While omega-3 fatty acid supplementation is widely used for cognitive protection, its efficacy remains controversial, and its impact on core Alzheimer's disease (AD) pathologies in humans is not well-established.

This longitudinal study utilized data from the Alzheimer's Disease Neuroimaging Initiative (ADNI). 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).

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 association was not mediated by Aβ deposition, tau pathology, or gray matter atrophy. Instead, longitudinal FDG hypometabolism within AD-vulnerable regions served as a significant mediating pathway, accounting for 30.8%, 40.8%, and 19.0% of the total effect on the decline in MMSE, ADAS-Cog13, and CDR-SB, respectively.

Omega-3 supplementation may be associated with accelerated cognitive decline in older adults, potentially through adverse effects on cerebral synaptic function rather than classical AD proteinopathies. These findings challenge the prevailing view of omega-3 as uniformly beneficial and highlight the need for a cautious reassessment of its widespread use for cognitive protection.