ARSENIC. LEAD. In old mystery novels, this was the stuff of which murders were made Agatha Cristie and the butler aside, times have changed. Today these sinister substancesare proving to be the good guys in the story on metabolism. These nutrients, in fact, are absolutely required for the body to function properly.

Arsencis and lead -- long with such substances as zinc, silicon, selenium, molybdenum and chromium--are commonly referred to as "trace nutrients," those required by the body in such small amounts (micrograms or milligrams) that they can not be easily measured by scientists.

So the trace nutrient plot thickens. Because of the miniscule amounts involved, they often elude the most zealous researcher. The amount of zinc an adult needs can be compared to one second out of 300 years, a drop in 150,000 gallons of fuel, says Walter Mertz, the Hercule Poirot of the trace-nutrient detectives.

"This one drop of fuel does make a difference whether we get to our destination," says Mertz, the director of the U.S. Department of Agriculture's Beltsville Human Nutrition Research Laboratory. One atom in one molecule, he says, can make the difference between health and deficiency. The cobalt in vitamin B-12, for instance, prevents pernicious anemia. But even when a scientist exposes the vitamin's true nature, the way the body metabolizes that nutrient remains a mystery.

These trace nutrients are what make whole foods--as opposed to highly processed ones--advisable. Whole foods retain these nutrients, which are often lost in the refining process. When bread fortification began in the 1940s, vitamin B-6 was left out because it was so expensive. Although cheaper now, it is still not added to bread. The recent infant formula controversy--where formula lacking B-6 caused convulsions and brain damage in infants--demonstrates the need for trace nutrients. The B-6 requirement for adults is a mere 2 milligrams daily. Infants need less, no more than .3 mg.

Cooks during the '40s and '50s turned to fortified processed foods to solve kitchen crises. With fortification came bread with niacin but no B-6, juice with vitamin C but no folic acid. Because Mertz and his Beltsville Street Irregulars still have not discovered the mechanism by which many of these vitamins work, they caution against the vitamin pill mentality. "If you accept, as I do, that our knowledge in nutrition is not complete," says Mertz, "then you would rather have the whole food."

He points out that although infant formulas are "as good as we possibly can make them," he adds that "as our knowledge increases, they will get better. We can fortify only with nutrients we know now are required."

"You can pick out a few foods that are particularly good sources of certain elements," says Mertz. Oysters for zinc, nuts for copper, whole grains for just about all trace elements. Eating a variety of whole foods, however, leads to a better balance of not only the trace nutrients but others as well.

The mystery continues to unfold, says Mertz. While people have known for centuries that iron and iodine are necessary for good health, "we have not yet managed to eradicate iron deficiency," he says. "And we have just begun to understand the problems with iron absorption."

For example, he explains, the body readily absorbs the iron from meat, but not the iron in vegetables--unless, of course, one consumes them with orange juice. (Vitamin C enhances absorption--unless one consumes coffee at the same time. Tannin inhibits iron absorption.)

Twenty-five years ago, says Mertz, they "knew absolutely nothing" about chromium. Now they know that glucose metabolism uses chromium and that older people who have low levels of chromium have higher levels of glucose in their blood. In one experiment he conducted, chromium supplements seemed to help his subjects who had high blood sugar. However, he quickly points out that people with normal blood sugar received no benefit from chromium supplements. "You do not improve optimal function further," he warns, and many nutrients can be as deadly as nightshade when taken to excess.

What Mertz calls "the new trace elements"--lead and arsenic among them--have only been discovered since 1970. He says he believes that "zinc must be the most significant dietary problem next to iron." In the early '70s, zinc replaced vitamin C as the country's most popular nutrient when it was discovered that a zinc deficiency seemed to be stunting the growth of children in Colorado. Recently, scientists have been perplexed by problems with zinc absorption.

This does not mean Americans are dropping dead from zinc or any other trace-nutrient deficiency. What it does mean, says Mertz, is that some American bodies are not humming along at top efficiency.

It also means, says Dr. Paul LaChance, professor of food science and nutrition at Rutgers University, that moderate deficiency predisposes some to more serious consequences. Women who take oral contraceptives, for instance, need more vitamin B-6 than others. If their intake is already low, problems could arise. (Moodiness during menstruation, says LaChance, can sometimes be treated effectively with B-6 supplements.)

"If you look at people holistically, it means a lot more," says LaChance about trace-nutrient deficiency.

While iron consumption proves difficult for many population groups (specifically women from adolescence to menopause), USDA household food consumption surveys also reflect low intakes of vitamin B-6, zinc, magnesium and iron. LaChance adds folic acid, another B vitamin, to the list of "problem nutrients."

Nutrient deficiencies appear in many parts of the population but show up most frequently in women, who are often on weight reducing-diets.

What does it mean? "It means," says LaChance, with the conviction of Nero Wolfe, "people aren't eating green vegetables. It means they aren't eating whole grains."

He contends that most people don't eat bread because they have the misguided impression that bread contributes nothing to the diet but calories. He says people don't eat green vegetables because "they're reacting to the price." Shoppers forego expensive greens for cheaper processed food. LaChance would like to see government subsidization of vegetable farmers to lower the cost of fresh produce and potentially improve the American diet. "If they can subsidize sugar," he asks, "why can't they subsidize vegetables?"

New clues continue to reveal more of the true nature of these elusive substances as nutrient detectives tirelessly pursue them. Even if all mysteries are solved, however, Mertz' advice will probably remain the same: a variety of whole foods should provide the best access to most of these trace nutrients.