Well, perhaps that’s a bit of an overstatement. Preserved brain function and the Mediterranean diet were positively associated in a study involving Americans in Utah. This fits with prior observations that the Mediterranean diet prevents dementia.
In the study at hand, the DASH diet (Dietary Approaches to Stop Hypertension) also protected the brain:
Higher levels of accordance [compliance] with both the DASH and Mediterranean dietary patterns were associated with consistently higher levels of cognitive function in elderly men and women over an 11-year period. Whole grains and nuts and legumes were positively associated with higher cognitive functions and may be core neuroprotective foods common to various healthy plant-centered diets around the globe.
Advanced Mediterranean Diet 