Weight development from adolescence to adulthood: Associations with leisure time activities, weight-perception and obesity-susceptibility loci - The HUNT Study
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Background Approximately 1.8 billion humans, i.e. more than a quarter of the world’s population, are young people aged 10-24 years1. Adolescence comprises a period in which lifestyle and health behaviors are established, influencing adult behavior and health status2. In the European Union the number of children and adolescents who are overweight in BMI units are expected to rise by 1,3 million per year, with more than 300,000 of them becoming obese each year3. The most negative consequence of adolescent obesity is the tracking of obesity from adolescence to adulthood4, creating not only short-term health problems in adolescence but also long-term health problems in adulthood. In 2008, it was predicted that within 2030 the majority of the population would be overweight or obese if the current trends of excessive weight increase would persist5. Available data reveal that the accumulation of central body fat, measured with waist circumference, has risen even more steeply than the whole body fatness in BMI units6-9. Individual factors, like genetic make-up together with environmental factors, cultural differences and socio-economic status and their interaction are comprised in a socioecological framework which determines the variance in phenotypes10. Large proportions of the Western population are unsatisfied with their weight and this perception is more common than overweight11. Klein et al. showed in her analyses of different risk factors for weight gain in a socio-ecological model that perception of being overweight, despite being normal weight was the most important risk factor for weight gain12. Twin- and family studies have estimated increasing heritability values for BMI from 40% at childhood to 80% at adolescence or adulthood depending on consulted literature13. So far, large-scale meta-analyses of GWAS have identified 32 loci robustly associated with BMI14-20 and 58 genetic loci in total for different weight measurements in adults21. The evidence that these loci may affect weight in childhood and adolescence has been growing since18, 22-24. Choquet21 stated that obesity is an inherited disorder of central regulation of food intake25, highly sensitive to environmental conditions26. Both physical and cultural leisure time activities may, in addition to diet, play a critical role in maintaining energy balance. Social activities, on the contrary, are suggested to fuel the obesity epidemic27-30. However, the effects of every factor in a socio-ecological model solely seem to be generally modest. Though, when acting together their cumulative effect on energy balance may be substantial31. The aim of this thesis was, in a longitudinal perspective, to investigate the associations between weight gain and weight in adulthood and three frequent factors in adolescence: selfperceived overweight (paper I), genetic obesity-susceptibility variants (paper II) and leisure time activities including cultural and social activities (paper III) which all constitute part of a socio-ecological framework. We also investigated if physical, cultural and social activities were effect moderators. Material and methods HUNT, the Health Study of Nord-Trøndelag, is a large population based study carried out three times during the last 25 years in the county of Nord-Trøndelag, Norway (http://www.ntnu.no/hunt). The first survey was in 1984-86, the second wave in 1995-97, the last one was carried out in 2006-08. Young-HUNT is the adolescent part (13-19 years) of HUNT. The Young-HUNT1 survey was carried out between August 1995 and June 1997 as part of HUNT2. The follow-up studies presented in this thesis include participants in Young- HUNT1 who also participated as young adults (24-30 years) in HUNT3 carried out in 2006- 2008. Thus, 1805 adolescents who participated in both Young-HUNT1 (aged 13-19 yrs) and as young adults (24-30 yrs) in HUNT3 were included in the present thesis. In HUNT3 as well as in Young-HUNT1 anthropometric measures were taken by trained nurses. Lifestyle factors as eating behaviour, physical, cultural and social activities and demographic characteristics as pubertal development, socio-economic status and age were assessed using comprehensive questionnaires. Linear and logistic regression analyses and generalized estimation equations were used to estimate associations between the respective exposure variables and adiposity-related traits. Results In paper I, we found that self-perceived overweight in both normal weight girls and boys was positively associated with weight gain into young adulthood. Physical activity seemed not to influence this relationship. In paper II, we replicated in adolescents the effects of obesity-susceptibility loci, previously established using GWAS in adults, individually and accumulated in a genetic predisposition score (GPS). In regard to weight change our data could not reveal strong associations between individual loci or the GPS-based risk scores and change in BMI and WC as outcome measures. Except for one locus for which our data indicated a significant relationship with weight change at nominal level. Physical activity seemed to attenuate the identified effect. Notably in paper III in girls, we found an inverse effect of participation in cultural activities in adolescence with central obesity 11 years later. Participation in cultural activities seemed to attenuate the effect on body fat estimates of 12 obesity-susceptibility variants accumulated in a genetic predisposition score. Participation in social activities was in both girls and boys positively related to body fat estimates. Conclusions This thesis has in a socio-ecological framework elucidated some risk factors which may influence weight in the transition from adolescence to adulthood. Focus on weight and body image is supposed to play an important role and may with advantage taken in account in the development of public health strategies. Furthermore, obesity-susceptibility loci may influence weight already in adolescence. Cultural activities may possibly be equally important as physical activity in overweight prevention in adolescents. More longitudinal studies in a life course perspective are warranted to confirm or reject these findings.