METHODOLOGY The WALNUTB scientific approach consists of an ambitious randomized controlled trial in a large population-based sample. Sample: The trial will include 400 healthy teenagers (12-15 year old) from Barcelona city (1,900,000 inhabitants), who will be randomly assigned to two groups (intervention & control) and followed up for 6 months (de Ruyter, 2012) with two identical series (before and after the intervention) of multimodal assessments of brain function, and biomarkers of n-3 PUFA. Power calculation for neuropsychological outcomes: Assigning 200 participants each to the intervention and control groups provides 85% statistical power to detect an intervention effect of 25 points on working memory neuropsychological score (mean 126, typical deviation 100 in the general population). The following additional specifications were assumed for the power calculations: 10% loss of follow-up; intra-class correlation between repeated measures 0.2; and a type-I error of 0.05.