Cuypers et al. BMC Public Health 2012, 12:820 http://www.biomedcentral.com/1471-2458/12/820 Background Development of obesity is influenced by a complex interplay between genetic background, environmental factors, behavior, and socio-economic status [1]. Genes are believed to be expressed where the environment allows [2]. Physical activities have previously been sug- gested as important leisure time activities protecting against development of obesity [3], but a substantial part of the daily leisure time in adolescence is spent on other activities, like cultural and social activities. Cultural ac- tivities include activities believed to require mental activ- ities and may be performed without the company of others like reading a book, listening to or playing music or doing homework. Social leisure time activities imply leisure time activities performed together with friends as hanging out with friends or attend meetings or clubs. Population based studies and human intervention studies have shown that cultural activities are associated with increased survival rate [4], better health [5] and decreased risk of dementia [6]. Recent studies on humans [7] and rodents which were placed in enriched environments where they were offered mentally exer- cises (i.e. toys, change in food locations, more complex housing) [8] have revealed biological and psycho-neuro- immunological pathways which may help to explain the possible health effects of these cultural activities. On the other hand, social influences may be an important cause of low compliance in physical activity interventions [9], and obesity appears to spread through social ties [10]. Adolescence may be a period when risks for chronic conditions are established through learned health behav- ior or emotional experiences [11]. The transition be- tween adolescence and adulthood is a developmentally sensitive time during which adolescents may become overweight, which may last into later adulthood [12]. While the effects of physical activity have been exten- sively studied, the potential role of other leisure time ac- tivities in weight development from adolescents to adulthood, a period with large biological, psychological changes, has not been explored. The aim of this study was, therefore, to investigate the associations between participation in cultural and social leisure time activities in adolescence and body fat in young adulthood 11 years later. We also wanted to study whether cultural or social activities would moderate the cumulative effect of 12 obesity-susceptibility gene variants on adiposity mea- sures at follow-up. Methods Study population The Nord-Trøndelag Health Study, HUNT, is a large population based study, carried out in three surveys in Nord-Trøndelag County, Norway (http://www.ntnu.no/ hunt) during the last 25 years: HUNT1 (1984–86), Page 2 of 10 HUNT 2 (1995–97) and HUNT 3 (2006–08). The popu- lation (130 000 inhabitants) is Caucasian, homogenous, and fairly representative of Norway as a whole regarding geography, demography and occupation, but lacks large cities [13]. The average income and mean educational level is slightly lower than the national average, never- theless the socioeconomic inequality in mortality is at the national level [14]. Young-HUNT1 was carried out in 1995–97 as the adolescent part of HUNT2. A total of 8408 (participa- tion rate: 83%) of all students in junior and senior high schools completed a comprehensive questionnaire about lifestyle, health, and quality of life and underwent a clin- ical examination including anthropometric measure- ments. When excluding adolescents who reported to be mentally or physically disabled at baseline and who were pregnant at follow-up, 1450 people participated both in Young-HUNT 1 as adolescent (13–19 years) and in HUNT3 as young adult (24–30 years). These participants had blood samples taken in HUNT3 and were genotyped (Additional file 1: Table S1). Out of this group, 1123 parti- cipants had a baseline BMI within the normal range [15] (Additional file 1: Table S2-S3). In both surveys, self-administered questionnaires were used to collect information on physical and mental health, somatic complaints and lifestyle. Anthropometric measures were obtained by specially trained nurses using the same standardized protocols as in Young-HUNT1. In both Young-HUNT1 (baseline) and HUNT3 (follow- up) height and weight were measured to the nearest centimetre, respectively to the nearest half kilogram with light clothes, without shoes, jacket or outdoor garments. Waist circumference (WC) was measured to the nearest centimetre, applying a non-stretchable band horizontally at the umbilical level after the participants emptied their lungs, or midway between the last rib and the iliac cris- tae if the latter was largest. Hip circumference was like- wise measured at the thickest part of the hip [13]. Measures at baseline: young-HUNT 1 (1995–97) Weight status in adolescence was defined by BMI cut- offs for the appropriate age groups proposed by Cole et al. [15] Central adiposity was measured by waist cir- cumference (WC) and waist-hip ratio (WHR). Cultural activities included reading a book you liked, listening to music or playing an instrument longer than 15 min, watching TV or a video, and doing homework or school tasks longer than one hour. Social leisure time activities included visiting someone you know, receiving a visit, being out for more than two hours with friends, being at a meeting or training in an organisation or a club. In regard to cultural and social activities we assigned values from 1 to 4 for each answer-category regarding