Evidence for...folic acid for memory and dementia

Effect of 3-year folic acid supplementation on cognitive function in older adults in the FACIT trial: a randomised, double blind, controlled trial.
Durga J et al.
Lancet. 2007;369:208-16.
Folic acid supplementation for 3 years significantly improved memory, information processing speed and sensorimotor speed in 818 older adults taking 800 µg/day.

Folic acid and polyunsaturated fatty acids improve cognitive function and prevent depression, dementia, and Alzheimer's disease--but how and why?
Das UN.
Prostaglandins Leukot Essent Fatty Acids. 2008;78:11-9.
This study reports that low blood folate and elevated homocysteine levels are associated with poor brain function. It appears that folic acid enhances blood levels of docosahexaenoic acid and eicosapentaenoic acid (omega-3 essential fatty acids), which also appear to have some beneficial effects for promoting healthy brain function.

Changes in folate, vitamin B12, and homocysteine associated with incident dementia.
Kim J-M et al.
J Neurol Neurosurg Psychiatry. Published Online First: 5 February 2008.
In a community population prospective study, low folate concentrations appeared to predict the onset of declining memory and brain health over about 2 ½ years of follow-up. It appeared that those individuals with lowest levels of folate in their blood increased their risk of poor memory and brain health by about 350%.