Researchers in South Korea measured naturally occurring folate levels in 518 elderly persons, none of whom showed any signs of dementia, and then tracked their development over 2.4 years.
At the end of the period, 45 of the patients had developed dementia, including 34 diagnosed with Alzheimer's disease, said the study, published by the British Medical Association's Journal of Neurology, Neurosurgery and Psychiatry.
When the researchers, led by Jin-Sang Yoon of Chonnam National University in Kwangju, South Korea, remeasured folate levels, they uncovered a strong link with the dementia.
Even after other factors were taken into account -- including age, disability, alcohol consumption, weight change -- "the onset of dementia was significantly associated with an exaggerated decline in folate," the researchers concluded.
Folate and folic acid, another form of the compound, are essential for the creation of new cells in the body.
The compound occurs naturally in leafy vegetables such as spinach, turnip greens, lettuces, dried beans and peas and in certain fruits.
An study published last year in The Lancet showed an improvement in short-term memory, mental agility and verbal fluency among persons over 50 who took a daily dose of 800 micrograms (mcg) of folic acid. The US recommended daily dose is 400 mcg.
Taking folic acid before conception and throughout the first trimester helps a mother ensure that her child will not develop certain brain and spinal cord defects, including spina bifida, according to previous research.