WP 1: The walnut intervention: Design and setting: This trial cannot be blinded due to the nature of the intervention. The control group will receive no intervention, apart from general healthy eating recommendations (as in the intervention group) and follow-up assessments. Recruitment: Nurses will recruit teenagers (n=400) in collaboration with 20 high schools from Barcelona. Implementation and Adherence: The investigators will perform age- and gender-stratified random sampling to assign subjects to one of the two groups. All families will receive a basic guide on following a healthy diet in order to ensure implementation and adherence. Families in the intervention group will receive additional instructions on how to encourage the adolescent to eat 30 g of walnuts per day during 6 months, which the investigators will supply for free. The nurses will contact the families every 3 months for a short checking-interview and to administer a food frequency questionnaire (FFQ).