Eleven studies were included in the review; 10 were prospective cohort studies and one was a cross-sectional study (23,649 participants; range 192 to 8,085). The included studies were based on five cohorts. The median follow-up ranged from 2.2 to 8.0 years. Ten studies were rated positive for quality and one was rated neutral. The limitations were selection bias, missing baseline data, lack of power, short follow-up, lack of generalisability, and issues with statistical analyses.
Cognitive performance: Four studies reported cognitive performance. Adherence to a Mediterranean-style diet was not consistently associated with cognitive performance in two studies. The other two studies suggested that higher adherence slowed cognitive decline.
Mild cognitive impairment and progression to Alzheimer's disease: Mild cognitive impairment was assessed in two studies, with conflicting results; one found no association, and the other found that each additional unit of the MeDi score was associated with an 8% lower risk of developing mild cognitive impairment for patients who were cognitively normal at the start. For patients with mild cognitive impairment at the start, a higher MeDi score was associated with a lower risk of developing Alzheimer's disease.
Dementia and Alzheimer's disease: Four of the six studies that assessed the relationship between adherence to a Mediterranean-style diet and dementia, Alzheimer's disease or both, reported a significant risk reduction. The other two studies reported no association.
Deaths related to Alzheimer's disease: These were assessed in one study, which found a 21% to 24% lower risk of death with each additional unit of the MeDi score. After adjustment for cardiovascular risk factors, this association was weaker, but remained statistically significant.
MRI infarcts and white-matter hyper-intensities: The Mediterranean-style diet was associated with a 9% to 11% lower risk of MRI infarct, but no association was found between the diet and white-matter hyper-intensities (one study).