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Adhering to an alternative Mediterranean diet high in vegetables, whole grains, fish, and olive oil was linked to lower risk of cognitive impairment but not to slower cognitive decline in post-hoc analyses.

Data from two trials -- the Age-Related Eye Disease Study () and -- showed that high fish and vegetable consumption appeared to have the greatest protective effect on cognition, reported Emily Chew, MD, of the NIH's National Eye Institute (NEI) in Bethesda, Maryland, and colleagues.

At 10 years, AREDS2 participants with the highest fish consumption had the slowest rate of cognitive decline, they wrote in .

"We do not always pay attention to our diets," Chew said in a statement. "We need to explore how nutrition affects the brain and the eye."

AREDS and AREDS2 looked at the longitudinal effect of supplements on age-related macular degeneration (AMD). AREDS participants were randomly assigned to placebo, antioxidants, zinc, or the combination. AREDS2 participants were randomly assigned to receive AREDS supplements, alone or with additional lutein/zeaxanthin, and docosahexaenoic acid (DHA) plus eicosapentaenoic acid (EPA).

The AREDS post-hoc analysis included 3,029 participants with no AMD to unilateral AMD and an average baseline age of 69. The AREDS2 analysis had 3,326 participants with bilateral large drusen or unilateral late AMD and an average baseline age of 73. Both groups were about 57% female.

Researchers used food frequency questionnaires to assess diets of participants at baseline. To calculate a modified Alternative Mediterranean Dietary Index (aMED) score, researchers measured intake of nine diet components: whole fruits, vegetables, whole grains, nuts, legumes, red meat, fish, monounsaturated fatty acid to saturated fatty acid ratio, and alcohol. Intake quartiles were calculated for each component; the aMED score was the sum of quartile values for the nine components, ranging from 9 to 36.

AREDS tested participants' cognitive function at 5 years; AREDS2 tested cognitive function in participants at baseline and 2, 4, and 10 years later. Cognitive function tests included the modified Mini-Mental State Examination (MMSE) in AREDS and the Telephone Interview Cognitive Status-Modified in AREDS2.

In AREDS, closer adherence to a Mediterranean diet was associated with a lower risk of cognitive impairment on modified MMSE (P=0.0001) and composite cognitive scores (P=0.001). In AREDS2, closer dietary adherence was tied to better Telephone Interview Cognitive Status‐Modified scores and composite scores (each P<0.0001). Apolipoprotein E (APOE) genotype did not affect these relationships.

Fish intake was associated with higher cognitive function. In AREDS2, rate of cognitive decline over 5 to 10 years was not significantly different by aMED score, but was significantly slower (P=0.019) with higher fish intake. Significant protective associations were seen for vegetables in both studies, and for nut intake and moderate alcohol intake in AREDS2.

The study had several limitations, including post-hoc hypothesis generation. Diet was assessed only once, at baseline. Participants with substantial cognitive impairment were likely excluded from the studies, Chew and colleagues noted, and unmeasured confounders may have influenced results.

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