The Association between Energy Balance-Related Behavior and Burn-Out in Adults: A Systematic Review

Although it is believed that physical activity, sedentary, and dietary behavior (i.e., energy balance-related behavior) may decrease the risk of burn-out, the association between both is currently not well understood. Therefore, the aim of this systematic review was to synthesize studies investigating the relationship between energy balance-related behavior and burn-out risk. A systematic literature search was conducted in four databases, resulting in 25 included studies (ten experimental and 15 observational studies). Nine out of ten experimental studies showed that exercise programs were effective in reducing burn-out risk. Fourteen out of fifteen observational studies found a negative association between physical activity and burn-out risk, whereas one study did not find a relation. Two of the 15 observational studies also showed that being more sedentary was associated with a higher burn-out risk, and two other studies found that a healthier diet was related to a lower burn-out risk. No experimental studies were found for the latter two behaviors. It can be concluded that physical activity may be effective in reducing burn-out risk. The few observational studies linking sedentary and dietary behavior with burn-out risk suggest that being more sedentary and eating less healthy are each associated with higher burn-out risk. More high-quality research is needed to unravel the causal relationship between these two behaviors and burn-out risk.


Introduction
Over recent decades, the prevalence-but also the recognition-of burn-out has increased enormously [1,2]. Burn-out often leads to absenteeism and presenteeism at work [3][4][5], and so it is an increasing concern in today's workplaces [6]. In the European Union, work-related stress costs EUR 25.4 billion annually, whereas globally, burn-out and stress cost more than USD 300 billion every year [7,8].
Burn-out can be defined as a "prolonged response to chronic emotional and interpersonal stressors on the job, determined by three dimensions: emotional exhaustion, cynicism or depersonalization, and professional (in)efficacy or personal accomplishment" [9]. People experiencing burn-out are mainly mentioning feelings of mental and physical exhaustion, low mood and lack of energy, and therefore with people experiencing high exhaustion could not be made due to the lack of highly exhausted people. Previous research has already demonstrated the mediating role of neurotransmission in the relationship between diet and mental health. For example, the administration of tryptophan increases brain serotonin synthesis, which, in turn, influences serotonin-dependent brain functions such as mood [37]. Tryptophan can be found in foods such as poultry, milk and some seeds. Likewise, the administration of tyrosine increases the production and release of dopamine and norepinephrine [37], which was found to be diminished in exhausted people. Tryptophan can be found in foods such as poultry, milk and some seeds. Likewise, the administration of tyrosine increases the production and release of dopamine and norepinephrine [37], which was found to be diminished in exhausted people. An increased production and release of this amino acid can be useful in enhancing performance during highly stressful situations. Tyrosine can be found in foods such as dairy, meat and fish. Secondly, studies showed that glucose administration and dietary carbohydrates enhance cognitive performance [38][39][40], while Chung et al. [41] suggested that also a mixed-grain diet can be beneficial for cognitive performance. This latter study also found beneficial effects of mixed-grain diet on plasma BDNF levels, which are decreased in people with burn-out. Lastly, omega-3-supplementation is associated with mood state, which results in an increase in feelings of vigor and a decrease in feelings of anger, anxiety, fatigue, depression and confusion [42]. The abovementioned physiological mechanisms hypothesize the preventative and healing functions of energy balance-related behavior towards burn-out.
It is clear that-given the rise in incidence and prevalence of burn-out-effective interventions are urgently needed. Improving energy balance-related behavior may be a promising strategy to counter burn-out. Although two recent (contradicting) systematic reviews [1,6] on the single association between physical activity and burn-out have been published, to date, no overview of studies investigating the relationship between energy balance-related behavior from a holistic point of view (including physical activity, sedentary and dietary behavior) and burn-out is available. Therefore, the aim of the present systematic review is to synthesize studies investigating the association between energy balance-related behavior and burn-out.

Materials and Methods
This review is registered in PROSPERO with registration number: CRD42019124458.

PICO Statement
The present systematic review investigates the association between energy balance-related behavior (i.e., physical activity, sedentary and dietary behavior) (=exposure or intervention) and burn-out (=outcome) in adults (=population).

Eligibility Criteria
Results were limited to original English-language articles published in full-text format in peer reviewed journals until January 2019, assessing the direct relationship between energy balance-related behavior (i.e., physical activity, sedentary and dietary behavior) and burn-out or one or more of its components (i.e., emotional exhaustion, cynicism and inefficacy) in working adults between 18 and 65 years old. Meta-analyses, systematic reviews, methodological papers, congress proceedings, meeting abstracts and case studies were excluded from the search. Studies were also excluded when investigating other affective disorders than burn-out (such as depression, bipolar disorders and anxiety disorders), investigating the influence of an intervention including more than only physical activity (such a mediation exercises, workshops, etc.) on burn-out levels, including non-working adults such as people in prison, students, athletes, etc., investigating the association between burn-out and disordered eating behaviors (such as emotional eating, uncontrolled eating, etc.), or investigating the association between burn-out and alcohol.

Selection of Studies
After completing the search in each database, all references were imported into EndNote (=bibliographic software program) and then exported to Rayyan (=bibliographic software program designed to facilitate systematic reference selection), in which the study selection was conducted. The study selection included the screening of titles, abstract and full-texts and conducting a forward and backward search. The search and study selection were conducted in January 2019 by two researchers independently from each other (YV and EVDL). Any doubts or disagreements between the two researchers were discussed with a third researcher (TD). The followed methodology was reviewed and approved by the head of the university library (KA). When important information in the articles was missing, authors were contacted via e-mail.

Quality Assessment
The 'Standard quality assessment criteria for evaluating primary research papers from a variety of fields' [45] was used to assess the methodological quality of the included studies. The checklist consists of 14 items which were given a certain score depending on whether or not the specific criterion was met ("no" = 0, "partial" = 1, "yes" = 2). Depending on the study design, some items were not applicable and were therefore scored as 'not applicable (N/A)' and excluded from the calculation of the total score. The total sum was calculated by summing the total number of "yes" multiplied by 2 and the total number of "partials" multiplied by 1. The total possible sum was calculated as follows: 28 − (number of 'N/A' × 2). Lastly, the summary score was calculated by dividing the total sum by the total possible sum.

Results
In total, 18,536 articles were found. When removing all duplicates, 4907 articles remained. Titles and abstracts were screened for eligibility. Of the 109 remaining articles, sixteen studies met the inclusion criteria. In addition, a forward and backward search was performed through which we identified six more studies. Another two articles were included after screening the reference lists of two previously published systematic reviews investigating the association between physical activity and burn-out [1,6]. One more article was obtained through hand search [46]. So, in total, 25 articles were included in the final synthesis. The flow chart of the search process is displayed in Figure 1. Nutrients 2020, 11, x FOR PEER REVIEW 5 of 27 The 25 included studies consisted of ten experimental and 15 observational studies. The experimental studies included five randomized controlled trials (RCTs), one randomized clinical trial, one quasi experimental and three pre-experimental studies. An overview of the included experimental studies is presented in Table 1. The observational studies included six longitudinal and nine cross-sectional studies. An overview of the included observational studies is presented in Table  2. The 25 included studies consisted of ten experimental and 15 observational studies. The experimental studies included five randomized controlled trials (RCTs), one randomized clinical trial, one quasi experimental and three pre-experimental studies. An overview of the included experimental studies is presented in Table 1. The observational studies included six longitudinal and nine cross-sectional studies. An overview of the included observational studies is presented in Table 2. The exercise intervention lasted for 10 weeks (4-5 days per week). The exercise intervention: a combination of aerobic exercise (cycling, stair climber, treadmill running) and resistance exercise. Aerobic exercise: participants started at 40-50% of age-adjusted maximum heart rate with 40-min sessions for the first 2 weeks, followed by 50-60% with 30-min sessions during the next 3 weeks, and 70% or higher with 20-min sessions for the last 4 weeks.
Resistance exercise: 4-5 sets of 4 exercises with a 30-second recovery interval between sets. The entire circuit had to be completed within 40 min. CG: no intervention.
Pre, post (week immediately after the intervention).
Emotional exhaustion improved significantly after the 10-week exercise intervention (−10%).  Multimodel treatment for 12 months and a special focus was placed on PA counselling: an 8-week group stress management program. All patients were given background information on the causes and consequences of chronic stress during a 2-h lecture. They were visiting every 4-6 weeks a physician and the program consisted of a 2 h lecture about stress-related mental disorders and the consequences on the individual and organisational level. The participants were also given comprehensive information of the effects of regular PA on stress-related exhaustion, opportunity to self-select their participation in an 18-week coached group exercise-program. Exercise program: Nordic walking for 1 h and a light strength-training program performed at the clinic once a week. No CG.
Burn-out: SMBQ. PA activity: at baseline: with the SGPALS, at follow-up: "How often did you exercise during the last 3 months?" and "How hard did you normally exercise during the last 3 months?" and "How many minutes did you engage in activity?" Mild and strong compliers reported significantly lower burn-out at the 18-month follow-up than the non-complying group (pre-18 months: non-compliers: −27.7%; mild-compliers: −47.8%; strong compliers: −41.3%; p < 0.017).   Emotional exhaustion: it was assessed with one of the three subscales from an adapted Dutch version of the MBI. Strenuous PA: "How often within the past four months did you participate in strenuous sports activities or strenuous physical activities that lasted long enough to become sweaty?" The amount of sedentary work: participants reporting sitting during the largest part of the working day = workers with a sedentary job, participants not reporting sitting during the largest part of the working day = worker with a non-sedentary job.
All workers who engaged in strenuous PA at a frequency of one to twice a week were at a significant lower risk of emotional exhaustion than workers who engaged in strenuous PA less than once a month. This association was stronger in workers with a sedentary job. PA at a frequency of once to twice a week was significantly associated with a reduced risk of future emotional exhaustion, this was not the case for PA at a higher frequency.  Work-related fatigue: five-item 'exhaustion'-subscale of the Dutch version of the MBI. PA: questionnaire based on international standards for PA: "On how many days a week are you normally physically active during at least 30 min a day during your work and free time together (only count PA that is equally demanding as brisk walking or biking. Activities shorter than 10 min do not count)?" It was found that an increase in PA is associated with a decrease in work-related fatigue over time (β = −0.05, p <0.05). Cross-sectionally, work-related fatigue is negatively correlated with PA at T1 and T2 (T1: r = −0.08, p < 0.01; T2: r = −0.08, p < 0.001) Burn-out: CBI. Exercise behavior: 5 questions on a 5-point scale: 1 = never and 5 = many times in a week.
Example questions: "How many times in a week do you take part in sports that include aerobic exercise (e.g., basketball and running)?" and "How many times in a week do you engage in aerobic exercise for at least 30 min?" Work-related burn-out was negatively correlated with exercise behavior (r = −0. 22  Burn-out: SMBQ. PA: based on patients' self-reports. Consistent with the American College of Sports Medicine and the American Heart Association guidelines, they were asked how many days per week and how many minutes each session they engaged over the past month in strenuous PA (activity that increases the heart rate and brings on a sweat) during their leisure time.

Physical activity and sedentary behavior
Jonsdottir et al. [  The high-risk group has a more unhealthy lifestyle, meaning that they perform less physical exercise and they consume less healthy diets during work days compared to the low-risk group (sporting/physical exercise: high-risk group: 28% several times a week; p < 0.005; Healthy diet at working days: high-risk group: 29% every day; p < 0.005).

Physical Activity
All 25 studies assessed the relationship between physical activity and burn-out. Nine out of ten experimental studies found a positive effect of physical activity on risk of burn-out. More specifically, five studies found a reduction in risk of burn-out in general (i.e., all three dimensions combined) due to physical activity (reduction ranging from 6.9% to 41.3%) [49,[52][53][54][55]. Two studies assessed the effect of physical activity on all three dimensions of burn-out separately, reporting a positive effect on emotional exhaustion (−33.4% and −10%) and cynicism (−33.3% and −4%), but not on personal accomplishment [51,56]. However, one other study also assessed the effect of physical activity on emotional exhaustion, cynicism and personal accomplishment separately and did not find a significant effect on any dimension [50]. Two of the ten experimental studies assessed the effect of physical activity on emotional exhaustion only and showed a significant improvement in emotional exhaustion (−21.6% and −10%) [47,48].
Four experimental studies also conducted follow-up measurements, with three studies reporting a decrease in risk of burn-out three and six months after the physical activity intervention [47,53,56]. The fourth study equally showed a decrease after six months and this maintained after 12 months [52]. Four studies compared physical activity interventions with other interventions such as multimodal rehabilitation and basic care, but no significant differences in the decrease in burn-out risk between these interventions were reported [49,52,54,56].
All 15 observational studies assessed the relationship between physical activity and risk of burn-out, or one of its components. Fourteen observational studies found a negative association between physical activity and risk of burn-out, or one of its components, and one did not find a relationship. More specifically, ten studies assessed the relationship between physical activity and risk of burn-out generally [21,61,63,64,[66][67][68][69], of which nine found a negative association and one did not find a significant relation between both [65]. One observational study assessed both the relationship between physical activity and risk of burn-out and all three dimensions separately and found negative associations between physical activity and risk of burn-out, emotional exhaustion and cynicism and a positive association between physical activity and professional efficacy [57]. Four other studies found a negative association between physical activity and emotional exhaustion [46,58,60,70] and one of them also found a positive association between physical activity and cynicism [46]. Lastly, one of 15 observational studies concluded that the association between physical activity and risk of burn-out was stronger in workers with a sedentary job [58].

Sedentary Behavior
Two of the 15 observational studies including physical activity also assessed the relationship between sedentary behavior and risk of burn-out. Both studies found a positive association with risk of burn-out [67,68].

Dietary Behavior
Besides physical activity, two of the observational studies also examined the link between diet and risk of burn-out. In both studies diet was measured by a single question. The study by Alexandrova-Karamanova et al. [46] measured fast food consumption by asking how many times in a week people ate fast food, and concluded that fast food consumption was positively associated with risk of burn-out. In the study by Gorter et al. [69] dietary behavior was measured by asking how many healthy diets participants consumed during workdays, and found that the amount of healthy diets during workdays and the risk of burn-out were negatively associated.

Quality Assessment of the Included Studies
According to the "Standard quality assessment criteria for evaluating primary research papers from a variety of fields" [45], the mean article quality score was 0.82 ± 0.10 out of a total of 1 (Table 3). Eleven articles scored below the mean score with a minimum score of 0.61. Fourteen articles scored above the mean score with a maximum score of 0.95.

Discussion
In this systematic review, an overview of studies investigating the association between energy balance-related behavior and burn-out is provided. In total, 25 studies were found, of which 21 assessed the relationship between physical activity and risk of burn-out, two studies investigated the association between physical activity, sedentary behavior and the risk of burn-out, and two assessed the link between physical activity, dietary behavior and risk of burn-out. No articles using a holistic approach-i.e., investigating the relationship between energy balance and energy balance-related behavior as a whole and risk of burn-out-were found. Nevertheless, it is important to use a combined approach to gain more insight regarding, e.g., which behavior may have a bigger impact on burn-out risk. Moreover, as previously mentioned, van der Ploeg et al. [27] indicated that sedentary behavior and physical (in)activity should be targeted at the same time in public health strategies. The present systematic review, for example, shows that the association between physical activity and risk of burn-out was stronger in workers with a sedentary job, showing a possible interaction between both physical activity and sedentary behavior in relation to burn-out risk [58]. A similar interaction between both behaviors, but with mortality as the outcome measure, was found in a large-scale meta-analysis (including over one million men and women), where high levels of moderate intensity physical activity (about 60-75 min per day) eliminated the increased risk of death associated with high sitting time [71]. In addition, previous studies showed that increased screen time was associated with an overall poor diet quality [72,73]. The above indicates possible triangular interactions, again highlighting the importance of combining all energy balance-related behaviors when investigating their association with burn-out risk.
Furthermore, despite the fact that no studies investigating the relationship between energy balance (i.e., energy intake vs. energy expenditure [74]) and burn-out were found, a positive or negative energy balance may also be associated with burn-out. The interaction and co-existence of energy balance-related behaviors determine whether or not a positive or negative energy balance is experienced [75]. As energy imbalances may lead to the development of overweight and obesity (= physical health) [74], one may hypothesize that a similar imbalance may lead to decreased mental health as well. Previous research, in fact, demonstrates that obesity is associated with higher levels of burn-out [76].
Despite the heterogeneity of populations, assessment methods for both risk of burn-out and physical activity, and physical activity interventions, the vast majority of the experimental studies, including 5 RCTs, concluded that physical activity is effective in reducing the risk of burn-out, suggesting a causal link between both. The experimental studies showed a decrease in burn-out risk ranging from 6.9% to 41.3% [49,[52][53][54][55], a decrease in emotional exhaustion between 10% and 33.4% [47,48,51,56] and a decrease in cynicism of 33.3% and 4% [48,51]. One experimental study by Freitas et al. [50], showing a relatively low quality score of 0.63, did not find any significant effect of physical activity on risk of burn-out. This may be due to the small sample size (n = 21) and the duration of the physical activities performed in this study. The participants had to perform a 10-min workplace physical activity session on weekdays (no information on type or intensity was provided), while the World Health Organization (WHO) [16] recommends that adults should be physically active at a moderate intensity for at least 150 min per week or at a vigorous intensity for at least 75 min per week or an equivalent combination of both moderate and vigorous intensity activity. So, the total duration of the physical activities performed in the study by Freitas et al. [50] (ten minutes per day, for five days a week) was not meeting the above guidelines [77]. On the other hand, the study by Stenlund et al. [54], in which the performed physical activities (60 min, twice a week, at a moderate intensity) also failed to meet the WHO guidelines, did report a significant effect of physical activity on burn-out. It should be mentioned that, despite not meeting the recommendations, total physical activity duration in the latter study was still much higher compared to the study by Freitas et al. [50] (i.e., 120 min versus 50 min per week, respectively). In another experimental study [55], two groups of participants performed highand low-intensive physical activities, respectively, for 60 min twice a week (=120 min in total). The low intensive group also failed to meet the WHO guidelines. Nevertheless, both groups (high vs. low intensity) had more or less the same effect on burn-out (reductions of 8.1% and 8.2%, respectively) [55]. These findings suggest that lower (than recommended) amounts of physical activity of 120 min per week (at a low to moderate intensity) may already be effective in reducing burn-out risk. Furthermore, the role of intensity may be questioned. It should be mentioned, however, that the physical activities performed in the other experimental studies (showing a positive effect of physical activity on burn-out risk) were in line with the WHO guidelines, as they lasted 20 to 60 min for two to five times per week at a moderate to vigorous intensity [47][48][49]51,53]. The remaining two experimental studies did not give clear information about the duration of the physical activity sessions [52,56]. Further, four experimental studies [47,52,53,56] also conducted follow-up measurements and showed a decrease in risk of burn-out three, six and 12 months after the interventions, indicating long-term effectiveness, even when the physical activity intervention did not remain in place.
Our results are in line with the systematic review by Naczenski et al. [1] showing strong evidence for the effect of physical activity on reducing (emotional) exhaustion, but limited evidence for the effect on professional efficacy and cynicism. Further, Naczenski et al. [1] concluded that being physically active one or two times per week for four to 18 weeks has promising effects on reducing burn-out symptoms. The present systematic review slightly deviates from this conclusion, showing that positive effects on burn-out risk were achieved when being physically active two to five times per week for 20 to 60 min, for six to 18 weeks, with 18 weeks showing the biggest reduction in burn-out risk (−47.8%) [53]. The latter suggests that the longer the duration of the physical activity intervention, the higher the reduction in burn-out risk. However, due to the large variety in type, intensity, duration and frequency of the performed physical activities in the included studies, comparison of the effectiveness of the individual interventions remains difficult. Further research to unravel the respective effects of type, intensity, duration and frequency is therefore highly recommended. Furthermore, to better understand the relationship between physical activity and burn-out risk, it is also important to investigate the underlying physiological mechanisms. For example, the role of BDNF might be interesting as BDNF-levels increase when physical activity is performed, while on the other hand, BDNF-levels were found to be decreased in people having burn-out [22]. Four experimental studies [49,52,54,56] compared physical activity interventions with other interventions-such as basic care, cognitive interventions and a multimodal rehabilitation program-and did not find physical activity to be more effective compared to the other treatment arms. It should be mentioned that, in one of these studies [52], the aforementioned conclusion was based on the results of six and 12 months after the intervention was completed, possibly causing differences between treatment effects to have been diminished. Another possible reason for these results may be a shared effect between the interventions, resulting in no effect of intervention type [78]. As suggested by Heiden et al. [52], the same (psychosocial) attention was given to all patients in both interventions, suggesting (psychosocial) attention to be such a shared effect. Furthermore, as human interaction was also present in the other experimental studies (e.g., interaction between the researcher or therapist and the participant during the intervention phase), part of the intervention effects might be explained by the same psychosocial component [49,52,54,56].
These results might suggest that physical activity may be equally effective compared to other types of interventions, as long as there is a psychosocial component involved. Future research should further unravel the relative importance of physical activity versus other intervention components when aiming at reducing burn-out risk.
Regarding sedentary behavior, only positive relationships with risk of burn-out were reported, indicating that higher levels of sedentary behavior are associated with higher burn-out risk. It should be mentioned, however, that only two observational studies [62,67] were found, making it difficult to draw any firm conclusions. Nevertheless, the quality score of these two studies was high (0.86 and 0.95). Besides, it is important to mention that these studies primarily aimed to assess the relationship between physical activity and risk of burn-out. In both studies, the assessment method of physical activity was the 4-level Saltin Grimby Physical Activity Level Scale [79], of which the lowest level reflects sedentary behavior. This level means that participants were not participating in any leisure-time physical activities or sport activities, which is not in line with the definition of sedentary behavior [25]. Furthermore, no validated objective (e.g., inclinometers, accelerometers) nor subjective (e.g., more detailed context-specific questionnaires) assessment methods for sedentary behavior were used. Despite the fact that sedentary behavior may influence mental health and despite its impact on the risk of the occurrence of common mental disorders such as depression, decreased mood and anxiety, no experimental studies investigating the influence of sedentary behavior on burn-out risk were found. Furthermore, the study by Bernaards et al. [58] concluded that the association between physical activity and burn-out risk was stronger in workers with a sedentary job. This suggests a possible moderating role of sedentary behavior in the physical activity-burn-out risk relationship pathway. So, the interaction between physical activity and sedentary behavior should be further investigated while explaining burn-out risk.
Two observational studies [46,69], with quality scores of 0.80 and 0.91 respectively, found that a healthier diet is related to a lower risk of burn-out. Because only two studies were found and the fact that these two studies were investigating different aspects of dietary behavior, namely fast food consumption and the amount of healthy diets during work days, it is difficult to draw reliable conclusions regarding this relationship. Moreover, these two studies did not use valid and reliable assessment methods to measure both aspects. Alexandrova-Karamanova et al. [46] measured fast food consumption by one single self-constructed question, namely "How many times in a week do you eat fast food?". In the study by Gorter et al. [69] dietary behavior was measured by asking how many healthy diets participants consumed during workdays, while "healthy diets" was not defined. These methodological shortcomings make it even more difficult to draw firm conclusions.
It has been shown that diet may influence mental health. A systematic review and meta-analysis by Tolkien et al. [80] for example, concluded that an anti-inflammatory diet may play an important role in preventing or reducing depression risk and symptoms. Moreover, because of the link between burn-out and neurotransmission and the role diet may play herein, it is important to further investigate the link between dietary behavior and burn-out risk by conducting experimental research using valid and reliable assessment methods (e.g., food diaries, 24-hour recalls or food frequency questionnaires).
Because all studies regarding sedentary behavior, dietary behavior and burn-out risk had an observational design, no conclusions about the causal relationship can be made. As hypothesized above, being sedentary may increase the risk of burn-out through different physiological mechanisms, while a burn-out may also cause people to be more sedentary, and so reversed causality is possible. The same may be true for dietary behavior. This shows the need for more experimental studies investigating the causal relationship between sedentary and dietary behavior and burn-out risk.
There are several limitations to the included studies. A first limitation is the fact that some studies [57,58,69] measured physical activity with only one or two single self-constructed questions. Moreover, some measurement methods for physical activity had methodological shortcomings. The study by Liang et al. [61], for example, measured exercise behavior on a 5-point scale with anchors 1: 'never' to 5: 'many times in a week', while 'many times' was not defined. Future research should use validated questionnaires (e.g., International Physical Activity Questionnaire [81]), and preferably objective measures, such as accelerometers or pedometers. The same can be said for sedentary and dietary behavior. A second limitation is that less than half of the included studies used the Maslach Burnout Inventory (MBI), while the MBI is the gold standard assessment tool for burn-out [82]. All the other assessment tools for burn-out used in the included studies are based on other definitions of burn-out and are mostly measuring one dimension, namely (emotional) exhaustion. It is recommended that future research uses the gold standard assessment tool for measuring burn-out, in order to increase measurement homogeneity across studies. A third limitation is that some studies are mixing up the terms "physical inactivity" and "sedentary behavior". Two of the included studies [57,63] use a physical activity questionnaire to classify people as active or inactive, and considered inactive people to be sedentary, while literature clearly shows that physical inactivity and sedentary behavior are two different concepts [27,28,83]. A fourth limitation is that the majority of the included studies did not take physiological, psychological and sociological confounders into account, and so conclusions may have to be interpreted with caution. A fifth limitation is that most studies consisted mostly of female participants, which may have influenced the results. Research shows that women generally eat healthier but are less physically active compared to men [84][85][86][87][88][89]. Moreover, a meta-analysis showed that women are more likely to report burn-out [90]. More specifically, women reported to be more emotionally exhausted than men, while men were more depersonalized [90]. Hence, future studies investigating the association between energy balance-related behavior and burn-out should take sex into account. Lastly, some articles were missing some relevant information, such as how physical inactivity was measured, so the authors had to be contacted. Unfortunately, we did not always get a response leaving some queries unanswered.
There are also a few limitations to the present systematic review. Since non-English written publications were excluded, we may have missed out on important scientific articles in other languages. Additionally, the used quality assessment tool does not distinguish between experimental and observational studies. Experimental studies can be considered higher in quality and so they should receive a higher score in the quality assessment. However, when calculating the mean quality score per study design, a higher mean quality score of 0.87 ± 0.08 for the observational studies was found, compared to a mean quality score of 0.74 ± 0.08 among the experimental studies.
A strength of this systematic review is the fact that this is the first systematic review aiming to include articles of all study designs investigating the relationship between energy balance-related behavior as a whole (i.e., the combination of physical activity, sedentary behavior and dietary behavior) and burn-out risk. As-in the present review-no studies using this holistic approach were found, and because of the hypothesized role these three components may play in reducing or preventing burn-out, further research on this topic is needed.

Conclusions
This systematic review shows that any type of physical activity, lasting 20 to 60 min and performed two to five times per week for six to 18 weeks, may be effective in reducing the risk of burn-out. The few observational studies linking sedentary and dietary behavior with burn-out risk suggest that engaging in frequent sedentary behavior and eating less healthy are each associated with higher burn-out risk. More high-quality research is needed to unravel the causal relationship between sedentary and dietary behavior and the risk of burn-out.