Study design and participants
This cross-sectional study is part of a large school-based, cluster-randomized intervention study, “Active and Healthy Youth.” The target group was the total population of students in the 1st grade at high schools in the south of Norway. In agreement with school boards/school principals, a total of 17 out of 23 schools (73.9%) decided to participate in the study. The main reason why six schools did not want to participate was due to a lack of time and a participation in other public health projects. A total of 2,619 out of 2,653 eligible students agreed to participate (98.7%) and responded by filling in a questionnaire. The data collection was conducted in classrooms or auditoriums, with at least one member of the project team present to inform about the project and answer possible questions. Before the questionnaires were handed out, the participants were informed that their responses would be treated as anonymous and that it was voluntary to participate. The students were given both oral and written instructions on how to fill out the questionnaire, and the students used approximately 30 minutes to answer the questionnaire. During the data analyses, a total of 187 students were excluded, as they did not meet the age requirements for participation in the study (15–17 years of age). Thus, the data analyses in the present study were based on a total number of 2,432 participants, 1258 girls (51%) and 1187 boys (49%). The Regional Committee for Medical Research Ethics approved the study protocol, and written consent was obtained from the students prior to participation in the present study. Although the Regional Committee for Medical Research Ethics did not require parental consent for adolescents aged 15 years and older in the present study, all students were encouraged to inform their parents about their participation and show them the information letter that described every detail of the study. Furthermore, the data collection was conducted during school time and principals, teachers and school nurses were given written information about the study.
Instruments
The questionnaire included questions about gender, body weight and height, parental education, selected food and beverages, meal frequency, leisure time physical activity level, active commuting, smoking- and snuffing habits and academic score.
In order to test the reliability of the questions used in the present study, we conducted a test-retest study, including 143 adolescents between 15–17 years old. The results exhibited a good test-retest reliability, with an intra-class coefficient (ICC) ranging from 0.66 to 0.99.
Self-reported weight and height were used to calculate body mass index (BMI) (kg/m
2). To estimate the prevalence of overweight and obesity, adolescent BMI categories were calculated using sex- and age-specific International Obesity Task Force (IOTF) cut-off points for defining overweight and obesity in children and adolescents aged 2–18 [
20]. The ICC was 0.98 (95% CI 0.98-0.99) for weight and 0.99 (95% CI 0.98-0.99) for height.
The parental educational level was assessed with the question: “What level of education do your parents have?” The question had four response alternatives: (i) elementary school, (ii) high school, (iii) college or university (≤3 years) and (iv) college or university (>3 years). These response alternatives were then dichotomized into lower and higher education levels (lower = no college or university education; higher = having attended college or university). The ICC was 0.83 (95% CI 0.77-0.88) for maternal education and 0.80 (95% CI 0.72-0.85) for paternal education.
Academic achievement was assessed using grades from three core mandatory academic classes in high school, including Norwegian, English and Mathematics. The grade system in Norwegian High School is from 0 to 6, in which 6 is the best possible grade to obtain. Based on the self-reported school grades from these classes, mean school grades were calculated and used in the analyses. The ICC was 0.76 (95% CI 0.65-0.84) for achieved grades in Norwegian, 0.83 (95% CI 0.75-0.89) for achieved grades in English and 0.82 (95% CI 0.73-0.88) for achieved grades in Mathematics.
Meal frequency was assessed by questions such as: “How often do you have breakfast each week?” The same was asked for lunch, dinner and evening meals. Response alternatives ranged from never to seven days a week, and these were dichotomized into having meals fewer than seven times a week and having meals every day. Adolescents who were having these main meals every day were classified as regular breakfast-, lunch-, dinner- and evening meal consumers. These dichotomous variables were then combined to create a summary variable referred to as “all regular meals,” i.e. those eating all meals every day vs. those skipping meals, respectively. The ICC was 0.91 (95% CI 0.86-0.94) for breakfast, 0.78 (95% CI 0.68-0.85) for lunch, 0.68 (95% CI 0.55-0.79) for dinner and 0.89 (95% CI 0.83-0.93) for the evening meal.
Diet and beverage intake was assessed by asking: “How often do you eat/drink….?” All items had eight different response alternatives: never, less than once a week, once a week, twice a week, . . . , 6 times a week, every day, several times every day, and for the statistical analyses, they were scored 0, 0.5, 1, 2, . . . , 7, 10 times per week. Intakes of healthy food items, including fruits, berries and vegetables, were dichotomized to less than once a day and once a day or more. A consumption of once a day or more was seen as an acceptable frequency of consumption for these food items. Intakes of unhealthy food items and beverages, including lemonade, sugar-sweetened soft drinks, diet soft drinks, candy and salty snacks were dichotomized into three times a week or less and more than three times a week, in which the first mentioned category, was seen as an acceptable frequency of consumption. The ICC was 0.70 (95% CI 0.57-0.80) for fruit and berries, 0.73 (95% CI 0.61-0.82) for vegetables, 0.81 (95% CI 0.71-0.87) for lemonade, 0.85 (95% CI 0.77-0.90) for sugar-sweetened soft drinks, 0.75 (95% CI 0.64-0.83) for diet soft drinks, 0.80 (95% CI 0.70-0.86) for candy and 0.75 (95% CI 0.63-0.83) for salty snacks.
Leisure time physical activity level was assessed by asking: “How many hours per week do you spend on doing sports/physical activity in a way that makes you breathless or sweat?” The response alternatives were: “0 hours, 1–2 hours, 3–4 hours, 5–7 hours, 8–10 hours and 11 hours or more.” For the statistical analysis, the response alternatives were dichotomized into 0–4 hours per week and 5 or more hours per week of leisure time physical activity. The ICC was 0.91 (95% CI 0.87-0.93) for physical activity.
Information about active commuting was enquired about as follows: “How do you usually commute to/from school?” The response alternatives were: “Walking, cycling, bus, car, MC/scooter, other alternatives (open alternatives).” This variable was dichotomized into active commuting, which represented walking or cycling and non-active commuting. The ICC was 0.85 (95% CI 0.79-0.90) for commuting to school.
Smoking and snuffing habits were assessed by the question: “Do you smoke/use snuff?” The response alternatives were: “Have never smoked/snuffed; have tried smoke/snuff, but not anymore, have smoked/snuffed regularly, but not anymore; smoking/snuffing, but not regularly and smoking/snuffing regularly and about__ cigarettes/day.” For the statistical analysis, those who reported smoking or snuffing occasionally or daily were classified as being a smoker/snuffer. The ICC was 0.95 (95% CI 0.92-0.97) for smoking and 0.93 (95% CI 0.90-0.96) for snuffing.