Krølner et al. BMC Public Health 2012, 12:191 http://www.biomedcentral.com/1471-2458/12/191 Many intervention studies suffer from methodological limitations such as lack of randomisation, lack of baseline measurements, low response rates, inadequate adjustment for clustering of schools, short duration of intervention, short follow-up period, lack of explicit theoretical basis, lack of an explicit programme theory (causal model), and lack of process evaluation [13,36,38,41]. Furthermore, eva- luation studies of multi-component interventions often report the collective, synergistic effect of the intervention as it is challenging to disentangle the relative importance of specific components. Evaluation of different combina- tions of strategies using multi-arm controlled designs requires substantial statistical power [19,20] and there is a need for new approaches to decompose the effects. Many FV intervention studies have used determinants as proximal outcomes and measured changes in these. Fewer studies have examined if changes in FV intake were pre- ceded by changes in determinants (mediation analyses) and even fewer have linked changes in determinants to implementation of specific intervention components. In order to make valid interpretations of the proximal out- comes (determinants) of the intervention and examine which methods are efficient in changing each of these determinants we need to know whether the specific com- ponents were effectively administered [32,42]. Moreover, there is a lack of insight in programme reach and differen- tial effects on proximal (determinants) and distal out- comes (FV intake) in population subgroups with low intake such as boys and low income groups. Few studies have examined how and if these subgroups benefit from interventions [43] and if intervention components reach all subgroups equally (effect modification analyses). It is important to promote access to FV in the multiple settings of adolescents’ lives to consistently support them in making healthy food choices throughout the day. Despite this fact most nutrition interventions have addressed the school-or home environment and not the neighbourhood and leisure time activities. There is a great potential in sports-and youth clubs to promote healthy eating habits to adolescents in their leisure time [44]. Cross-sectional studies from Australia, Finland and Den- mark have pointed at factors promoting unhealthy eating habits in sports clubs [44-47]. The same challenges are found in youth clubs in Denmark [48]. A Cochrane review from 2008 concluded that there is a lack of controlled stu- dies evaluating policy interventions aiming at enhancing health behaviours through sporting organisations [49]. Given the rationale above, the aim of the Boost study was to develop, implement and evaluate a comprehensive school-based and community-integrated intervention which produces a persistent increase in FV consumption among teenagers. The key component of the intervention programme was to improve the determinants for FV intake, especially to enhance availability of FV in the Page 3 of 25 multiple settings in which teenagers take part. The devel- opmental work was to be guided by theory, evidence and best practise. This paper presents the development of the intervention, the design of the effectiveness study, and the baseline characteristics of pupils at intervention-and con- trol schools. Conceptualisation, design and methods The Boost intervention builds on a socio-ecological frame- work which recognises adolescents’ eating behaviour such as intake of FV as affecting and being affected by multiple levels of interacting influences [50]. Ecological models emphasise the structural, physical and political context while incorporating social and psychological influences. A practical implication of the ecological framework is multi- level interventions which use multiple strategies to increase adolescents’ consumption of FV and ensure that healthy messages are consistent and coherent throughout the settings in which the adolescents take part. The Boost study recognises the importance of availability of FV as an enabling factor. We used main principles from the Inter- vention Mapping protocol to plan the intervention, imple- mentation and evaluation in a systematic fashion, and the study was guided by advice from a steering committee of internationally recognised experts from the field [51]. The six consecutive steps of the Intervention Mapping plan- ning process are described below. In reality, we did not use the protocol rigidly but moved back and forth between the steps as suggested by the authors (iterative planning process) [51]. Needs assessment (step 1) As part of the needs assessment a national planning group was established, the programme theory was drawn up, target group and programme objectives were speci- fied and the needs, values and views of the target group were analysed. The planning group The planning group consisted of parents of teenagers, practitioners and experts with experiences in implement- ing school fruit breaks, public food provision, participatory research with teenagers, teaching, development of teaching material, and lobbying with stakeholders. The planning group discussed the scope and feasibility of initial ideas of the intervention programme, assisted the project group in making contacts with important stakeholders and was included in deciding the final programme theory and intervention model. Target groups We narrowed our target group to 13-year-olds (school year 7) because of the decline in FV intake observed in this age group [4,5]. Moreover most intervention studies had previously focused on younger children [12,13]. The Boost intervention was designed to reach all 13-year-olds