"Oats, peas, beans and barley grow," goes the old nursery song, but in this age of dwindling truck farms, megalopolises and Wendy's, there's a generation and a half that never heard the song or, if it did, gave the words no more heed than if they had been nonsense syllables.

Now our health gurus are telling us we'd better renew the relationship -- not, of course, so much with the song, as with its subjects.

And Dr. James W. Anderson of the University of Kentucky Medical College and the Lexington (Ky.) Veterans Administration Medical Center is now providing solid evidence that of all the fibrous foods in the song, oats are really the ones to sing about.

Fiber is generally defined as one or another of the largely indigestible substances, sometimes euphemistically known as "bulk," that are found in fruits, vegetables, cereals and grains.

But there are several different kinds of these basically carbohydrate and carbohydrate-like food components. And although the current imperatives coming from cancer and heart specialists are generally urging an increase in "dietary fiber," different fibers have different physiological effects. The principal difference between classes of fiber are their solubility in water. And the water-soluble fibers appear to have broader health benefits than those that are not.

For instance, wheat bran, the kind found in most products that say they contain "bran" but do not specify what kind, is not water soluble. It and other nonsoluble fibers tend to affect primarily the intestinal tract, and epidemiological evidence strongly suggests that they are protective against colon cancer and some other gastrointestinal disorders. They are clearly enhancers of intestinal regularity.

But Anderson, who is a diabetes specialist, has shown that water-soluble fibers, especially oat bran, have a more global effect on the body, lowering blood cholesterol levels, enhancing the absorption of sugar, even helping to lower blood pressure in some cases and circumstances.

Anderson has been working with oat bran since the mid 1970s. It was largely his early results demonstrating its benefits to diabetics -- it reduced the amount of insulin needed, for example -- that persuaded the Quaker Co., which manufactures most of this country's oat products, to market its oat bran, rather than dispose of it in animal feed or simply discard it. Quaker began marketing, but tentatively, and even now, Quaker's "Mother's Oat Bran" is not always available in supermarkets across the country (once limited to a few health food stores in this area, it may now be found on many Safeway and Giant cereal shelves).

Rolled oat products (such as oat meal, quick oats and granola) contain oat bran as well.

In the past few years, Anderson's work has received increased attention from his colleagues and his latest studies have demonstrated that a few ounces of oat bran a day -- Anderson recommends between two and three ounces a day -- "pack more wallop," as Anderson puts it, "than any other substance we've looked at in terms of blood cholesterol, in terms of having good patient acceptance and good patient tolerability."

In a series of studies, recently confirmed with yet another, Anderson and his colleagues are finding that in groups of men with high cholesterol readings, three ounces of oat bran daily will lower the cholesterol by about 19 percent. Moreover, says Anderson, "we noted that oat bran was working to selectively lower what we call the bad guys, the LDLs low-density lipoproteins while tending to raise or leave at the same level, the good guys -- the HDLs high-density lipoproteins ."

In fact, he says, the newest studies show a 25 percent drop in the LDL component. Specialists have found that the HDL component of cholesterol appears to be protective against cardiovascular disease while the LDLs adhere to the sides of veins and arteries and form the dangerous plaque that can eventually block blood flow and result in heart attacks and strokes.

"Of course," says Anderson, "it's one thing to do this in a hospital setting, but we wanted to see what happened when the patients went home." One group of men Anderson and his colleagues followed for 10 years demonstrated that the benefits obtained in the hospital setting were maintained.

On a recent sabbatical at the Massachusetts Institute of Technology, during the 1983-84 academic year, Anderson and a group at MIT studied 12 "free living students," comparing oat bran and wheat bran in their diets.

He found that when the students ate about 1 1/2 ounces of oat bran a day, their cholesterol dropped about 9 percent even though they were eating more fat-containing products, such as eggs, than they did usually. However, as other researchers have shown, the wheat bran ("a good fiber G.I.-wise," as Anderson puts it) did nothing to blood cholesterol.

Says Anderson, "for middle-aged men, a 9 percent reduction in cholesterol would translate into an 18-percent reduction in risk of a heart attack. In young people it is probably a higher ratio, so a 9 percent reduction of cholesterol of people in their 20s would translate into as much as a 30 percent reduction in their risks of an early heart attack.

"So if we can get the word out about the benefits of oats to young people, it could do a lot of good."

Anderson has also found that the regular daily consumption of a "big bowl" of oatmeal can also work to lower cholesterol. Most oat products, says Anderson, are already partially cooked when you buy them, and whether cooked or raw, oats and oat bran seem to have the same effect on lowering blood cholesterol. However, Anderson has found that for purposes of lowering blood sugar, uncooked oat bran seems to be a little better. A few tablespoons in a bowl of cold cereal, or a container of yogurt, can be eaten almost without notice.

Currently, Anderson is comparing the cholesterol-lowering effects of oats and oat bran with other fibers. "So far," he says, "the sort of foods that are going to have good cholesterol-lowering properties are beans and those other sources of soluble fiber like some of the commercially sold stool softeners such as Metamucil."

That may be a good adjunct, Anderson says, for people who have trouble eating beans or oat products.

He has also found that canned beans tend to lower cholesterol, but are not as selective in lowering the LDLs as is oat bran. And the commercially canned beans are not quite as potent as dried beans. However, "they provide a good alternative for people and they do add variety."

Anderson, who is also working in the field of obesity, a particularly serious problem in diabetes, is also beginning to find that high-fiber, high-carbohydrate and low-calorie diets are particularly useful in controlling blood pressure. "We do find a fiber effect," he says.