WP 3: Study the intervention effects and the role of omega-3 polyunsaturated fatty acids: In order to investigate the effects of the intervention, valid measures of the outcomes are necessary (WP2), as are valid measures of the intermediate factors, in this case plasma n-3 PUFA levels. Blood samples will be collected at baseline and after the intervention, centrifuged, and plasma samples stored at -80°C until analysis. Statistical Analysis: The following statistical analyses will be carried out: 1) Assess the effect of the intervention on the ratio of n-6 PUFA to n-3 PUFA (n6-n3 ratio) using linear regression models adjusted for baseline levels. 2) Assess the effect of the intervention on brain development, using the method described in 1) above; 3) Assess the effect of n6-n3 ratio on brain development at baseline, and of n6-n3 ratio change on brain development change from baseline to follow-up. 4) Assess the proportion of the effect of the intervention on brain development that is mediated by n6-n3 ratio change. This proportion can be calculated by comparing results from models with and without the mediator. Analyses 1), 2) and 3) (i.e. those involving the intervention) will be based on intention-to-treat, i.e. using intervention (no/yes) regardless of compliance, in order to estimate the effects of the policy (i.e. to encourage eating 30 g of walnuts per day), rather than the effect of walnut consumption.