Early in this century, scientists searching for the cause of such diseases as beriberi, rickets and scurvy made a startling discovery: They did not come from germs in the air or toxins in the food but from something missing from the diet.

That "something" is now known as a vitamin.

Vitamins are organic (carbon-containing) molecules that the body cannot do without and does not produce on its own. They are chemical catalysts -- they make things happen.

"A vitamin is like a traffic cop at a busy intersection with cars shooting past," says Dr. Victor Herbert, chief of the hematology and nutrition research laboratory at the Bronx Veterans Administration Medical Center.

In Herbert's analogy, the cars are the carbohydrates, fats and proteins that the body uses for energy. Vitamins speed up the flow of traffic -- the chemical reactions involving those sources of energy.

Vitamins don't provide energy, any more than a traffic cop provides traffic, says Herbert, a member of the National Academy of Sciences' committee on recommended dietary allowances. But they help determine the rate at which the body consumes fuel.

Prolonged lack of any vitamin in the diet causes serious illness. The best-known vitamin-deficiency diseases are beriberi (from lack of vitamin B1), night blindness (from lack of A), pellagra (from lack of B3), rickets (from lack of D), pernicious anemia (from lack of B12) and scurvy (from lack of C).

Vitamins are big business. Americans spend nearly $3 billion a year on vitamin and mineral pills to supplement their intake from ordinary food. More than one in three Americans, surveys show, take vitamin supplements regularly.

Of the $3 billion, Herbert estimates that "probably about 1 percent" is medically necessary. The rest, he says, is a waste.

"People have an almost irrational love affair with vitamins," says William Jarvis, chairman of the department of public health science at Loma Linda University in California and president of the National Council Against Health Fraud. "These things are like sacred cows."

The word itself has come to stand for vim, vigor and vitality and, as marketers have learned, a kind of can-do magic. It was coined in 1911 by the Polish biochemist Casimir Funk, who discovered that rice husks seem to prevent beriberi in pigeons. He thought he had found the nutritional missing link.

Actually, what Funk had found was a substance later identified as thiamine, or vitamin B1. He called it "vitamine" -- a combination of the Latin word "vita," (meaning "life") and "amine" (because he erroneously thought that it was in a class of chemical compounds called amines).

In the 1920s, the "e" in "vitamine" was dropped, because scientists found that not all vitamins were amines after all.

Vitamins were identified by letter because their exact chemical composition was not known. The original B vitamin turned out to be several different vitamins, and numbers had to be added. Later, it was discovered that some substances identified as vitamins were not essential to human health, and they were removed from the vitamin list, leaving gaps in the numbers.

In all, 13 vitamins have been identified as essential to the human diet. The last, B12, was discovered in 1948, and most experts say it is unlikely that more vitamins will be found. Since patients have survived for more than 10 years on intravenous feedings fortified only with the 13 known vitamins, the existence of a 14th essential vitamin seems doubtful.

Since 1943, the National Academy of Sciences periodically has published recommended dietary allowances for essential nutrients, including vitamins and their inorganic chemical cousins, minerals. (Essential minerals include calcium, phosphorus, magnesium, potassium, sulfur, sodium and chloride and -- in tiny or "trace" amounts -- iron, zinc, selenium, manganese, molybdenum, copper, iodine, chromium and fluorine.) The latest NAS report, published in 1980, was due for revision this year, but the new recommendations have been delayed repeatedly by disagreement over proposed changes.

Vital as they are, vitamins are needed by the body in only the tiniest amounts. A few vitamins, such as vitamin A, occur in more than one form and are counted in international units (IUs), a measure of biological activity. Most are measured in milligrams (thousandths of a gram) or micrograms (millionths of a gram).

The recommended dietary allowance (RDA) of vitamin B12 for adults is six micrograms a day; a single ounce of B12 would satisfy the daily need of 4,724,916 people. And one person's RDA for all 13 vitamins would fill about an eighth of a teaspoon.

Vitamins come in two main categories: fat-soluble and water-soluble. Fat-soluble vitamins (A, D, E and K) are absorbed through the intestinal walls and can be stored in the body fat for long periods of time. Because they accumulate in the body, fat-soluble vitamins can become toxic if consumed in excessive amounts. As little as five times the RDA for vitamin A, taken regularly, can cause brain and liver damage (a plant form called beta-carotene, which the body converts to vitamin A, is much less toxic).

Water-soluble vitamins (B-complex and C) cannot be stored in the body for long, because they dissolve in water and are released in the urine.

The rapid excretion of surplus water-soluble vitamins can easily be demonstrated, as Herbert and Dr. Stephen Barrett point out in their book, "Vitamins and 'Health' Foods: The Great American Hustle." The simple test involves riboflavin (B2), which has an RDA of 1.6 milligrams a day and is bright yellow. Unusually bright yellow. All one has to do is swallow a couple of vitamin pills containing five milligrams of riboflavin, wait an hour, urinate and notice the color.

"The reason why Americans have the most expensive urine and the best-nourished toilets in the world," say Herbert and Barrett, "will then be obvious."

"More is not necessarily better," says Dr. Aaron Altschul, director of the diet management clinic at Georgetown University Medical Center. "It's true of calories, vitamins, minerals, anything."

On the subject of vitamins, most nutrition experts today agree with Hippocrates' advice to his fellow physicians 2,400 years ago: "Let food be thy medicine."

"The best source of vitamins is food," says Dr. Arthur Frank, assistant professor of medicine at George Washington University Medical Center. "It's regrettable that most people think you have to get them from a pill. If you get any kind of reasonable variety in your diet, you don't need to take vitamin pills.

"It's actually fairly difficult to be vitamin-deficient."

A stalk of broccoli contains between two and three times the RDA of vitamin C for an adult; a baked potato or a tomato contains about half the RDA, and one orange takes care of the whole RDA. The RDA for vitamin A can be filled by two ounces of beef liver once a week, a cup of spinach every five days or a cooked carrot every other day. (Content of water-soluble vitamins in food is reduced by canning, prolonged storage or cooking; boiling vegetables can eliminate nearly half their vitamin C.)

Vitamin supplements are appropriate for some people, such as pregnant women and nursing mothers, who have special nutritional needs, Frank says. These also include "people who are on very restricted diets for medical or religious reasons, or people eating very poorly, or the alcoholic who's getting all his nourishment out of a bottle, or the person who lives on potato chips and Coke and McDonald's hamburgers." A strict vegetarian also needs to supplement for vitamin B12, which exists naturally only in meat and fish.

"But the ordinary person eating ordinary food," Frank says, "doesn't need vitamin supplements."

And vitamin megadoses -- more than 10 times the RDA -- are unwarranted for anyone, he says. At best, they are a waste of money. At worst, they can be highly toxic.

Some people on strict weight-loss diets need supplements because vitamins get cut out along with the calories, says Georgetown's Altschul.

The stricter the diet, the Georgetown clinic has found, the greater the risk of vitamin deficiency. Adults consuming more than 2,500 calories a day generally get all the vitamins they need; those consuming between 2,000 and 2,500 calories a day can easily arrange their diet to include all the necessary vitamins; those consuming between 1,500 and 2,000 calories a day often are vitamin-deficient, and those consuming less than 1,500 calories a day are especially likely to be missing out on needed vitamins.

Department of Agriculture surveys show that millions of Americans get less than the RDA for several vital nutrients, including vitamins A and B6, iron, calcium, magnesium and zinc.

The best remedy, says Michael Jacobson, executive director of the Center for Science in the Public Interest, would be for people to eat a more healthful diet, including more vegetables, whole grains, lean meat and dairy products. But it "certainly wouldn't hurt" to take a multivitamin and mineral supplement as an insurance policy, he adds.

"Nutritionists since time immemorial have been urging people to eat a more balanced diet and forget the junk," Jacobson says. "But people generally don't listen to them. They eat what's in the vending machine."

Acknowledging that vitamin supplements are often "a total waste of money," Jacobson says that boosting dietary intake with moderate supplements -- not megadoses -- is safe and costs only about $100 a year.

"That's two meals on K Street. That's 30 cents a day . . . If the person can afford it, why not?"

But some experts worry that use of vitamin supplements encourages the beguiling delusion that every problem can be solved with a pill.

"Training yourself to eat better, exercise more and eat less all takes a lot of discipline," says Loma Linda's Jarvis. "Some of these quick-fix approaches -- take a pill and solve your problems -- tend to undermine the commitment that's necessary for real life style changes."

And Jacobson agrees that vitamins should be considered in the larger context of overall dietary health.

"People shouldn't worry so much about whether they should take three milligrams or four milligrams of vitamin B6 while they're gobbling a lot of sour cream and cheese and losing sight of the danger of a high-fat diet," he says. "Chances are, if they cut down on fat, they'll increase their intake of other nutrients."

Promotion of vitamin supplements by health food stores and other suppliers is "unconscionable," says GW's Frank. "People are consuming huge amounts of these things in the mythic hope that something good will happen, that they'll feel better, live longer, live better. And really -- none of the above.

"Stress vitamins? That's ridiculous. There is no such thing as stress vitamins. Special vitamins for joggers, for tennis players? Vitamin E to cure impotence and everything from baldness to cancer? It's a modern-day snake oil. These are the peddlers of the nostrums in 20th-century dress."

Nor is a "vitamin" always a vitamin. Laetrile, an extract of apricot pits, was marketed as "vitamin B17." Pangamic acid, another substance without proven health benefit, was dubbed "vitamin B15." An oral form of the painkiller novocain was even marketed as "vitamin G3" and promoted, says Jarvis, as a treatment "for everything from Alzheimer's disease to zits."

Besides the 13 undisputed vitamins for humans, there are some substances that are essential growth catalysts for bacteria and other forms of life but not for humans. They are nonetheless often included in dietary supplements.

These include paraminobenzoic acid (PABA) and bioflavonoids (sometimes mislabeled vitamin P) such as herperidin, choline, inositol, lipoic acid and ubiquinone. They are not vitamins because they either are not needed by humans or are produced in adequate amounts by the body.

Yet all of these non-vitamins are sometimes referred to as vitamins in the promotional literature distributed in some health food stores.

"Part of that is an attempt to get out from under the authority of the government to regulate drugs," Jarvis says, "and part of it is to cash in on the mystique of vitamins."

Because of a 1976 amendment to the food and drug laws, the FDA has no authority to regulate sales of dietary supplements that make no specific drug claims on the label.

Vitamin supplements made from "natural" sources are often promoted as more effective, faster-acting or less dangerous than synthetic versions. But usually they are merely more expensive.

"The reality is, synthetic or natural, vitamin pills are refined products," says Joan Gussow, associate professor of nutrition and education at Teachers College, Columbia University. "Fundamentally, they're the same structure, synthetic or natural. There's no conceivable reason for paying the extra cost."

Overdoses of fat-soluble vitamins can be toxic, she says, whether from natural or synthetic sources.

"It's no better to get an overdose from cod liver oil than from a synthetic vitamin," Gussow says.

Another problem is the "outrageous" combinations in multiple vitamin supplements, she says. The most logical proportion of ingredients in multiple vitamins would be the same as the ratio of their RDAs. For example, since the RDA for niacin is 10 times the RDA for thiamine, there should be roughly 10 times as much niacin as thiamine in a B-complex vitamin.

"But there are all kinds of vitamins with 100 milligrams of each in them," she says. "Look down at the percentages of RDA, and they're all over the map. It's irrational nutritionally."

For those who need a supplement, she advises reading the labels carefully and choosing a balanced multiple vitamin containing vitamins roughly in the same proportions as they are needed by the body -- for example, with about 100 percent of the RDA of all the nutrients it includes.

"Just using high doses of whatever happens to come on the market is nuts."