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Despite the strong evidence, Snowdon cautions, it is still possible that folate does not play a direct role in the disease. The link could be coincidental. Or it could be that the same mechanism that triggers the disease also destroys the body's ability to retain folate. But folate and the amino acid it controls, homocysteine, have already been implicated in a broad range of diseases, as well as certain neurological birth defects (see box).
To nail down the low-folate, high-homocysteine link to Alzheimer's, researchers will have to explain how it might be doing damage. They are pursuing a growing list of theories. It may be that folate bestows some protective effect directly on the cell. Or it may be that without folate to control it, homocysteine reaches levels that are toxic to neurons or to the cells that line blood vessels. This could lead to the type of ministrokes found in Snowdon's earlier studies.
Indisputable confirmation will require large-scale clinical trials. Luckily, these may not be long in coming. The National Institute on Aging has already modified a trial currently under way that looks at the influence of supplemental B vitamins, including folic acid, on heart disease in 3,000 older women. To make the Alzheimer's connection, the NIA simply added an annual test of cognitive function to the heart study to see whether the vitamins lower the rate of expected dementia in the group. Results from that study should be available within three years.
Meanwhile, how much vitamin B should you take? That's what the nuns asked too. Unfortunately, there is very little information about how such nutrients are metabolized in the elderly. Snowdon suggests that taking double the current recommended daily allowance is probably a safe precaution. But it is possible to take too much of a good thing. Excessive folic acid can be dangerous if it masks symptoms of other diseases.
So the best advice may be the same that your mother gave you--and that Sister Mary Aloysius, former dietitian at the Mankato, Minn., convent, has been giving the sisters there for the past 30 years: Eat a balanced diet, including plenty of beans and leafy green vegetables. The advice may be the same, but Mary Aloysius reports that ever since the nuns heard about Snowdon's folate findings, they have been crowding around the salad bar.