We employed linear mixed-effects models to assess the association between omega-3 supplementation and longitudinal cognitive decline, and mediation analyses to examine whether this relationship was mediated by core AD pathologies (Aβ-PET, tau-PET, T1-MRI, FDG-PET).<h4>Results</h4>Omega-3 supplementation was associated with significantly accelerated cognitive decline, as evidenced by a faster decrease in MMSE scores (β = -0.266, p < 0.001) and a faster increase in both ADAS-Cog13 (β = 0.823, p < 0.001) and CDR-SB scores (β = 0.205, p < 0.001). This evidence concerns the gene MAPT and Alzheimer disease.