Participants and data-collection
We selected a random sample of schools (n=26) from a government database of secondary schools in Flanders, a region in Belgium consisting of around 6 million inhabitants. Eight schools (31%) agreed to collaborate in the study. The main reason for not participating was no time to set-up the survey at the school within the desired time frame. The study took place between November 2014 and May 2015. Within each school, classes were randomly selected. We aimed to collect data among all grades 7-12 (aged 12-18), which was not always practically feasible. Data collection took place at school, during one class hour. The anonymous paper-and-pencil survey was administered by the researchers, who explained at the start of the survey that students were under no obligation to participate and could withdraw at any time. Students were assured that their responses would be confidential and that no information would be shared with teachers, parents, or fellow students. Five students declined to participate, none of the parents declined consent. The study received approval from the Ethics Committee of the Ghent University Hospital (2012/307, B670201214183). Adolescents provided written informed consent, parents provided passive informed consent.
Measures
General socio-demographic information
Items were derived from the HBSC 2009/10 questionnaire, a cross-national survey supported by the World Health Organization (46). Socio-demographic variables included gender, age, type of education, country of birth, family living situation, self-reported weight and height (used to calculate Body Mass Index, BMI).
Family affluence
This part of the HBSC questionnaire also comprised the validated adolescent self-report ‘Family Affluence Scale’ (FAS), to identify family material wealth and socio-economic status (SES) of children and adolescents (47). The FAS is used as an indicator of SES. It has widely been used to explore and explain socioeconomic inequalities in a wide range of health indicators in the HBSC study over the last 20 years (48). FAS is validated against other measures of SES and macro-economic indicators at country level (47, 48). The FAS was developed to overcome the problem of inaccurate perceptions and missing data among children and adolescents of their family’s finances, especially among lower socio-economic groups which could thus lead to an underestimation of socioeconomic inequalities (28, 48). It was proposed as a less intrusive, more comprehensible approach to identify the family’s socioeconomic status (49) than inquiring about parents’ educational, occupation or income levels (47, 50). It is indicated that in contrast to for example parental occupation, the proportion of missing data on FAS items is low (48). The FAS consists of four items: number of cars, own bedroom, computers owned and number of holidays per year (47, 49). A composite FAS score (ranging from 0-9) is calculated for each adolescent based on his or her responses to these four items. The following, international, cut-off points were used: score of 0, 1, 2 classified as low affluence; score of 3, 4, 5 as medium affluence; and a score of 6, 7, 8, 9 classified as high affluence (47).
Healthy lifestyles
Items to assess healthy lifestyles, except for sleep duration, were also taken from the HBSC survey. Several health-related lifestyle behaviors among adolescents are interrelated. Based on Principal Component Analyses on these data reported elsewhere (11), healthy lifestyles were grouped into two factors: ‘energy-balance related behaviors’, consisting of physical activity and a healthy diet, and ‘addictive behaviors and sleep duration’, consisting of alcohol consumption, smoking and perceived sleep duration. These factors will be used to discuss the results, individual behaviors are however retained in the analyses.
Energy-balance related behaviors
Physical activity was measured by the number of days they achieved ≥60 minutes of moderate to vigorous physical activity, was defined in the questionnaire as: “bodily movements that make your heart beat faster and make you feel out of breath at some moments”. A healthy diet was measured by assessing the number of days per week adolescents had have breakfast. Eating a regular, healthy breakfast has been found to contribute to the daily recommended intake of essential nutrients (51, 52). Moreover, daily breakfast consumption can serve as an indicator to identify adolescents at risk for unhealthy lifestyle behaviors. For example, daily breakfast intake has proven to be associated with both daily fruit and vegetable consumption and an inverse relationship was found with daily soft drink consumption (42).
Addictive behaviors and perceived sleep duration
Alcohol use was assessed by summing the frequency of six different types of alcohol consumption: beer, wine, spirits/liquor, alcopops and any other drink that contains alcohol (0-never; 4-daily. Range of summed score 0-24). An index combined several questions on tobacco use frequency (range 0: ’never tried smoked’ to 4: ‘smoking ≥11 cigarettes per day, considered as daily high dose smoker (=median among daily smokers)). To calculate sleep duration (number of hours slept per night), adolescents were asked to report at what time they usually go to bed and get up.
Mental health
Mental health was measured through feelings of depression, anxiety and stress and self-esteem. Feelings of depression, anxiety and stress were measured with the Depression Anxiety Stress Scales (DASS-21) which has good psychometric properties to measure adolescent mental health outcomes (53, 54). It consists of seven items per subscale (54). Total scores per subscale were used as dependent variables, with high reliability for each of the subscales (αdepression=0.90; αanxiety=0.84; αstress=0.87). Focusing on self-esteem is considered a core element of mental health promotion and a fruitful basis for a broad-spectrum approach (55). Positive global self-esteem was measured by a single item from the Rosenberg Self-Esteem Scale (RES), namely ‘I take a positive attitude toward myself’. Global self-esteem can be measured by a single item (56) and this specific item is a main contributor to global positive self-esteem (57, 58).
Analysis
Multiple linear regression analyses assessed the association between healthy lifestyles and mental health outcomes (RQ1); and the moderating role of family affluence in the relation between healthy lifestyles and mental health outcomes (RQ3). Analyses were controlled for individual background factors that significantly influenced mental health outcomes (namely, BMI, gender and age). Analyses were conducted stepwise, by first examining the influence of family affluence and background variables, next the healthy lifestyle variables, and lastly the interaction effects between healthy lifestyle variables and family affluence. Collinearity diagnostics were conducted examining Variance Inflation Factor (VIF) (≤10) and tolerance (≥0.1). Cross-tabulations were checked for empty combinations of cells or low expected frequencies (59). Continuous independent variables were mean centered. Moderator variables were created by multiplication of interaction variables. Graphical presentations of moderator analyses were made using PROCESS 2.16.3 for SPSS, and based on parsimonious model results. Post-hoc analyses of significant interaction effects were performed using the Johnson-Neyman method. All analyses were conducted in SPSS 25.0.