Objective: The investigators hypothesize that walnut intake will increase n-3 PUFA availability in the body to a level that enhances the development of the brain during adolescence. The technicians will conduct a long-term (6 months) population-based randomized controlled trial in teenagers (n=400), and determine the effectiveness of the intervention (30 g of walnut kernels per day, ~1.5g of n-3 PUFA) in enhancing brain development.