Cuypers et al. BMC Public Health 2012, 12:820 http://www.biomedcentral.com/1471-2458/12/820 We chose the components included in “cultural activity” and “social activity” based on knowledge of adolescent’s preferred activities at the time from previous literature [16,18] and from the choices as found in a study of the first author including adolescents from the same area [17]. Also, all the components chosen for the “cultural activity”-index were activities believed to require mental activity. TV-viewing is an important leisure time activity in adolescents and was included. However, the direction of the association between TV/video-viewing and obesity is equivocal and may be biased by snacking and psycho- social problems [39-41]. We chose the components included in “social activity”-index because we assumed the adolescents to socially meet during these activities. In order to be sure of directional consistency in our data each component in both the cultural and social index was tested in separate models all showing the same direction in each index. In additional GEE-models we investigated interaction between participation in social and cultural activities by introducing the interaction terms (cultural activities*so- cial activities) in addition to the main effect of the leis- ure time activity to see whether participation in one kind of leisure time activity modified the effect of the other. Gender specific models were calculated due to statistically significant interactions between gender and both leisure time activities. With linear regressions models, we studied the associ- ation between the GPS and z-scores for BMI and WC at follow-up respectively. The interaction effect between the respective leisure time activities (cultural and so- cial) and GPS on the association with z-scores BMI and z-scores WC at follow-up was assessed. We checked interaction by introducing cross-product terms (participa- tion in social activities*GPS and participation in cultural activities*GPS) in addition to the main effect of the re- spective leisure time activity and GPS. Additionally, we also investigated the relationship between the z-scores of Page 4 of 10 BMI and WC and the GPS in those with high and in those with low participation in social and cultural activities. Interaction between GPS and gender was tested by includ- ing the cross-product term (GPS*gender) in the model, in addition to the main effect of GPS and gender on the adi- posity traits. Statistical analyses were performed using IBM SPSS Statistics, version 19. A p-value at or below 0.05 and a 95% confidence interval denoted statistical significance. In regard to the interaction terms we used the p < 0.10 level, because an interaction term is a multiplication of two variables which both include measurement error, resulting in a multiplicative error and subsequent high standard error. Results In the analyses of associations between participation in cultural or social activities and adiposity measures at fol- low-up, a total of 775 girls and 675 boys were included. More girls than boys were highly engaged in cultural ac- tivities, while participation in social activities was equally distributed between the genders. Descriptive analyses (Table 1) showed an inverse rela- tionship between frequency of participation in cultural activities in adolescence and overweight in young adult- hood in both normal weight girls and boys. P for trend in the four quartiles was beyond the 0.01 level for all three adiposity measures in young adulthood. In con- trast, there was a positive association between participa- tion in social activities in adolescence and overweight in young adulthood; however, the P for trend did not reach statistical significance. Participation in cultural activities or social activities and adiposity measures in adulthood Including all girls and boys (n = 1450), an inverse associ- ation between participation in cultural activities at base- line and z-scores of WC and WHR in adulthood was Table 1 Mean adiposity measures and standard deviation at young adulthood in the four quartiles of participation in leisure time activities in adolescence from 1 (lowest) to four (highest) 1 2 3 4 P Cultural activities BMI 25.0 25.1 24.9 24.3 0.006 3.2 3.5 3.6 3.5 WC 88.6 87.7 87.2 84.3 <0.001 9.3 10.6 11.2 10.9 WHR 0.869 0.860 0.854 0.834 <0.001 0.056 0.072 0.070 0.069 Social activities BMI 24.6 24.6 24.9 25.0 0.10 3.3 3.4 3.5 3.6 WC 85.8 85.7 86.7 87.1 0.06 10.9 10.6 10.6 10.8 WHR 0.849 0.845 0.853 0.853 0.23 0.074 0.066 0.069 0.069 Girls n = 603; Boys n = 520. Excluded: overweight and underweight, pregnant, disabled physically or psychologically in daily activities. P: p for trend. Cultural leisure time activities includes reading a book you like, listening to music or played an instrument longer than 15 min, watching TV or a video, and doing home work or school tasks longer than an hour. Social leisure time activities included visiting some one you know, receiving a visitor, being out for more than two hours with friends, and being at a meeting or training in an organization or a club. We assigned values from 1 to four for each answer-category regarding frequency Note once (1), Once (2), 2–3 times, (3), 4 times or more (4) in the last seven days and made a continuous variable by adding the scores for each activity (range 4–16). Then 4 quartiles were constructed from low to high participation.