Obesity in children is increasing over the last decades and the majority of children in Europe fails to have a healthy diet. In the Netherlands, the majority of primary schools do not provide a (healthy) school lunch. However, previous research shows some positive effects. Within the current study the investigators want to implement healthy school lunches in the city of Rotterdam, a city with a very diverse youth population. The main objective of the healthy school lunch project is to evaluate the effectiveness and feasibility of providing healthy school lunches.

This is a prospective, single center, intervention study using a cross-over design with 3 measurement moments to collect data on effects and feasibility. Additional feasibility data will be collected throughout the study via questionnaires for teachers.

The study population is derived from primary schools participating in the "Enjoy Being Fit!" program in the city of Rotterdam. Children (4-12 years old) from these schools are eligible to participate in this study. Children from participating primary schools will take part in observations and complete questionnaires (only grade 5 to 8). Parents and teachers will complete questionnaires.

Together with individual schools and stakeholders a healthy school-lunch concept will be developed and implemented for half a school year. Data will be collected on three time points during the school year.

Questionnaires for children and parents will focus on dietary behaviour/food intake, taste preferences, perceived health, and the satisfaction with the healthy school lunch (also for teachers). Atmosphere in the class during lessons and during lunch will be measured in questionnaires for children and teachers. The questionnaire for parents also focuses on affordability of a healthy school lunch. Observation forms will be used to score on one day what snack and lunch children bring to/get at school.

Inclusion Criteria:

• Schools: Schools will be included if they are a LF! school, have a high deprivation score and ≥ 20% overweight/obesity, and are willing to participate in the project irrespective of starting in the intervention or control period. Schools should be willing to facilitate retrieving consent from parents and should provide students time to complete the questionnaire during school hours. They should also be willing to participate in a co-creation event, in which the lunch concept for their school will be designed together with school staff, parents, and children. Schools should be willing to facilitate the implementation of a healthy school lunch and should not provide any extra activities regarding healthy school lunches during the control period.
• Children: All children of the participating schools will be eligible to receive a healthy school lunch, also if parents do not provide informed consent for the research project. Data will be gathered for all children for whom informed consent is provided for participating in the data gathering. Informed consent must be signed by the parents of children who are allowed to participate in the research activities (observations and questionnaires) in the school. Additionally, when children are 12 years or older they have to sign an informed consent themselves.
• Parents: All parents with children in the participating schools will be included into the project to complete questionnaires. Informed consent must be signed by the parent.
• Teachers: All teachers in participating will be included into the project to complete questionnaires. Informed consent must be signed by the teacher.

Exclusion Criteria:

• The non-compliance of the inclusion criteria will be considered as exclusion criteria.

A data sharing statement will be finalized when known which percentage of participants consent for sharing data outside of our own organization.