Procedure and study sample
We investigated cross-sectional data of N = 7.930 working participants (6,204 full-time and 1,726 part-time employed) enrolled in the Gutenberg Health Study (GHS) from 2007 to 2012 who had received the Copenhagen Psychosocial Questionnaire (COPSOQ). The GHS is a population-based, prospective, observational single-center cohort study in the Rhine-Main-Region in western Mid-Germany. The study protocol and study documents were approved by the local ethics committee of the Medical Chamber of Rhineland-Palatinate, Germany (reference no. 837.020.07; original vote: 22.3.2007, latest update: 20.10.2015) and by the local and federal data safety commissioners. The primary aim of the study is to evaluate and improve cardiovascular risk stratification. The sample was drawn randomly from the local registry in the city of Mainz and the district of Mainz-Bingen. The sample was stratified 1:1 for sex and residence and in equal strata for decades of age. Inclusion criteria were age 35 to 74 years and written informed consent. Persons with insufficient knowledge of German language, or those who reported that they were not able to visit the study center on their own (due to their physical and/or mental condition) were excluded. The response rateFootnote 1 was 60.3% for the first 5.000 participants. Due to the ongoing recruitment of the GHS, which is conducted in waves, a final statement concerning the response rate cannot be made at this time. The design and the rationale of the Gutenberg Health Study (GHS) have already been described in detail elsewhere [21].
Materials and assessment
The 5-h baseline-examination in the study center comprised evaluation of prevalent classical cardiovascular risk factors and clinical variables, a computer-assisted personal interview, laboratory examinations from a venous blood sample, blood pressure and anthropometric measurements. In general, all examinations were taken out according to standard operating procedures (SOPs) by certified medical technical assistants.
Measures
The Personal Burnout Scale (PBS) is part of the Copenhagen Psychosocial Questionnaire with 6 Items assessing physical and mental exhaustion, independently from work. It assesses the frequency of the following items („How often do you feel …“: tired, physically exhausted, emotionally exhausted, unable to go on, weak and prone to illness). Ratings are done on a 5-point scale 1 = never/almost never, 2 = rarely, 3 = occasionally, 4 = often, 5 = always (COPSOQ [22]). Data were transformed to a metric scale (1 = 0; 2 = 25 to 5 = 100) („high burnoutFootnote 2“). The scale is reliable (Cronbach alpha of the German version = 0.91 [23]); a mean score ≥ 50 was considered evidence for the presence of fatigue [24].
In order to cover a broad range, work-related stressors (work overload, piece/shift work, insufficient vacation, frequent conflicts with supervisors or colleagues and unemployment of the partner) were assessed by single items using 5-point scales (0 = no, does not apply, 1 = yes it applies, but it does not stress me, 2 = yes, it applies, and it stresses me slightly (3 = moderately, 4 = strongly). Items were recoded combining 0 and 1 (no strain or no stress); 2 = slightly, 3 = moderately and 4 = severely stressed.
Depression was measured by the Patient Health Questionnaire (PHQ-9); caseness of depression was defined by a score ≥ 10 with a sensitivity of 81% and a specificity of 82% for depressive disorder [25]. Further depressive symptoms can be classified as “minimal” (score 5 to 9), “mild” (score 10 to 14), moderately severe (score 15 to 19) and severe (score ≥ 20). The somatic-affective and cognitive-affective dimensions of depression were defined according to prior studies [26]. Four PHQ-9 items related to problems with sleep, fatigability, appetite, and psychomotor agitation/retardation were classified as somatic-affective symptoms, whereas 5 items, related to lack of interest, depressed mood, negative feelings about self, concentration problems and suicidal ideation, were classified as cognitive-affective symptoms of depression [26].
Computer-assisted personal interview
During the computer-assisted personal interview, participants were asked whether they had ever received a definite diagnosis of any depressive disorder by a physician (medical history of lifetime diagnosis of any depressive disorder, medical history of depression). Severe medical disease was defined by the presence of coronary heart disease, myocardial infarction, stroke, peripheral artery disease, heart failure, diabetes, cancer, COPD, rheumatic, chronic kidney or liver disease. Diabetes was defined in individuals with a definite diagnosis of diabetes by a physician or a blood glucose level of at least 126 mg/dl in the baseline examination after an overnight fast of at least 8 h or a blood glucose level of at least 200 mg/dl after a fasting period of less than 8 h. The presence of coronary heart disease was assessed by the question: ‘Were you diagnosed with a stenosis of your coronary vessels?’ Other chronic medical diseases were assessed correspondingly. Cardiovascular risk factors were defined as follows: Smoking was dichotomized into nonsmokers (never smoker and ex-smoker) and current smokers (occasional smoker, i.e. <1 cigarette per day, and smoker, i.e. > 1 cigarette per day). Obesity was defined as a BMI of at least 30 kg/m2. Unhealthy alcohol intake was defined as habitual alcohol intake of more than 20 g per day for men and more than 10 g per day for women.
The socioeconomic status (SES) was defined according to Lampert and Kroll’s (2009) scores of SES with a range from 3 to 21 (3 indicates the lowest SES and 21 the highest SES) [27].
Statistical analysis
Data are presented as numbers/percentage, mean (and 1.96-fold standard deviation) or median (and 1st, 3rd quartile) as appropriate. We performed non-parametric and parametric tests as appropriate to compare participants with and without fatigue. In order to identify determinants of fatigue, we computed separate linear regression models with fatigue as the dependent variable. For each of a set of potentially explanatory variables we fitted a series of linear models including that variable and successively more variables for adjustment. Unadjusted effects and all increasingly adjusted effects are reported. Models were pre-specified in a statistical analysis plan; no data-driven model selection procedures have been applied. In a stepwise manner, we adjusted for age, sex and SES, work-related strains, medical disease, health behavior and depression. In face of small proportions of missing values and a large sample size we preferred to perform complete case analysis with respect to set of variables of each fitted model. We reported the number of cases for each model fit.
To determine relations between work-related strains, fatigue and depression we computed Pearson partial correlation coefficients partialling out depression, respectively fatigue from the associations with work-related strains. The difference of the size of partial correlations was determined by Steiger’s Z test [28]. P-values are given for descriptive reasons only and should be interpreted with caution and in connection with effect estimates. All p-values correspond to 2-tailed tests; the levels of significance was set at p < .05. Statistical analysis was carried out using IBM SPSS Statistics 20 (IBM, Chicago, IL).