Folic acid, a nutrient found in green leafy vegetables, liver, kidney, whole grains and nuts, is important in the development of the central nervous system and in the maintenance of blood vessels. Lack of this nutrient can cause birth defects in the developing fetus. Women who have low levels of folate, the by-product of folic acid found in the blood, appear to be at greater risk of Alzheimer's disease.
In the study of 30 nuns who participated in a long-term study of Alzheimer's disease, half had brain changes consistent with the memory-robbing illness at autopsy. The women, aged 78 to 101 when they died, had lived at the same convent for most of their lives.
The authors note that the study could not determine whether low levels of folate actually cause Alzheimer's. And the findings do not provide any evidence that taking folic acid supplements can prevent the disease or slow it down. It is possible that the women had low blood levels of folate due to problems absorbing or metabolizing the nutrient. The women all ate in the same kitchen and, presumably, had similar intakes of folic acid.
American Journal of Clinical Nutrition April 2000;71:993-998
More important confirmation of increasing the amount of fresh vegetables in our diet which is the main source of folate. I would NOT use this study as an endorsement of taking folic acid supplements to prevent Alzheimer's disease. It is far more likely that other phytonutrients in the vegetables that are associated with the folic acid are responsible for the observed benefit.