Animal research has well established that a link exists between variations in corticosteroids and the proneness to excess body fat accumulation. Accordingly, it is known that adrenalectomy is an efficient approach to counteract weight gain in most animal models of obesity. In humans, the association between variations in corticosteroids, its stress-related environmental effects and the predisposition to obesity is more difficult to demonstrate. In this paper, we propose that this relationship is accentuated by globalization and modernization which favour a labour context imposing additional stress and changes in life habits promoting a positive energy balance. Our main hypothesis is that the increase in knowledge-based work, and the decrease of quality and duration of sleep both induce an increase in cortisolaemia and glycaemia instability, which results in an increase in food intake, a reduction in energy expenditure and body fat gain. The authors of this paper believe that, from a socioeconomic perspective, globalization leads every nation of the world in conflict with itself and may consequently represent a real problem. On one hand, there are preoccupations related to productivity and money making. On the other hand, people have to adopt a daily lifestyle leading to hyperphagia and decreased energy expenditure in order to maintain their economic competitiveness.
Some decades ago, researchers acknowledged the important role of stress in physiological changes of the body. Already in 1929, Cannon described how mechanisms responsible of the fight or flight response could induce bodily changes such as rapid beating of heart, hurried respirations and trembling of muscles (1). With time, some studies started to further investigate the effects of stress on numerous mechanisms of the body functioning (2). Science then led to a better understanding of the underlying reactions to stress and how these responses could interact with energy balance. Nowadays, it is accepted that glucocorticoids (GC), hormones released in stressful situations, may be a dominant factor responsible for the predisposition to obesity and more specifically to abdominal obesity (3,4). For example, Shively and her colleagues exposed cynomolgus macaques to chronic stress and subsequently showed higher fat deposition in the central portion of the body in socially subordinate animals who received constant aggression by dominant monkeys (5). In a similar manner, the ever-growing requirements of the society in terms of productivity may restrain an individual to a career-oriented life and to the sedentary lifestyle that results from it. As the need to succeed systematically strives to achieve higher standards, there is now evidence that stress in humans increases because of such expectations (6). Yet, stimulation of the hypothalamo-pituitary-adrenal axis (HPA) by stressors leads to the release of GC, mainly cortisol, in the blood circulation, as demonstrated by Bjorntorp (3). This central hormone has important effects implicated in the metabolism of adipose tissue (7).
Other sources of perturbation such as sleep curtailment, low levels of physical activity and demanding mental work have emerged from modernization. These new physiological stressors can accentuate energy intake (EI) and fat anabolism. The consequences of such conditions on health can ultimately favour the development of abdominal obesity and diabetes. In this regard, the main preoccupation of this paper is to discuss recent literature relevant to the new modality of labour, particularly in a context of globalization, and to describe how this phenomenon, associated with modernization, may exaggerate the predisposition to obesity.
Globalization and modernization: a new reality accentuating the importance of mental work
The term globalization is used to describe the increased mobility of goods, services, labour, technology and capital throughout the world (Government of Canada 2007). With the development of technology and communications, globalization has known an important progress since the 1970s. This new reality mostly translates itself by a rapid economic expansion of financially integrated countries. From 1970 to 1999, average output per capita rose almost three times in the group of nations financially integrated (7). This pattern also applies to consumption and investment growth (7). Accordingly, Berg and Krueger (8) concluded in a recent study that trade openness also greatly contributed to growth. For these reasons, we can conclude that globalization is an omnipresent trend that has enhanced the economy of numerous countries. In the same way, it contributed to the wealth of nations, globalization has also influenced the work organization through modernization (9). Indeed, the Institute for Occupational Safety and Health (NIOSH) attributes the intensification of work and the rapid change of demands to globalization (9). A model was developed by NIOSH to illustrate how the external context, which includes globalization and modernization, affects working modalities via the organizational context. Indeed, the physical and mental characteristics, as well as the social-relational aspects of work, are influenced, although indirectly, by globalization and modernization (10). The same document even describes a range of new organizational practices that employers have implemented in response to globalization in order to compete more effectively. High performance work systems are an example of organizational restructuring that was implanted following globalization, according to NIOSH. Some argue that modernization offers increased responsibility and learning opportunities, or in other words, increased mental effort through longer or irregular working hours (8). The Tokyo Declaration focuses on the consequences of these modernized changes. According to these experts (11), the frantic pace of work and life, the erosion of leisure time, and the blending of work and home time are driven by economic and technological changes caused by globalization and motivated by short-term productivity and profit gain.
The authors of this paper believe the ‘quest for the best’ mentality greatly valued by developped societies compels individuals to constantly strive to perform at their best. With industrialization and computerization almost totally replacing physical work, this competition can take more and more the form of cognitive performance. Concretely, Bockerman et al. (2008) reported that the proportion of sedentary jobs nearly doubled in Finland (24.9% and 28.4% for men and women in 1972 vs. 48.3% and 52.9% for men and women in 2002, respectively) to the detriment of physically demanding jobs (54.5% and 38.1% for men and women in 1972 vs. 31.7% and 20.5% for men and women in 2002, respectively) over the last 30 years of the 20th century (12). In the context of modernization, the occurrence of such mental performance is now rapidly growing among professional practices. At least 76 million people in the United States used a computer during working hours in 2003, which accounts for at least 56% of all employed adults (13). A growing proportion of computer-user workers are in professional positions and it has been estimated that more than 25% of US employees now work on a computer for more than half their working day (14). Considering that this kind of labour could be considered a form of mental effort (15), the stressful effects of mental performance are inescapable. Furthermore, the impact of irregular working hours is a precipitated and unforeseen consequence of globalization (16). An earlier paper reported that many European countries experienced a rise in weekend work during the 1992–2001 period (17). The proportion of shift workers is now estimated to be over 20% (16). According to 2004 data from the Bureau of Labor Statistics (18), almost 15 million Americans now work full time on irregular schedules. Thus, irregular working schedules and long hours spent on computerized equipment appear to be increasing occupational health problem throughout the modernized society. According to the Australian Institute for Family Studies, there is an increase in the average hours worked by full-time employees in Australia since the late 1970s (19). A more recent study found that the average weekly working hours in 2004 for male lawyers was 50.9 h and 48.1 for female lawyers (20), supporting Yang in his claim that computerized work environment may have the effect of increasing the workload within the office (21). Swedish statistics demonstrate the cases of musculoskeletal injuries induced by computer work increased by 20% between 1992 and 1998 (22), thus suggesting an elevated number of hours spent on computer devices. Taken together, these observations suggest that the technology made available by computerization of office-based work may enhance stress through high mental work.
A recent study (23) assessed the excretion rates of stress hormones in occupational groups under mental work. Very high excretion rates of adrenaline, noradrenaline and 11-oxycorticosteroids were observed in occupational groups working under high mental effort, high responsibility, making important managing decisions and having low control (24). While adrenaline peaked early, noradrenaline levels were higher later on during the mental task. A twofold increase in adrenaline was observed in men in comparison with women during mental work. Gender differences in the response to mental stress are then possible and implications for body weight regulation are further addressed in section 4.1. Peters et al. (25) investigated the effects of mental work and the controllability of a task on cardiovascular response and found that high mental effort and low control over a task lead to higher cortisol, blood pressure and noradrenaline response.
It is now of common knowledge that long-term effects of such mental stimulation, if related to stressors, could be related to cardiovascular disease (15,26–29). Some studies have demonstrated that symptomatic or silent myocardial ischaemia in patients with coronary artery disease can be induced by mental stress (30,31). The relationship between chronic mental stimulation and cardiovascular disease is solidly established to the point that an excessive heart rate elevation provoked by mild mental stress can even predict long-term risk for sudden cardiac death (32). Many studies describe the cardiovascular response to knowledge-based work (KBW) and show a common pattern that can be resumed as an increase of heart rate and blood pressure (15,26–29). In addition, the difficulty of a memory challenge can alter cardiovascular response (26). Under moderately difficult mental conditions, there is an increasing high blood pressure response in high mental tasks when success importance is high. In a context in which mental work may influence cardiovascular response, this also raises the question as to how globalization can ultimately increase the risk of other diseases such as diabetes.
Risk of glycaemic instability, glucose intolerance and diabetes with globalization and modernization
Mental work solicits neurons which increase their glucose utilization under normal feeding conditions. As the carbohydrate stores are limited, it can be deduced that a relationship might exist between mental work and increased fluctuations in plasma glucose and insulin levels. In a preliminary study, a participant undertook two independent sessions of 60 min. In the first session, he had to spontaneously dictate a text submitted for a grant application while in the other, he rested. Every 15 min, plasma glucose and insulin concentrations were determined. As illustrated in Fig. 1, the results revealed absolute changes in plasma glucose and insulin of, respectively, 2.2 and 8.3 times greater in the oral communication task (33). These results were confirmed in female university students who displayed an increase in plasma glucose and insulin instability when performing two tasks imposing a significant cognitive effort (34).
While an elevated glucose concentration in blood can diminish hunger (35), a decreased level can stimulate episodes of feeding. Even though hypoglycaemia has strongly been associated with hunger, decreasing values of blood glucose that do not necessarily go as low as those of a medical diagnosis of hypoglycaemia may trigger desire to eat (36). This state of mild hypoglycaemia may predispose individuals to glucose intolerance and type 2 diabetes (37). Long-term instability of plasma glucose, possibly observed in chronic mental work conditions, can predict the onset of diabetes (38). Indeed, as lower glucose concentrations can stimulate hunger and food intake, this phenomenon can be related to weight gain over time (37) and insulin resistance (39). With glycaemic instability being an important characteristic of diabetes, the risk of developing such a health problem becomes prominent (40). As discussed in the next section, the phenomenon is also linked with the impact of mental effort on energy balance and its related risk of overweight.
Globalization and modernization influence energy balance
In the context of modernization, working manners have progressively changed from physically demanding tasks to mentally challenging work. This evolution is not without consequence on energy balance and might even be partly responsible for the obesity and type 2 diabetes epidemics by contributing to a positive fuel balance in numerous ways. Energy balance, regulated by EI and energy expenditure (EE), is greatly affected by the consequences of globalization such as increased sleep deprivation, low levels of physical activity, elevated mental work and stress response. For example, a study from Mathieu et al. (41) analysed the impact of activity-related factors on body weight status in 300 children. The criteria related to these factors were a minimum of 10 h of sleep per night, at least 16 500 steps per day of physical activity, 2 h or less of screen time and over 30 min of school homework per day. Children were more prone to have abdominal obesity when they performed over 30 min of school homework per day. The results also showed a significant increase in prevalence of obesity when children were exposed to over 2 h of screen time per day. Two-thirds (64%) of children who did not adhere to any of these four guidelines had a body weight status corresponding to obesity (41). Thus, it seems that the profile of daily activities which prepares children to deal with globalization is also related to a greater risk of excess body weight.
Effects of knowledge-based work on energy intake
An increase in desire to eat with mental work was observed in our preliminary case study (33), which agrees with the results of Wallis and Hetherington who reported a 15% increase in chocolate intake after a cognitive task compared with a control session (42). In animals, emotional stress was found to accentuate saccharine consumption whereas the opposite change was induced by physical stress (43). Similarly, busy periods in a department store which corresponds to a working schedule of 47 h a week with a substantial stress level was associated to higher sugar and EI than with low-workload periods (32 h per week) (44). Although we might be tempted to attribute this higher EI to a possible elevated EE in a busy department store, a study confirms that even sedentary employees occupied in writing computerized documents show a similar pattern in high-workload periods (45). In addition, irregular working hours, imposed by the pursuit of productivity and flexibility of competitive industries, are associated with obesity. Weight gain in shift workers can possibly be explained by the consequences of modernization, such as higher calorie intake, lower physical exercise and changes in sleep habits (16).
A longitudinal study confirmed that increased mental strain at work is associated to weight gain. Over a 10-year period, men with increased mental solicitation gained 4.2 kg vs. 2.4 kg for the ones with stable or reduced mental strain (P < 0.05) (46). As pointed out earlier, there are gender differences in increments of adrenaline levels (21) and this could potentially explain why weight gain did not differ for women exposed to different levels of mental solicitation between 1973 and 1983 (46). Therefore, gender specificities would warrant specific considerations in further studies.
The data of Chaput and Tremblay (2007) allowed a quantification of the acute effect of KBW on EI (47). In this study, 15 students participated in a two-session protocol of 45 min followed by a buffet meal giving free access to food. The sessions were either a rest period in a sitting position or a cognitive task, also performed in a sitting position, which consisted of reading a document and writing a summary of 350 words using a computer. It was found that mean ad libitum EI after the mental task exceeded by 959 kJ the intake measured after the resting session. As the subjects did not compensate by eating less for the rest of the day and the energy cost of the cognitive task exceeded by only 13 kJ the cost of the control session, there was a substantial net positive energy balance.
Our previous investigations also allowed to compare the acute effect of mental work on energy balance to that of physical exercise of different intensities. As depicted in Fig. 2, results indicate a negative energy balance for high and low-intensity exercise and a positive energy balance for KBW (47). This was established by calculating relative EI, i.e. by subtracting EE of the task above resting metabolism from ad libitum EI in the buffet-type meal after each task. Thus, mental work has the potential to increase EI and can further exaggerate the positive energy balance that occurs in inactive individuals. This is in opposition with the negative energy balance induced by physical activity which contributes to the prevention of obesity. Physical activity also improves body functioning by stimulating key enzymes, increasing hormone sensitivity and facilitating substrate transport through cell membranes (33). Physical activity practised on a regular basis has also been proven to attenuate hypoglycaemia (48). Therefore, the transition from physical to mental work in our daily activity schedule is not a good news regarding the control of EI. In fact, it replaces a source of stimuli facilitating appetite control by another one which rather acutely promotes hyperphagia.
The experimental work performed by Mayer provides a plausible explanation of this acute effect of cognitive effort. It suggests that the detection of variation in blood glucose concentrations by glucoreceptors can ultimately influence energy balance by affecting short-term control of hunger and food intake (36). Accordingly, KBW solicits the brain which only uses glucose as a source of energy under habitual feeding conditions (49). Because the reservoir of this nutrient is limited to the daily intake of carbohydrates (49) and the ability of the body to synthesize this macronutrient is limited (49), a reduced availability of glucose caused by its brain utilization during mental work could lead to perception and expression of hunger and food intake (36). This hypothesis agrees with the glucostatic theory of appetite control which states that hypoglycaemia and a low level of liver glycogen can induce meal initiation (36). Taken together, these observations suggest that mental work might have the ability to destabilize the regulation of plasma glucose and to increase food intake.
Furthermore, Fig. 3 depicts the impact of KBW on cortisol levels (mmol L−1), EI (kcal) and glucose instability (mmol L−1) in relation to the intensity of mental work, as measured by the reaction time to a second task. As illustrated, a demanding cognitive work, assessed by a longer reaction time, is associated with higher cortisol levels, elevated EI and more pronounced variations in glycaemia. This is in accordance with other studies which showed that higher anxiety perceived and cortisolaemia measured during a mental task is linked to higher subsequent EI (50,51).
Cognitive work and energy expenditure
Some studies assessed the energy cost of agricultural activities in male workers and found mean values of daily EE of 10 868 kJ d−1 when agricultural activity was low and 14 200 kJ d−1 for high activity (52). Another study demonstrated that the energy cost of physical activity in Gambian women during peak agricultural activity was 2.5 times greater than comparative values from inactive women living in similar conditions (53). In contrast, the mean EE while seated at work in an office chair is about 301 kJ h−1(54). Accordingly, Chaput and Tremblay (47) found an EE of 243 kJ during a 45-min resting period and 256 kJ for a KBW trial. Currently, direct measurements show that sedentary workers sit on average nearly 10 h per day (55). To restore the positive energy balance of obese desk workers, Levine and Miller (2007) introduced a vertical computer station that allows individuals to walk on a treadmill while working (54). The EE at rest, working in a seated office chair, standing and working on the vertical workstation while walking at a self-selected pace on the treadmill were compared in 15 sedentary obese individuals. The EE for the ‘walking while working’ session ‘burned’ an additional 497 kJ h−1 compared with the seated working session. EE can also be increased by 6% simply by replacing the regular office chair by an exercise ball (56). Thus, the data again provide clear indication that the regulation of energy balance becomes more difficult in a context of modernity. Indeed, even if cognitive effort might be very demanding, its additional energy cost is trivial.
Knowledge-based work, stress and fat metabolism
Mental work may accentuate EI (33,47) because its related cognitive effort represents a neurogenic stress which increases food consumption. As hypercaloric load can inhibit stress responses in the HPA axis (57), the explanation in which food serves as a consolation to emotional stress is possible.
Mental stress related to cognitive performance can affect fat metabolism in numerous ways. GC play a significant role in the control of HPA axis activity and the termination of the stress response (3,4,58). Cortisol and insulin together stimulate lipoprotein lipase and inhibit hormone sensitive lipase leading to storage of triglycerides and decreased mobilization of free fatty acids. In addition, increased glucocorticoid receptor density, enlarged adipocytes, increased cortisol concentrations and decreased lipolysis accentuate energy storage (3,4,58). GC can also suppress growth hormone (GH) and antagonize their action on lipolysis. In the end, visceral fat accumulation occurs, which inhibits GH secretion and causes further insulin resistance, thus participating in a vicious cycle (3,4,58). GC are normally regulated by the HPA axis but may also be produced locally from adipose tissue therefore inhibiting, once again, lipid mobilization in the presence of insulin. Furthermore, GC can enhance the neuropeptide Y concentration which has an orexigenic role and induces leptin secretion as well as leptin resistance, thus blunting the reduction of food intake (3,4,58).
Sleep and the proneness to diabetes and obesity
Another potential effect of modernity lies in poor quality and short-duration sleep. Elements responsible for sleep deprivation which include longer work days, an increase in evening and night work, the advent of computer and the Internet and a decrease in physical activity (59), are all in some way related to globalization and modernization. Statistics from ‘Sleep in America’ reveal that 44% of working adults report sleeping less than 7 h, and 16% less than 7 h on weeknights (60). Moreover, an Australian study confirms that an overall decline in sleep duration of approximately 30 min has been observed between 1985 and 2004 (61). Thus, sleep curtailment is a common behaviour in modern societies. Similarly, the elevated incidence of obesity and diabetes mellitus has developed over the same period of time as the progressive decrease in sleep (62). With sleep having a major role to play in the secretion of hormones and glucose metabolism, some evidence indicates that sleep deprivation can promote obesity and type 2 diabetes (63). Spiegel et al. (63) described how the effect of sleep loss on hormone secretion depends on the stage of sleep affected. For instance, low slow-wave sleep duration is related to diminished glucose tolerance. After restricting sleep to 4 h per night, glucose tolerance decreased by 40% because of a reduction of 30–40% in glucose effectiveness and a near 30% decrease in the insulin response to glucose (64). Self-reported short sleep duration was even associated with the prevalence of diabetes (65).
In six studies that have assessed the effect of sleep duration and quality on the risk of diabetes, an elevated incidence was predicted by short sleep duration in four of them, whereas poor-quality sleep was associated with an increased risk of diabetes in five of the six studies (66). Some evidence indicates that sleep curtailment and obesity are related. Reduced sleep up-regulates ghrelin, an appetite-stimulating hormone, and down-regulates leptin, a hormone responsible for the sensation of satiety, and therefore increases hunger and food intake. In a study in which participants slept two nights of 4 h and then two nights of 10 h, results showed a decrease in leptin levels of 18% and increased hunger of 23% in the short-sleep trial (67). Reduced physical activity attributable to reduced percentage of time spent in high-intensity activities is another important change resulting from sleep deprivation (68). The results obtained in our population studies agree with the notion that short sleep duration increases the risk of excess body weight. In the Quebec en Forme Project, a negative association was observed between body weight and sleep duration in boys (69). In addition, short sleep duration was a better predictor of overweight compared with TV viewing and sedentariness (69). Interestingly, the increase in body weight related to short sleeping was accompanied by an increase in waist circumference, suggesting a preferential abdominal fat accumulation (70). The testing of men and women participating in the Quebec Family Study confirmed the greater predisposition of short sleepers to be overweight and revealed that their plasma leptin levels are significantly lower than what is predicted by their body fat mass (71). This study also demonstrated a more pronounced state of hypoglycaemia in short sleepers (72) which explained to a significant extent their greater risk to develop glucose intolerance and diabetes (73).
In summary, the recent study of the effects of variations in sleep duration and quality on glucose metabolism and energy balance emphasizes the importance of adequate sleeping on metabolic fitness. Unfortunately, the turbulent environment to which we are currently exposed is more inclined to deteriorate than to improve this relationship.
Globalization and modernization have been an opportunity of significant economic growth for many countries of the world. As described in this paper, such an achievement cannot be reached without a major personal involvement for many individuals. In this regard, it is particularly relevant to emphasize that global competitiveness in a modern world necessarily relies on KBW and cognitive effort. Many studies have shown that KBW is a human activity that perturbates energy metabolism and its hormonal control in a way that promotes hyperphagia and increases the risk to develop some diseases. The reality of globalization is stressful not only because of its related cognitive work but also because of some related practices that are also a source of metabolic perturbation. Moreover, the decrease in sleep duration and sleep quality that has been observed in many countries since several decades also represents a source of stimulation favouring an increase in hunger and the risk to be overweight.
We propose in this paper that a major underlying mechanistic explanation of these new unsuspected determinants of obesity pertains to a role of GC which represent one of the most potent orexigenic messengers of the body. In our opinion, globalization and modernization may contribute to reinforce this reality. In humans, it creates new modalities of living that impose neurogenic stress and that likely increase the impact of GC as a determinant of the obesity epidemic. This also helps perceiving that the obesity epidemic is a reality that leads mankind in a conflict with itself as globalization and modernization seem to promote per se some stimuli favouring a positive energy balance.
Conflict of Interest Statement
There is no conflict of interest to declare.
Lysa Huneault was supported by a research grant from CIHR. Angelo Tremblay is partly supported by the Canada Research Chair in Environment and Energy Balance.