The findings are unexpected as folate, a B vitamin, helps break down the amino acid homocysteine, high levels of which are linked to Alzheimer's disease.
High levels of homocysteine have also been linked to an increased risk of stroke.
The study measured cognitive decline in more than 3,700 elderly people living in Chicago, aged at least 65 years-old at baseline. They were followed up after three years, and again after six years, using the average score of four different cognition tests. Folate intake was assessed using a food frequency questionnaire.
Those with the highest folate intake, an average 742mcg per day, had more than twice the rate of cognitive decline as those in the lowest fifth of intake (186mcg per day), said the researchers.
"A faster rate of cognitive decline was also associated with high folate intake from food and with folate vitamin supplementation of more than 400mcg daily compared with nonusers," write the researchers in this month's issue of the Archives of Neurology (vol 62, pp641-645).
The findings could add fuel to the European debate over fortifying food with the vitamin to reduce incidence of neural tube defects. In the US, where the research was carried out, flour has been fortified with folic acid since 1998, increasing intake of the vitamin across all segments of the population.
The UK opted not to introduce such a policy, for fear of masking B12 deficiency in the elderly. The new study could offer some signs of this effect.
However Paul Finglas, senior research scientist at the Institute for Food Research, told NutraIngredients.com that the study data was "prone to large error".
"They've assessed the folate intake using a food frequency questionnaire, which could underestimate or overestimate folate intake by as much as 50 per cent," he said.
A blood sample would have helped to validate the information gathered by the questionnaire and could also assess folate status over the long-term, he added, giving the hypothesis better support.
"The study does not show that high folate intake leads to faster cognitive decline and I don't think evidence for the association is particularly strong," added Finglas, also co-ordinator for the European Union-funded research project on folate, FolateFuncHealth.
The researchers also tested for the impact of vitamin B12 and found that high intake of the vitamin was only associated with slower mental decline among the oldest study participants.