"Eat your fish," my mother would say, "It's brain food." She was not a nutritionist. She was merely citing an item of food folklore that has been around for ages.

Judith Wurtman, on the other hand, is a nutrition scientist at Massachusetts Institute of Technology. When her daughter was studying for an important exam, Wurtman recommended tuna "to keep her mentally alert." And when Wurtman travels, she carries cans of tuna fish for snacks.

Wurtman and her colleague and spouse, Dr. Richard J. Wurtman, director of M.I.T.'s Clinical Research Center, have measured the reactions of the brain to different foods. They and other researchers have discovered that food affects the mind in powerful and surprising ways. What you put in your mouth can change your mood, alertness of memory and clarity of thought.

"It remains peculiar to me," Richard Wurtman has said, "that the brain should have evolved in such a way that it is subject to having its function and chemistry depend on whether you had lunch and what you ate. I would not have designed the brain that way myself." But nature apparently did, and what scientists have learned about it can be of great value to us.

Use their findings in the course of an average day, and you may well gain mental acuity:

Breakfast. The first thing in the morning, many of us feast on carbohydrates such as congee and rice dishes. These starches increase the presence in the brain of the soothing neurotransmitter serotonin. As a result, we might not reach our normal morning energy peak.

Eggs, sausage and bacon contain high fat and cholesterol. They are slow to digest, diverting blood from the brain and thereby reducing mental sharpness. "When I see people eating these foods for breakfast," says Judith Wurtman, "I feel like shaking them and saying, 'Why are you eating that food? Don't you know how you're going to feel?'"

A good breakfast, scientists now believe, features food low in fat. This means choosing lean meat instead of sausage or bacon and fresh fruit or juice instead of sugary foods. The fructose sugar in fruits digests slowly and does not trigger the reaction that table sugar, corn sweeteners and honey do.

How about caffeine? After one or two cups of coffee or tea at breakfast, you will be more alert, have better reaction time and score better on some performance tests. After three or so cups of coffee, however, caffeine over-stimulation can begin making you less sharp and clear-headed.

Lunch. Few people are aware of the effect of a carbohydrate lunch featuring only such foods as bread, noodles or rice, and sweet desserts. One study, headed by psychologist Bonnie Spring, found such meals made women sleepy and men calmer and lethargic. Moreover, says Spring, "we found that men and women over age 40 were less able to keep their minds focused on work for up to four hours after eating carbohydrates than were those who ate a high-protein meal instead."

Why? Protein-rich meals of poultry or fish, for example, charge your bloodstream with amino acids, including tyrosine. Tyrosine is carried across the protective filter called the blood-brain barrier. In the brain this amino acid is available for conversion into the alertness chemicals dopamine and norepinephrine. If stress exhausts the brain's supply of these neurotransmitters, the result may be confusion, indecisiveness, anxiety and depression.

Another key nutrient that is carried across the blood-brain barrier is choline, which is found in fish, meats, egg yolks, soy products, rice and peanuts. Choline is a chemical precursor of the brain neurotransmitter acetylcholine. The latter plays a major role in memory.

Dinner. Unless you need stimulation and energy to work or study through the night, avoid proteins — such as meat or fish — at supper. Instead choose carbohydrates. These foods alter brain chemistry indirectly: by triggering a release of the hormone insulin, they cause muscle cells to take up most amino acids in the bloodstream. One amino acid not taken up is tryptophan, a scarce chemical that competes with other amino acids for transport through the blood-brain barrier. With its competition reduced, more tryptophan enters the brain, where it is converted into the soothing neurotransmitter serotonin.

Carbohydrates may have a deeper influence on the mind than scientist used to suspect. According to Dr. Samuel Seltzer in the United States, in addition to making a person drowsy, tryptophan can also help reduce sensitivity to pain.

Indeed, the Wurtmans and others believe that many of us unwittingly use sweets and other carbohydrates to make us feel better. As the seasons change and the amount of natural light decreases, for example, some of us plunge into the severe condition called Seasonal Affective Disorder (SAD), characterized by depression, hours of extra but unsatisfying sleep each night, and weight gain. This seasonal weight gain comes mostly from eating carbohydrates in response to cravings that are strongest in late afternoon or evening.

During the shorter days of autumn and winter, the pineal gland manufactures more of the sleep-wake cycle hormone melatonin. According to Richard Wurtman, "This hormone may have some effect on brain serotonin." To compensate for feelings of irritability, discomfort, moodiness and depression, SAD sufferers begin overeating carbohydrates to increase brain serotonin levels and enjoy a temporary lift out of their dark mood.

Smokers who break the habit often seek comfort in sweet and other carbohydrates, too. In one recent study, psychologist Spring found that typtophan, with its ability to boost brain serotonin actually prevented nicotine withdrawal symptoms.

For SAD victims, ex-smokers and others, it's important to remember that as little as 1.5 ounces of carbohydrate can start the reaction that produces calming serotonin in your brain. Eating more will probably make you fatter but no calmer. That effect is also delayed. It may be an hour before sweets and carbohydrates soothe your mind and ease your appetite.

Eating the right food at the right time of day can make a considerable difference in your states of mind. But instead of simply adjusting their diet, some people experiment with "smart pills" said to improve brain function. Some are as commonplace as choline, a highly impure form of which is marketed as lecithin. Others include tyrosine, which in its pure form — that is, without the other amino acids normally accompanying it in dietary supplements — can cause dangerous changes in blood pressure. Trytophan pills used to be sold as nutritional supplements, but they were banned in the United States following at least 28 deaths caused by contamination. Hong Kong of China and Singapore have followed suit.

Scientists warn against taking such mind-alerting substances. The brain's chemistry works as a delicate balance in which an excess of one vital chemical can cause a shortage of another. Large amounts of tyrosine, for example, can reduce the amount of tryptophan crossing the blood-brain barrier.

The best advice, says Judith Wurtman, is to feed your head with food, not pills. "It is impossible to overdose on the amino acids in foods," she says, "Get them the way nature intended — in real food."

In recent years, we've learned to choose foods that help the heart and foods that prevent cancer. Now we should learn to choose them with our brain in mind.

(Selected from Reader's Digest (Asia edition), September 1992, written by Lowell Ponte)