It is commonly found that gastrointestinal complaints are more prevalent among rotating and night shift workers compared to day workers [13
]. These complaints include: disturbances of appetite; irregularity of bowel movements; constipation; dyspepsia; heartburn; abdominal pains; grumbling and flatulence. It has been stated that between 20–75% of shift workers, compared to 10–25% of day workers, report such complaints [14
]. There are wide ranges in the reported prevalence rates of these symptoms, particularly amongst shift workers. Between-study heterogeneity is also the case when the prevalence of peptic ulcers amongst shift workers is considered. For example, Segawa et al.
] found that the prevalence of gastric ulcers was 2.4% in current shift workers compared with 1.0% in day workers. Nevertheless, Costa [16
] reported that the prevalence of peptic ulcers could be eight times greater for shift workers compared to day workers. These differences in prevalence rates may be due to variability in the nature of the clinical examination employed as well as the quality of interview data collected from the shift workers. Notwithstanding these inter-study differences in magnitude of event rates, the fact remains that gastro-intestinal problems, which could obviously impinge on ‘normal’ eating habits, are more common in shift workers.
There are reports of altered lipid profiles in shift workers, [17
] although the results of these studies are not completely consistent in terms of which specific lipid concentrations are altered. Romon, et al.
] found that shift workers had higher levels of serum triglycerides compared to day workers (1.26 vs 1.03 mmol/l), but levels of cholesterol and high density lipoprotein cholesterol (HDL) were similar between the two groups as well as comparable to normal population reference ranges. Karlsson et al.
] found a high concentration of triglycerides (>1.7 mmol/l), but low concentrations of HDL (<1.0 mmol/l), in a shift working population. Ghiasvand, et al.
] reported that cases of high concentrations of serum total cholesterol (>5.13 mmol/l) and low-density lipoprotein cholesterol (>3.33 mmol/l) were both more common amongst shift workers. The latter finding is supported by the results of Knutson et al.
] who found that the ratio between Aplolipoprotein B and Aplipoprotein A increased during shift work. Apolipoprotein A is the main protein component of HDL and promotes the transfer of cholesterol into the liver, where it is metabolized and then excreted from the body via the intestine.
In analysing the causes of the above epidemiological observations, it is clear that a shift worker may be restricted behaviourally, in terms of the desired amount and type of food that is eaten during working hours, and because traditional meal times in the home are disrupted. In addition, there may be a biological disruption to circadian rhythms relevant to the metabolism of food eaten during shift work. Both these behavioural and biological factors, which might combine to lead to unhealthy consequences, will now be considered in detail.
2.1. Eating behaviours of shift workers
In order to gain comprehensive knowledge about the eating behaviour of shift workers, past researchers have measured overall energy intake and macronutrient composition over 24 hours, the prevalence of snacking during different shifts, the number of meals consumed, and the timing of these meals and snacks. It appears from the results of these dietary surveys that the timing of meals, rather than their overall content, is affected most by shift work. Lennernas et al.
] used a repeated 24-h recall strategy that was administered to shift workers over 5 days of a rotating three-shift system (morning, afternoons, nights). The total amount of food ingested over a 24-h period did not differ between the three shifts. Nevertheless, when data were analysed over individual 8-hour work periods, food intake was higher on the morning shift than during the evening shift. This observation may be mediated by circadian factors related to appetite or by a decreased availability of meals during the night shift. Lennernas et al.
] also compared workers on 3-shift and 2-shift schedules to day workers across a complete work cycle. No differences were found with respect to total energy intake or dietary composition (calculated as % energy from protein, fat, total carbohydrates and sucrose) as well as intake of vitamins and minerals).
In contrast to the findings of Lennernas et al.
], Sudo & Ohtsuka [22
] found that total energy intake of Japanese female shift workers was 200–400 kcal/day less than that of day workers. Shift workers ingested less protein, fat and carbohydrate. The authors attributed these differences to a lower meal frequency and poorer quality of meals on the night shift. Takagi [23
] also showed that shift work led to a reduction in the number of meals eaten on the evening and night shifts, while workers on the day shift adhered to the more traditional three-meals-per-day. The apparent disagreement in research findings between Japanese and Scandinavian studies may be due, in part, to differences in data collection methods and analysis, making comparisons between studies tentative at best. At present there is no section devoted specifically to eating habits within the commonly used data collection tool, the Standard Shiftwork Index [24
]. Such a section in this Index would be informative, but has not been included before, probably because of concerns about lengthening an already comprehensive suite of questionnaires.
Shift work may also increase ‘snacking’ of energy-dense foods, depending on the shift that is worked. De Assis et al.
] found no significant difference in total food intake between morning, afternoon and night shift workers; nevertheless, morning shift workers ingested more energy and macronutrients in the morning than the other shift workers (). Afternoon and night shift workers had a greater energy intake at noon and at dawn, respectively, than morning shift workers. Night shift workers, like morning shift workers, consumed high-energy foods and drinks, high energy being obtained from carbohydrates, during their work shifts, rather than during the afternoon. It can be concluded from these data that, while total energy intake and composition is little affected by shift working over a 24 hr period, meal frequency is reduced but the prevalence of high-energy snacking is increased on the particular shift that is worked [25
Figure 1 Circadian rhythms of ingestion of total energy, proteins, fats and carbohydrates during three work shifts (morning, afternoon and night). Total energy intake did not vary between the shifts, but the timing of energy intake and macronutrients differed, (more ...)
Important factors which impinge on the dietary habits of the shift worker are the specific type of job, the work environment and the availability of food in the work place. Fisher et al.
] reported a significant influence of working arrangements and sleeping habits on meal times. Availability influenced most types of foods consumed. Other researchers have also found that work schedules have a greater influence than feelings of hunger in determining the timing and type of food intake [29
] (). Stewart & Wahiqvist. [30
] studied workers in the steel and aluminium industry and found inadequate canteen facilities, resulting in a greater reliance on vending machines. Shift workers who regularly brought food in to work generally consumed more lower-energy dense food items (defined as those containing < 1642 kJ or < 17 g of fat) than those using canteen facilities. On the night shift, workers were more likely to consume more items of higher-energy dense food than those on afternoon or morning shifts, and this also coincided with higher usage of vending machine facilities.
Table 1 Mean proportion (%) of night workers (n=43) who stated the reasons for not eating or eating, and determinants of food eaten during work and rest periods. The data are expressed as a proportion of the possible occasions when the option applied. Participants (more ...)
The decisions surrounding the diet of shift workers are obviously influenced by external factors such as work schedules and availability of food. However it is also important to consider the impact of shift work on dietary habits from a socio-behavioural perspective. Persson & Martensson [31
] used a critical-incident approach to explore nurses’ dietary habits, and found that this group of workers was influenced particularly by social interactions and by the disruption to their circadian rhythms. For example, the nurses often chose ‘healthy’ foods to help alleviate some of the gastrointestinal symptoms they experienced during the night shift. Other reasons for food choices included using high-sugar foods to help overcome tiredness and cravings whilst working on shifts.
Shift work may also disrupt the normal diurnal pattern of social and family life. This disruption may be particularly significant for females with children. These workers often exhibit a clear increase in the off-job workload, caused by domestic duties [32
]. Some of these duties may include the preparation of food for family mealtimes, which cannot be rescheduled to suit the shift worker and may be at odds with an individual’s sleep pattern or appetite. Indeed, it is these types of problem that should be considered when establishing key interventions to help improve the nutritional health of shift workers. In order to be successful in reducing both the health and social costs of the modern 24-hour society, recommendations with regard to meal timings and composition, aimed at alleviating the negative health consequences of shift work schedules, should also take into account individual and social barriers.
2.2. Biological disturbances associated with eating during shift work
It is important to consider how the altered eating behaviours of shift work impact on metabolic and endocrine responses, and other nutritional health parameters which are of particular importance in the development of disease. From a circadian point of view, the body is not designed for a nocturnal intake of energy and nutrients. Glucose tolerance decreases from morning to night and nocturnal eating increases the LDL/HDL cholesterol ratio, [33
] which is a cardiovascular risk factor. Plasma triacylglycerol, the major lipid component of dietary fat, also appears to be influenced by the circadian clock, with higher levels observed during the night [34
]. Nocturnal eating has been shown to cause an increased triacylglycerol response after a nocturnal meal compared to one eaten during the day. Morgan et al.
] administered a constant routine protocol to participants and reported that the concentration of plasma triacylglycerol increased by 0.2 mmol/l at night. It was postulated that such an increase might be due to the decrease in nocturnal insulin sensitivity that was also observed by these researchers. Insulin is an important activator of lipoprotein lipase, the key regulatory enzyme involved in triacylglycerol clearance and hence a reduction in sensitivity may result in a similar reduction in lipoprotein lipase activity. This notion is supported by the results of a later study on permanent night shift workers in Antarctica. Test meals were used to determine postprandial hormone and metabolic responses during the daytime, at night and at the beginning of a night shift. The lack of adjustment of circadian rhythms to the night shift was associated with increased insulin resistance and lipid intolerance [36
]. The hypothesis that reduced insulin sensitivity may be a factor in reducing triacylglycerol clearance during the night is also supported by more recent work [37
]. Similarly, further evidence that shift work is associated with elevated lipid levels has been presented by past researchers [3
] and in more recent studies [19
While reduced insulin sensitivity may mediate a reduction in triacylglycerol clearance, it is also a risk factor itself in the development of diabetes. At night, there is a reduced glucose utilisation, lowered insulin secretion rates and a reduction in insulin sensitivity [37
]. The fall in glucose utilisation during early sleep is due to a decrease in brain glucose metabolism during slow wave sleep (which is concentrated into the first part of sleep) as well as reduced muscle tone and the anti-insulin effects of growth hormone [42
]. This finding has implications for shift workers who eat during the night, since it is known that there is less insulin secretion for a given glucose stimulus in the night compared to the daytime [43
]. Sleep deprivation itself may also cause decreased insulin sensitivity. Spiegel et al.
] found that the peak glucose response during the few hours after breakfast was higher by 15 mg/dl during six 24-h periods of restricted (4 h) sleep compared to a period of normal sleep.
Gastrointestinal activity may also be disturbed as a result of nocturnal eating. There is a reduction in gastric emptying time at night [44
] and a reduction in pancreatic polypeptide, which is an indicator of gastrointestinal tract activity [45
]. Hirota et al.
] found that exposure to bright light in the evening reduced the absorption of dietary carbohydrate. These last findings suggest that entrainment signals such as the light-dark cycle, which have an altered timing relative to the body clock during shift work, may affect metabolism of food. Such a hypothesis has not been formally tested to date.
It is also relevant to consider the impact of shift work on endocrine function, especially those hormones involved in energy balance. The adipocyte-derived hormone, leptin, tends to inhibit food intake, whereas ghrelin tends to stimulate it [47
]. Leptin and ghrelin secretions may be related to the sleep-wake cycle and exhibit a circadian rhythm. Both sleep deprivation and shifts in the timing of the sleep period alter plasma concentrations of these hormones [48
]. Some authors have suggested that mealtimes influence leptin levels to a greater degree than ghrelin. Fogteloo et al.
] showed that changing meal frequency from 3 to 8 times per day influenced the 24-hour rhythm in leptin. Schoeller et al.
] also observed phase shifts in plasma leptin levels as a result of changing meal patterns. In an investigation more relevant to shift work, Qin et al.
] studied the effects of a nocturnal lifestyle on serum leptin. Night-time consumption patterns chosen for this experiment were similar to those observed in shift workers. Participants consumed more than 50% of their total energy intake in the evening and night, with a greater frequency of eating but less food eaten at each time. This pattern of eating was thought to have resulted in an inadequate suppression of appetite, as evidenced by the decrease in the nocturnal peak of plasma leptin levels.
2.3. Dietary interventions during shift work
In a report by the World Health Organisation, the links between diet and nutrition on chronic diseases such as obesity, type 2 diabetes, cardiovascular disease, cancer, dental disease and osteoporosis were highlighted. It was proposed that nutrition ‘should be placed at the forefront of public health policies and programmes’ [52
]. There is a clearly-established link between inadequate diet and the development of disease for people living a ‘normal’ diurnal existence. Given the altered eating habits as well as metabolic and endocrine profiles of shift workers outlined in the previous section, it seems even more important to provide evidence-based nutritional recommendations for this population. While there are a number of studies relevant to the effects of shift work on dietary habits and intake, few researchers have been able to formulate evidence-based recommendations for shift workers. The formulation of any recommendations should be based on knowledge about the metabolic consequences of shift work as well as tolerance to shift work at the individual and socio-behavioural levels.
In one of the few studies that we could locate, Love et al.
] investigated the effects of altering meal composition on subjective alertness scores during shift work. These scores improved significantly with a test meal of 46% carbohydrate and 42% fat during a night shift, compared with a ‘baseline’ meal (56% carbohydrate; 28% fat). These findings concur with those from studies on cognitive performance during the daytime. Nevertheless, the long-term consequences of raising the amount of dietary fat whilst working shifts are still unknown. Again, there is a need for more multi-disciplinary research (involving shorter-term and longer-term outcome variables) on nutrition during shift work.
Future researchers should also address the fact that shift work and night work impose a fundamental clash with normal diurnal existence. It is not possible or practical for many shift workers and night workers to adjust their circadian rhythms or lifestyles sufficiently to be able to synchronise with such an altered existence, especially when working rapidly-rotating shift systems.