http://www.medpagetoday.com/PrimaryCare/DietNutrition/tb/1795

The investigators found that folate, in particular, protected against
cognitive decline. Men with higher plasma levels of folate and higher
dietary folate intake showed significantly less loss on cognitive
tests, according to data from the Veterans Affairs Normative Aging
Study of Boston-area men, begun in 1963.

On the other hand, a higher homocysteine level, a biomarker for
vascular disease, was associated with reduced cognitive function, the
Tufts researchers reported in the September issue of the American
Journal of Clinical Nutrition. Increased folate levels have been linked
to reduced homocysteine levels.