Fornix fractional anisotropy mediates the association between Mediterranean diet adherence and memory four years later in older adults without dementia

Background The mechanisms in the brain that explain the benefits of adherence to the Mediterranean diet (MeDiAd) for cognition are incompletely understood. Here, we investigated whether fractional anisotropy (FA) in hippocampus-relevant white-matter tracts mediates the association between baseline MeDiAd and verbal episodic memory over four years.

Methods Participants with baseline diffusion-weighted imaging data from the DELCODE cohort study were selected, including healthy older adults with and without subjective cognitive decline and patients with mild cognitive impairment (n = 376; age: 71.47 ± 6.09 years; 48.7% female). Demographic, MeDiAd, and diffusion data were obtained at baseline. Verbal episodic memory was assessed at baseline and four yearly follow-ups. The association between baseline MeDiAd and verbal episodic memory’s mean and rate of change over four years and the mediation of that association by baseline white-matter tracts were tested with latent growth curve modeling. Potential mediators were selected based on the association with hippocampal volume.

Results Baseline MeDiAd was associated with verbal episodic memory four years later (latent intercept; 95% confidence interval, CI [0.01, 0.32]) but not with its rate of change (latent slope) over this period. Only baseline Fornix FA, among four potential mediators (the cingulum ventral, corticospinal tract, and superior longitudinal fasciculus II), mediated this association (latent intercept; 95% CI [0.002, 0.09]).

Conclusions Higher Fornix FA explains the association between higher baseline MeDiAd and better memory four years later in pre-dementia stages. Fornix FA may be a useful response biomarker of Mediterranean diet interventions on memory.