The long term effects of the school-based Lekker Fit! (translated 'enjoy being fit') intervention on BMI, weight status, waist circumference, physical fitness, lifestyle, psychosocial health and academic performance.

The aim of this study is to evaluate the long-term effects of Lekker Fit! on a). the primary outcome BMI, and b). the secondary outcomes waist circumference, weight status, physical fitness, lifestyle and lifestyle determinants, psychosocial health and academic performance. In order to determine this long term effectiveness, a naturalistic effect evaluation with a retrospective, controlled design will be conducted. Furthermore we will explore whether any encountered effects differs for gender and elapsed time since exposure to the intervention. Also we will explore the association between more years of intervention and the set of outcome variables. The intervention will not be applied to participants in this study, but its long term effects will be retrospectively studied by comparing students originating from 'enjoy being fit' Elementary schools with students originating from regular Elementary schools on the set of outcome variables.

Approximately 2500 students will be included within this study. The study is a naturalistic effect evaluation in which the long term effects of the 'enjoy being fit' intervention are determined. The intervention itself however will not be applied to participants but lies in the past (in elementary school). Measurement instruments in this study are a digital questionnaire, combining several existing valid questionnaires, and anthropometric measurements (BMI and waist circumference) and the 20m-shuttle run test. All measurements take place during school hours in cooperation with the secondary schools in Rotterdam. Finally potential individual and environmental confounding variables will be collected to correct for in analysis.

We expect that this study will give more insight in the long term effectiveness of the 'enjoy being fit' intervention. Furthermore findings from this study will provide foundation for further decision-making by the municipality of Rotterdam regarding their policies for youth and health.

Our hypothesis is that adolescents that have been educated on an 'enjoy being fit' elementary school exhibit healthier scores than adolescents that have been educated on a regular Elementary school on the set of outcome variables. However, we do not know the relationship between longer follow-up and the intervention effects, due to the scarce literature on the sustainability of such intervention effects into adolescence. Furthermore, we hypothesize that more years of Lekker Fit! intervention on an elementary school leads to even more healthier scores on the set of outcome variables due to the prolonged exposure to this behavioral changing intervention.

Update: due to the Corona pandemic, the research has suffered some delay. Therefore time points have been updated. Recruitment will take place until September 2021. All prospective measurements will be conducted once for every participant within the academic years 2020-2022. Weight status baseline measurements will be requested from the Center for Youth and Family (CJG) Rijnmond.

This study is a naturalistic effect evaluation in which the long term effects of the 'enjoy being fit' intervention are being determined. The intervention itself however will not be applied to participants but lies in the past (elementary school). Measurement instruments in this study are a digital questionnaire, combining several existing valid questionnaires, and anthropometric measurements (BMI and waist circumference) and the 20m-shuttle run test. All measurements take place during school hours in cooperation with the secondary schools in Rotterdam.

The 'enjoy being fit' intervention consists of several components of which the following are the most important:
1. the implementation of an additional third physical education lesson every week
2. professional physical education teachers instead of regular classroom teacher providing the physical education lessons
3. voluntary additional physical activities out of school hours
4. the promotion of drinking water over sugar-sweetened drinks
5. the promotion of healthy nutritional and lifestyle behavior