Although it’s mechanistically plausible that folic acid supplementation could improve cognitive function, in clinical trials it does not seem to have a significant effect.
High levels of homocysteine and low levels of folate have been associated with an increased risk of dementia and Alzheimer’s disease.[1][2] While this doesn’t prove that high homocysteine and low folate can cause cognitive decline, mechanistically such an effect could make sense.
Folate deficiency can lead to high levels of homocysteine which in turn can result in oxidative stress and stimulation of N-methyl-D-aspartate (NMDA) receptors, both of which could have toxic effects on neurons.[1] Additionally, folate is involved in the development and repair of nerve cells.[3] Despite this, clinical trials have generally been anticlimactic, and folic acid supplementation has demonstrated little to no benefit on various measures of cognitive function, including memory, in older adults with or without dementia.[4][5]
In fact, folic acid fortification in North America has been associated with an increased risk of cognitive impairment in older adults.[6][7] This may be because folic acid can mask the hematological signs of a vitamin B12 deficiency, potentially leading to an undiagnosed deficiency which may result in irreversible cognitive decline in the long term.[6]