Can you remember phone numbers as well as you used to? What about "folding flap A into notch B" to put together a gift box or some contraption you brought home from the hardware store? If your memory and ability to complete spatial tasks isn't what it once was, perhaps you're a little low on some of the B vitamins.

Nutritionists have known for some time that severe deficiencies in the B vitamins can seriously affect cognitive abilities--mental processes involving memory, perception, judgment and reasoning. Tufts University researcher Robert Russell, for instance, relates the story of an older woman whose middle-aged daughter was frantic because her mother had gotten to the point where she couldn't remember anything from one minute to the next. The daughter was afraid it was Alzheimer's. It wasn't. Her short-term memory lapses, it turned out, were fueled by a severe inability to absorb vitamin B12. Once she was given absorbable B12 supplements, her failing memory largely reversed itself, and her mental decline slowed considerably.

Of course, that was an unusual case. Nutrition is only one of many factors that affect how well the mind functions. In the vast majority of instances, if someone starts to lose his or her mental facilities, a vitamin pill alone is not going to solve the problem. But in the same way that scientists are learning how food choices can influence the health of organs such as the heart, they are starting to make inroads on how foods might influence the health of the brain.

One of those discoveries is that when blood levels of various B vitamins fall even to the low end of the so-called normal range, mental dexterity may be compromised. The effects might not be glaringly apparent, but they could affect quality of life nonetheless.

Consider a Tufts study of men in their fifties and older who were asked to perform a battery of cognitive tests. Those whose blood levels of the B vitamins folate and B12 were on the relatively low side (but still normal) did not perform as well as men with higher levels of those two nutrients on a spatial task that involved copying cubes and other geometric shapes. Those with relatively low levels of B6 were not as good at memory tests, such as listening to a series of numbers and then repeating them in backward order.

The study is preliminary, but it's certainly not a bad idea to make sure you get plenty of B vitamins. Good sources of folate include legumes (such as kidney beans, chickpeas and lentils), green leafy vegetables, grain-based foods and orange juice. B6 is also widely available in vegetables as well as in fruits and can be found, too, in beef, poultry and seafood.

With B12 it gets a bit complicated. While it comes in every animal food from meat to fish to cheese to poultry, up to 20 percent of people over age 60 and 40 percent of those over 80 don't have enough stomach acid to break it apart from the protein in those foods and thereby absorb it. That's why older people should eat cereals fortified with B12 or take a multivitamin supplement. It's not attached to protein in those sources and therefore is easily available to the body from those sources, stomach "juice" or not.

Along with the B vitamins, vitamins C and E and beta carotene may optimize brain health. In 1997, scientists working on federally funded research called the Alzheimer's Disease Cooperative Study found that people with Alzheimer's disease who took large doses of vitamin E delayed such outcomes as the loss of ability to feed or groom themselves.

In addition, a large research project in Switzerland found that people in their sixties and older who had the highest blood levels of vitamin C and beta carotene (both available in fruits and vegetables) did better on tests of memory than those with lower levels. Beta carotene also appeared protective against cognitive impairment in research looking at older adults in the Netherlands. Similarly, vitamin E (found in nuts and oils) and vitamin C seemed to stave off losses in cognitive function in Japanese-American men living in Hawaii, according to another study.

Scientists speculate that these vitamins, called antioxidants, may reduce oxidative stress to brain cells called neurons. Such stress occurs when highly volatile forms of oxygen damage cell structure in the course of normal chemical reactions that occur throughout the body. That damage keeps neurons from functioning as well as possible, according to the theory.

Yet another eating-related factor that can affect the brain's function is high blood pressure. This serious medical condition can, of course, severely compromise mental capacity by causing a stroke. But scientists are finding that elevated blood pressure, or hypertension, can also impair mental acuity in much more subtle ways over time.

Granted, hypertension is more common among middle-aged and older people than in young adults, and age itself impacts brain function. That's because as the decades pass, the brain undergoes reductions in volume and other changes which, while they don't make people feeble-minded, do slow the processing of information. (That's why an older person might become frustrated that he or she can't, say, memorize a new phone number quickly enough.) But when aging is accompanied by high blood pressure, mental functioning can be compromised further, researchers are learning.

A National Institute on Aging study conducted on people in their mid-fifties demonstrated the point. Those with high blood pressure scored lower on language and memory tests than those whose blood pressure was in the normal range. According to the researchers, they underwent more brain atrophy and also experienced reductions in gray matter. In addition, there was a reduction in the thalamus--part of the brain structure that potentially influences everything from motor and sensory functioning to the ability to pay attention.

Losses in mental acuity were also seen in men followed in a study occurring in several cities around the country. Those who had high blood pressure for at least 15 years starting in middle age had 50 percent more cognitive decline than men with normal blood pressure. Men who had high blood pressure for at least 25 years and were in their sixties and seventies underwent a doubling in cognitive decline. The reason appears to be related to the fact that they ended up with twice as much brain fluid--a sign of small, undetected strokes.

The high blood pressure that linked to mental decline was not necessarily very high, suggesting that even slight variations in blood pressure that were once thought to be meaningless could have significant impact on brain function.

What does all this have to do with eating? The answer is that high blood pressure can be controlled to a large extent through dietary measures. Losing excess weight, if necessary, is the most important lifestyle weapon against high blood pressure. Even a 10-pound loss can reduce blood pressure significantly. Other recommended dietary steps for getting blood pressure down: have no more than two drinks of alcohol a day (one for women) and consume no more than 2,400 milligrams of sodium daily. Finally, eating more potassium-rich foods helps, too. That means more fruits and vegetables--the same items that contain folate, B6, vitamin C and beta carotene.