Many of us occasionally find ourselves eclipsed by mental fog. Our mind wanders in a lecture, and we miss its key point. We cannot focus on writing an article or preparing a presentation. We are unreasonably slow to calculate a waiter's tip at a restaurant--and then suddenly fail to recall a colleague's name when introducing her to a friend.

Mental slipups and slowdowns are a part of life, but we may be able to prevent some of them by paying attention to what we eat. Our diet affects not only our overall health and emotional well-being [see "Feeding the Psyche," by Michael Macht, on page 64] but also our ability to think, studies show. Nutrients in foods--or a lack of them--can influence memory, learning, concentration and decision making.

The brain operates best, for example, when blood glucose is stable. Consuming complex carbohydrates rather than simple sugars, researchers say, can help stabilize glucose in the blood and guard against mental lapses. In addition, consuming adequate amounts of iron is important for staying mentally sharp, because that metal chaperones vital oxygen to the brain. And studies indicate that protein-packed fare seems to boost attention, whereas certain fatty acids found in fish buttress brain function.

When we eat is also important. Research confirms, as the old adage advises, that breakfast is critically important for mental function in the morning. Snacking throughout the day can also be helpful in keeping blood glucose levels stable.

In general, what is good for the body is also good for the brain. Although much remains to be discovered about food's effect on mental function, the findings to date strongly suggest that good nutrition can help us more fully realize our intellectual potential.

Fuel for Thought
The brain takes shape in the womb and continues to mature in infancy and beyond. Its growth and development depend on adequate energy and on a variety of specific nutrients such as certain fats and proteins from the mother's diet. These molecules are especially critical for building the fatty membranes of nerve cells and the layer of insulation--called myelin--that encases nerve fibers. Studies of nutritionally deprived children in the developing world show that such malnourishment depresses IQ.

Regular food consumption also ensures that the 100 billion nerve cells in the adult brain remain active at all times. Although the brain makes up a mere 2 percent of our body weight, it uses 20 percent of the body's metabolic fuel. The brain, unlike muscles, cannot store carbohydrates, and so it requires a constant supply of glucose. When blood glucose drops, say, from lack of food, our faculties fade and we lose the ability to concentrate.

If you fast for several days, the cognitive crisis worsens, because the brain must get the energy it needs by metabolizing compounds called ketone bodies, which are derived from the breakdown of body fat. That metabolic process requires the synthesis of specialized enzymes, a time-consuming activity that can lead to further lapses in concentration.

Very high blood glucose levels can also inhibit mental function. In a study reported in 2005, psychologist Daniel J. Cox and his colleagues at the University of Virginia Health System found that about half of the 230 diabetics they were monitoring performed basic verbal and math tasks more slowly and less accurately when their glucose levels rose above a certain threshold. Hyperglycemia might cause cognitive difficulties, the scientists speculate, by altering the structure of blood vessels at the blood-brain barrier, for example, or by triggering changes in the production of chemical messengers in the brain.

Such research indicates that a moderate, stable blood glucose level is the best way to buttress intellectual functioning. Most people can prevent wild fluctuations by being selective about which carbohydrates they consume.

Simple sugars such as sucrose (table sugar) and lactose (milk sugar) elevate blood glucose levels powerfully and quickly. Sweet foods can supply the body with quick energy--a useful pick-me-up when blood glucose plummets, such as from extreme physical exertion--but the effect does not last. The pancreas responds to a rapid rise in glucose by secreting the hormone insulin, which accelerates glucose uptake by body tissues--and glucose levels plunge once again. Starch- and fiber-rich foods such as whole-grain breads, legumes and vegetables, on the other hand, are thought to be better for brain function because they raise glucose levels slowly and moderately. Fiber, which is indigestible, also slows uptake.

Recent data support the idea that starchy and fibrous fare promote mental endurance. A research team led by cognitive psychologist Keith A. Wesnes of Northumbria University in England gave 64 children (aged six to 11) on two mornings either a sugary breakfast cereal that quickly boosted blood glucose to high levels or a starchier cereal that gradually raised concentrations. The investigators administered attention and memory tests at hourly intervals thereafter and found that although all the kids' test scores declined as the morning wore on, the decline was markedly less steep in the children who had breakfasted on the complex carbohydrates.

The Iron Effect
To metabolize glucose, our brain cells require oxygen, which is transported to the brain by hemoglobin, the large iron-containing protein in red blood cells. Consuming enough iron is thus important for mental function. In childhood, iron deficiencies impair brain development and lead to measurable deficits in speech, reading and math skills. In a 2005 review of the literature, for example, pediatrician Howard Taras of the University of California, San Diego, found that severely iron-deficient children are at an academic disadvantage and that iron supplements can reduce that intellectual deficit.

Among adults, women of reproductive age need to consume the most iron; the U.S. recommended daily allowance of this element for such women is 18 milligrams, compared with just eight milligrams for men and postmenopausal women. In a March study of reproductive-age women, nutrition scientists Laura E. Murray-Kolb, now at Johns Hopkins University, and John L. Beard of Pennsylvania State University found that concentrations of iron in the blood can also influence mental function in adulthood.

At the start of the study, only 42 of the 149 subjects had sufficient iron in their blood, and these women performed cognitive exercises better and faster than the women who were iron-deficient. In addition, the researchers found that 16 weeks of iron supplementation closed the intellectual gap for the anemic women who received it, improving their cognitive performance between five- and sevenfold.

Red meats such as beef or lamb contain the most easily absorbed iron, the so-called bivalent form. Plant seed oils, yeast, and some herbs and legumes carry trivalent iron, which is harder for the body to use. Foods rich in vitamin C--such as oranges and garlic--can aid iron absorption, however, so vegetarians can improve their iron status by eating such foods in conjunction with iron-containing plant foods.

Other vitamins, minerals and trace elements are important for brain function, too [see table on next page]. Potassium, sodium and calcium are used for nerve cell signaling and metabolic reactions in the brain. Vitamin B1, in particular, enables glucose metabolism. Even slight vitamin and mineral deficits--which may result, for example, from a diet of fast food--can lead to fatigue, forgetfulness and concentration problems.

An overdose of vitamins and minerals, however, is unlikely to turn you into Einstein. Correcting a vitamin deficiency may raise a child's IQ, but it is not clear whether supplements can boost intelligence in people whose nutrient intake is adequate, according to a 2004 review of the literature by nutrition researcher France Bellisle of Htel-Dieu Hospital in Paris.

Protein Power?
The adult brain is also dependent on amino acids, the building blocks of proteins, for producing enzymes, transport molecules, structural materials and neurotransmitters (the brain's chemical messengers), among other essential molecules. For example, the amino acids tyrosine and phenylalanine are needed to produce the hormone epinephrine and the neurotransmitter dopamine, both of which contribute to alertness. A boost in these amino acids could partly explain why small high-protein meals featuring, for example, low-fat dairy products, fish, lean meats and legumes, may make people more alert and attentive, as some studies have indicated.

Protein may also boost attention by stabilizing blood glucose levels. In a 2002 study of 15 healthy male students who ate meals with differing ratios of carbohydrate to protein, nutrition scientist Karina Fischer of the Swiss Federal Institute of Technology in Zurich and her colleagues found that, relative to the carbohydrate-rich meal, the balanced and protein-rich meals led to more accurate short-term memory and improved attention beginning one hour after the meal was consumed. The meals with more protein seemed to cause less variation in glucose metabolism, implying that proteins may be useful in part because they help stabilize glucose levels.

High-protein meals, however, have a paradoxical effect on levels of another amino acid, tryptophan. Tryptophan is a precursor of the neurotransmitter serotonin, which not only helps to stabilize mood [see "Feeding the Psyche," by Michael Macht, on page 64] but also may influence cognitive processes, particularly learning and memory. Because most food proteins contain less tryptophan than other amino acids, which compete with tryptophan for transport into the brain, high-protein meals actually decrease the brain's tryptophan levels.

How such a decrease affects cognition is controversial. Some human and rat studies indicate that tryptophan depletion leads to deficits in long-term memory and information processing, whereas other data suggest that depleting the body of tryptophan has a beneficial effect: it improves decision making.

Boosting tryptophan levels in the brain, on the other hand, can benefit cognition under some circumstances. The consumption of carbohydrates pushes tryptophan into the brain. Although carbohydrates do not contain tryptophan, they trigger the release of insulin, which stimulates muscles to take up competing amino acids. Tryptophan then becomes relatively abundant in the blood--and more likely to get into the brain. Work by experimental psychologist C. Rob Markus, now at Maastricht University in the Netherlands, and his colleagues shows that carbohydrate-rich diets that increase the amount of available tryptophan improve cognitive performance--but only in stress-prone people. In such cases, some researchers speculate, the resulting swell of serotonin may provide a mental edge in part by decreasing a person's anxiety about performing intellectually challenging tasks.

Smart Seafood
Unsaturated fats are also good brain food, especially the polyunsaturated omega-3 fatty acids found in fish such as mackerel, tuna, herring and salmon. These fish oils are components of nerve cell membranes and myelin, and they help to keep blood vessels in the brain healthy. Statistics show that eating as few as one to three portions of fish per month significantly decreases the risk of stroke.

Recent studies have found that fish consumption benefits the fetal brain, too. Nutrition scientist Joseph Hibbeln of the National Institute on Alcohol Abuse and Alcoholism and his colleagues surveyed 11,875 pregnant women about their seafood intake and evaluated the behavioral and cognitive development of their offspring from six months to eight years of age. They found that the children whose mothers had eaten less than 340 grams of fish a week during pregnancy were more likely to have lower IQs and poorer communication, fine-motor and social skills than were the children of mothers who had ingested more fish during pregnancy. These findings imply that nutritional benefits of eating fish may outweigh the risks of exposure to trace contaminants.

A person may gain similar health benefits from linseed, canola, soy and walnut oils. These oils contain significant quantities of alpha-linolenic acid, a shorter-chain lipid that the body converts into omega-3 fatty acids. But one should avoid using these plant oils for frying or sauting because high heat turns them into trans-fatty acids, which may have detrimental effects on learning and overall health.

Nutrients from omega-3 fatty acids (or any other food) can reach the brain in adequate amounts only if the body gets enough fluid. Studies have shown that even slight dehydration slows the rate at which nutrients can enter the brain, producing short-term memory deficits and reasoning difficulties among other cognitive problems.

Caffeinated beverages such as tea and coffee have an additional advantage in limited quantities: caffeine can improve short-term concentration and facilitate learning and memory. Coffee's effect takes hold within about 20 minutes and lasts for two to three hours. Tea has a weaker but longer-lasting impact because it contains less caffeine than coffee and its caffeine is released more slowly. Drink too much caffeine (four cups of coffee or more), however, and your ability to concentrate will likely decline, studies suggest.

Perfect Timing
When a person eats can also influence cognitive performance. Eating breakfast is particularly important for cognitive function, yet some 10 to 30 percent of American and European children skip this meal. Results from 22 studies of the link between breakfast consumption and academic performance in school-age youth show that breakfast eaters have better memories, test scores and school attendance rates, according to a 2005 analysis by food scientist Gail C. Rampersaud of the University of Florida and her colleagues. A second 2005 literature review led by Taras shows that school breakfast programs improve cognitive functioning and academic performance among severely undernourished populations.

Between-meal snacks can ensure a consistent blood glucose level and thus prevent or reduce performance troughs. Choose combinations of complex carbohydrates and protein--say, fruit and yogurt or whole-grain bread and low-fat sausage or cheese. A protein-rich snack such as a tuna fish sandwich eaten shortly before a quiz or important meeting may help ward off inattention. If the exam or meeting will last longer than 20 minutes, avoid simple sugars to prevent a blood glucose drop before it ends.

But any snack boosts glucose, and at least one study shows that the mere act of chewing can improve memory. Cognitive neuroscientist Lucy Wilkinson and her co-workers at Northumbria reported in 2002 that subjects who chewed sugar-free gum were better able to remember words than subjects who did not chew anything, perhaps because chewing improves blood flow to brain areas that are important for memory.

Eat a low-calorie, protein-rich lunch that also includes lots of vitamins and minerals--say, fish or chicken with a salad. This repast will maintain attention and memory and minimize afternoon energy troughs. And at dinnertime, eat lightly and avoid caffeine. Studies show that rice, noodle, or grain dishes boost blood tryptophan levels, which can hasten sleep. Or simply relax with Grandma's recipe: honey dissolved in hot milk.