Omega-3 fatty acids demonstrated some interest as nutraceutical therapies for CNS disorders, such as Alzheimer's disease, where cerebral omega-3 levels (DHA) are known to be deficient in brain regions associated with disease, but these results are still discussed. Results from MAPT, the largest randomized controlled trial in this field, highlight omega-3 role on prevention of cognitive decline in subjects with lower RBC omega-3 levels. These results need to be confirmed in a second trial. Omega-3 treatment can provide a cost effective therapy, well tolerated, without any severe known side effects. Our aim is to prevent cognitive decline before the onset of dementia in omega-3 deficient elderly subjects by re-establishing and maintaining homeostatic levels. In MAPT 2, we propose to explore this targeted approach by testing the efficacy of a supplementation of DHA+EPA (versus placebo) on cognitive decline in subjects 70 years and over with lower RBC DHA/EPA index (≤ 4.83%) at study entry for a period of 18 months. After 18 months all participants will receive the same amount of omega-3 in an open phase for one year