beverages were assessed with open-ended questions.
The questionnaire has been validated against seven-day food diaries and algorithms to estimate total fat (proportion of energy, E%), saturated fat (E%), sucrose (E%), fibre (g), vitamin C (mg), iron (mg) and vitamin D (μg) have been created.22
Classifications and measurements
Employees were categorised into three working time groups. Working hours between 6:00 and 18:00 were categorised as day work. Pilots or flight attendants with irregular working hours were categorised as in-flight (shift) workers. A shift work group included employees who were not day workers or in-flight workers. Day workers were mainly working in light office duties (77%). Shift workers were working in aircraft service (53%) and customer service (13%). In-flight workers were working as flight attendants (88%) or pilots (12%). Regular and irregular shift systems were assessed by the question: Is your shift system regular? (Yes/no). The number of night shifts was assessed by the question: How many night shifts (night shift=at least three working hours between 23:00 and 06:00) do you have per month?
Education was categorised into three groups: low (comprehensive school), intermediate (secondary education) or high (polytechnic or university). Sleep disturbance during the past 3 months was assessed by questions from the Nordic Sleep Questionnaire:23 ‘have you had difficulties in falling asleep’, ‘do you feel tired during the day time’, and ‘how often do you awake during the night’ with five-scale rating (never or less than once a month to daily or almost daily). Sum of the three questions’ responses were used as a continuous indicator of sleep disturbance. Stress level was assessed with the question ‘Stressed person may feel strained, restless, nervous and/or distressed or he may have difficulties to fall asleep. I feel that kind of stress’. The four response alternatives were ranging from ‘never’ to ‘very much’.24
Analyses were conducted separately for men and women. In addition, we analysed the data in age groups, divided by the mean age. The cut-off age was 48.0 years for men and 47.6 years for women.
As the nutrient intakes were not normally distributed, we used the Box-Cox power transformation to normalise the distributions before the analyses. We used analysis of variance for the continuous and χ2 test for categorical variables to analyse differences between the working time groups. Linear and logistic regression was used for post hoc analyses. We adjusted analyses for potential confounders as appropriate: age, education, stress, sleep and participating in the lifestyle intervention offered during the programme. The significance level was set at α=0.05. Analyses were performed using the statistics package Stata V.11.2 (StataCorp, College Station, Texas, USA).
The working time groups differed statistically significantly in many characteristics (table 1). Non-flight shift workers were less educated than the other working time groups. On the basis of metabolic parameters, in-flight workers were healthier than other workers and the women non-flight shift workers had more stress and a more sedentary lifestyle. Shift systems varied between the shift work and in-flight work group. A larger proportion of in-flight workers worked an irregular shift system (men 98.5%, women 98.5%) compared with shift workers (men 44%, women 81%). Also, in-flight workers had night work more often than shift workers did (table 1). Of shift workers, 2.2% (n=9) of men and 0.7% (n=1) of women worked permanent night shifts.
Men shift workers were less likely to consume vegetables and fruits daily than men day and in-flight workers. They also consumed more high-fat milk products, but this association disappeared after adjustment for confounders. However, their use of rye bread was higher than in the other working time groups (table 2). Men and women in-flight workers were less likely to consume vegetable oil-based spread on bread than the other workers. Women in-flight and shift workers used high-fat milk products more than did day workers (table 2).
As shown in table 3 presenting the results of the logistic regression analyses, the daily vegetable and fruit intake tended to increase with age among men (vegetables: OR 1.63 95% CI 1.22 to 2.18, fruits: OR 1.70 95% CI 1.27 to 2.27) and women (vegetables: OR 1.51 95% CI 1.02 to 2.26, fruits: OR 1.58 95% CI 1.12 to 2.24).
In men, estimated energy intake from fat and saturated fat was the highest in the shift work group and lowest in the in-flight work group (fat 33.0 vs 31.7 E%, p=0.003; saturated fat 12.1 vs 11.6 E%, p=0.005; table 4). Men in-flight workers had a higher intake of vitamin C than the other men workers. Fat and vitamin C intake differences between the groups remained statistically significant after adjustments for education, age, sleep, stress and participating in the lifestyle intervention. The analyses separately for the younger (age <48) and older (age ≥48) age groups indicated that in men the nutrient intake did not differ between the working time groups, except for the young shift workers who had a higher intake of saturated fat than the in-flight workers (12.4 vs 11.8 g, p=0.038) and young in-flight workers who had a higher intake of vitamin C than the day workers (95.5 vs 82.1 mg, p=0.03; table 4).
In women, energy intake from saturated fat was higher among shift workers compared with day workers (12.6 vs 12.2 E%, p=0.023; table 5). The association remained after adjusting for confounding factors. In younger women participants (age <47.6), the intake of iron was higher in the in-flight work group than in the day work group (table 5). In older women participants, energy intake from fat and saturated fat was higher in the shift work and in-flight work groups than in the day work group and the association remained statistically significant after adjusting for confounding factors (p<0.001; table 5).
Finally, we analysed the differences in nutrient intake separately for shift work and in-flight work groups by amount of night shifts per month. In shift working men who did not work night shifts, sucrose intake was lower (8.0 vs 8.6 E%, p<0.001) compared with men who worked at least three nights per month. In women, vitamin D intake was higher (5.4 vs 4.8, p=0.03) when work did not contain night shifts.
In this study, our aim was to explore and compare the dietary quality of workers in an airline where two-thirds of the personnel were working in shifts. We categorised the workers into day, shift and in-flight workers because we wanted to investigate the differences between two rather different shift work groups. We have previously shown that in-flight workers, especially women, are healthier than other working groups in the airline.20 Our present analyses show that there are some differences also in food and nutrient intakes between the groups of different shift systems. The proportion of energy from saturated fat was substantially higher in shift working women compared with women who were working daytime. High saturated fat intake associated strongly with higher total fat intake, especially among older women. Furthermore, male shift workers were less likely to consume vegetables and fruits daily than day workers or in-flight workers. Consequently, intake of vitamin C was higher in male in-flight workers than other working time groups.
Similarly, Morikawa et al16 found that vegetable intake was lower in shift workers with night shifts than day workers or shift workers without night shifts. De Assis et al,13 however, reported that energy intake from vegetables and fruits including fruit juices was higher in night workers compared with day or evening workers. The higher intake may be a result of shift workers’ tendency for increased consumption of sweet beverages.18 Gastrointestinal disorders, linked with a dysregulated metabolic circadian clock, are common among night shift workers.25 Irritable bowel syndrome has been diagnosed more often in rotating shift workers than day workers.26 High vegetable intake has been associated with severity of irritable bowel syndrome symptoms27 and could partially explain the low vegetable and fruit intake among shift workers. In our study, daily consumption of vegetables and fruits increased by age, and therefore occupational healthcare should emphasise abundant daily consumption of fruit and vegetables together with advice on vegetables, which may irritate the gastrointestinal tract especially among younger workers. A diet rich in vegetables may prevent weight gain,28 which could be one very important issue preventing weight gain in shift working people.29
Our results suggest that shift workers’ quality of dietary fat is poorer than that of day workers. Morikawa et al16 reported that younger shift working men had higher saturated and older shift working men had lower polyunsaturated fatty acids intake compared with day workers. In addition, the proportion of energy from fat has been found to be higher in shift workers than in day workers in some30 ,31 but not all studies.4 ,14 ,32 High saturated fat intake can increase the levels of total and low-density lipoprotein (LDL) cholesterol and thus increase the risk for coronary heart diseases. In this study, total or LDL cholesterol levels did not differ between the working time groups, although it has been shown that shift workers more often have lipid disorders than day workers.33 In-flight workers had higher high-density lipoprotein (HDL) cholesterol level than the other workers. This may indicate that despite the shift work exposure, a healthy lifestyle may prevent adverse effects on health. For example, female in-flight workers were more physically active and that may contribute to their higher HDL cholesterol levels.
Shift workers’ shift schedule may have an impact on workers’ food choices. In this study, night work increased sucrose intake in men and lowered vitamin D intake in women. Sleep debt has been shown to increase consumption of sweet or high-carbohydrate-containing snacks.34 ,35 In this study, men who worked at least three nights per month were presumably sleep deprived and that may have influenced their food choices.
Overall, study participants’ nutrient intake estimates were close to the national nutrition recommendations except for saturated fatty acids and, in women, fibre and vitamin D intake. Similar results have been reported by national nutrition studies, suggesting that the employees participating in this study have the same nutritional inadequacies as the general population but shift work makes the problem even worse.
Interestingly, educational level did not associate with food or nutrient intake in this study, even when it is known that socioeconomic position can influence health behaviour, especially eating habits.36 We speculate that shift work itself may affect food habits more than education. In-flight workers were highly educated in general, but still their food choices were not in line with the recommendations as regards the use of high-fat milk products and butter-based spread on bread. In this study, older age predicted a higher consumption of vegetables and fruits. That could possibly be related to the increasing knowledge, skills or interest in healthy eating over time. In addition, the variety and choices of available foods at the workplace influence eating habits and the quality of diet. The airline subsidies for catering services are offered in order to serve reasonably priced meals for the employees. For day workers, workplace meals including salads, a hot meal and bread are served in canteens. The flight attendants are offered a proper meal and also sandwiches and fruits are served during the working hours, but for shift workers proper meals are not always provided during evening or night shifts. It has been shown that having catering services in worksites may be associated with healthier eating habits.37 The shift workers consumed less vegetables than the other working groups. This may reflect the unavailability of proper meals including vegetables outside the regular working hours in canteens or that vegetables are not included in packed lunches.
Some limitations need to be addressed. For practical reasons, we estimated the nutrient intakes with a short food intake questionnaire which may affect the reliability of our intake estimates. The questionnaire has been validated among employees in the same airline where this study was completed. Therefore, we think that we can identify the nutrient intake differences between the working time groups by the used method. The method gives an estimate for only a few nutrients, but we believe that it captures the intake of the most relevant nutrients as regards the prevention of chronic diseases. The quality of dietary fat and carbohydrates is considered important in prevention of cardiovascular diseases. In addition, vitamin D intake may be an indicator of fish intake, which is an important part of a healthy diet, vitamin C is an indicator of vegetable and fruit intake and iron is an indicator of consumption of meat and whole grains. We think these nutrients give a good overview of the participants’ diet quality. In analysis, the effects of several possible confounders were taken into account. Still, residual confounding is possible and the cross-sectional nature of our comparison makes it difficult to conclusively attribute particular dietary habits to work schedules.
In conclusion, we have shown that shift work and working environment correlate with dietary habits and this association is not explained by other characteristics such as the educational level of the workers. The results of this study are valuable as they give new information about food and nutrient intake of personnel who are working in shifts. Shift workers’ increased risk for chronic diseases should be taken into account and lifestyle counselling including advice in nutrition should be incorporated into routine occupational healthcare of shift workers.
Contributors All authors participated in planning the study design, revised and approved the final version of the manuscript. KH was responsible for data analyses and preparation of the manuscript. JL and KH were involved in nutrient intake estimation calculations from the 16-item food intake questionnaire. KV, KH and SP participated in data collection.
Funding Tekes—the Finnish Funding Agency for Technology and Innovation (grant 1104/10); Yrjö Jahnsson Foundation (grant 6342); Academy of Finland (SALVE consortium, grant 129518); Juho Vainio Foundation; Finnish Work Environment Fund (grant 108320).
Competing interests None declared.
Patient consent Obtained.
Ethics approval The study was approved by the Ethics Committee of the Hospital District of Helsinki and Uusimaa.
Provenance and peer review Not commissioned; externally peer reviewed.
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