In 1998, when the U.S. government required manufacturers to enrich bread and grain products with the B vitamin folic acid, the goal was to reduce the number of babies born with neural tube birth defects. It worked—incidence has plummeted about 30 percent. But there's a hitch: A diet high in folic acid, if not also sufficient in vitamin B12 , can worsen your cognitive performance.

Both folic acid and B12 nourish the nervous system; researchers have long known that being deficient in either can impair thinking. The elderly are at heightened risk because they have a tougher time absorbing the B12 . Strict vegetarians may also fall short because the major sources of B12 are meat, seafood, and dairy products.

In a recent Tufts University study of 1,459 people over 60, researchers found that 25 percent were low in B12 —and that this group did poorly on a test assessing response time, attention span, memory, and learning. Those who also had high folic acid levels fared the worst, scoring about five times below people with normal levels. That's not to say that folic acid is the culprit: What's optimal is high levels of folic acid along with normal levels of B12 .

The researchers warn the elderly to monitor their intake of both B vitamins. The best approach for tracking B12 is to get a routine blood test, says Edward H. Reynolds, MD, consultant neurologist at King's College London School of Medicine. For vegetarians, taking a daily supplement is essential.