Cuypers et al. BMC Public Health 2012, 12:820 http://www.biomedcentral.com/1471-2458/12/820 behave in the opposite direction, the adiposity measures were very homogenous in the study sample and were not expected to affect the associations substantially. We repeated the analyses including only the 1123 ado- lescents with normal weight at baseline in the analyses, because the effect of environmental influences are sug- gested to be larger in individuals with phenotypic extremes during adolescence [37] and might bias our results. Additionally, overweight in adolescents tends to track into adulthood [42] and it is more likely that envir- onmental effects might be easier detectable in normal weight adolescents without onset of obesity. The results might indicate that the favourable effect of cultural par- ticipation in adolescence is more preventive against overweight and less a remedy for becoming normal weight again. A possible limitation might be that we did not include physical activity as a component in the index of social activity. We were mostly interested in the “social” part of the social activities, although we cannot exclude some overlap with physical activity in our definition of social activities. Therefore, we adjusted for the level of physical activity. The effect of cultural participation There might be several explanations for the inverse ef- fect of participation in cultural activities on adiposity measures. Firstly, participation in cultural activities is found to be associated with an overall regular lifestyle and healthy behaviour [43]. Secondly, relaxing leisure time activities like listening to music, reading and TV watching seem to be strong predictors of coping with stress, as suggested by Iwasaki et al. [44] and thereby counteract stress-induced central obesity. Thirdly, Lajunen et al. [16] argues that adolescents cannot eat while playing music, keeping the energy input and output in balance. Girls were obviously more engaged in cultural activ- ities compared to boys, and it seemed that only in girls the participation in cultural activities was protective against overweight. This gender difference may be due to factors like different food choices, body satisfaction, and physical activity , as well as differences in participa- tion in cultural activities [45]. Our findings might possibly be related to some bio- logical pathways, demonstrated in animal experiments. Rodents who were exposed to enriched environments where they were offered exercises both mentally (i.e. toys, change in food locations, more complex housing) and physically tended to switch from a white fat pheno- type to a brown fat phenotype, more likely to burn fat depots [46]. Furthermore, diet induced obesity has been shown to be prevented through the genetic and environ- mental activation of hypothalamic-adipocyte axis [8]. Page 7 of 10 These effects were specifically noticed in rodents exposed to an enriched environment compared to rodents exposed only to physical exercise, like running. Histological studies show that these mechanisms may also be relevant in humans [47]. Cultural activities, as defined in this study, were sitting activities. There is no unanimous conclusion on the negative effects of the different sedentary activities per se on development of overweight [43]. Although there is evidence that TV viewing is a risk factor for develop- ment of obesity, being more important in children than adolescents [48], the pathways of the association are un- clear [39-41]. We have, therefore, repeated the analyses without the component “Watching TV or a video” as a part of the index of cultural activities (Additional file 1: Table S6). The exclusion of “Watching TV or a video” from the cultural activity index did not change the results substantially. Specifically, excluding the compo- nent TV viewing from the index of cultural activities weakened the effect estimates slightly, but the confi- dence intervals remained nearly unchanged. “Watching TV or a video” was individually also inversely associated with central body fat estimates in girls in discordance with other studies [49,50]. TV viewing may in our popu- lation counteract stress in concordance to Iwasaki et al. [44]. Another explanation may be that TV viewing seemed not to displace physical activity in our popula- tion. Biddle et al. has stated that TV viewing may be re- flective of other unhealthy behaviours [51], but this does not seem to be the case for the girls in our study. Also, the relationship between TV viewing and obesity has been suggested to be influenced by other confounders like snacking, psycho-pathologies [39], socio-economic status [51] and family structure [48] which may not be ap- plicable to the girls in our population. Finally, Norwegian adolescents seemed to consume less TV during dinner than peers in other European countries [49]. The effect of social activities Consistent with the findings of Christakis et al. [10], but in discordance with the study of Lajunen et al. [16], our data suggest a positive association between adolescent participation in social activities and overweight in young adulthood. The social activities as used in the study of Lajunen comprised not solely social activities. Like Bara- basi et al. [52], we might suggest that the social network may be a factor in the spread of obesity through social ties. Some studies [6,53] proposed psychosocial pathways as one possible mechanism in the beneficial effects of social participation on psychological health in adults. Others [4], on the contrary, found that the relative risk of mortality increased equal to the extent of the social network. Likewise, our data indicate negative influence on health measures, i.e. adiposity measures. One can