TOASTED WHITE bread contains more fiber than untoasted white bread.

Stir-fried vegetables contain more fiber than boiled vegetables.

Bread made with coarse grains has more useful fiber than bread made with finely ground wholewheat flour.

All fiber is not the same.

As the fiber fad enters its second decade, our knowledge about what Grandmother used to call "roughage" has become much more sophisticated than it was in the early '70s.

But for the most part, we still don't know much about its properties as preventive medicine for a host of ailments it has been credited with curing . . . with two exceptions. Fiber definitely prevents constipation and there is now a substantial body of evidence that fiber may help prevent colon cancer.

Members of the conservative scientific organization, the Federation of American Societies for Experimental Biology (FASEB), have "agreed that most of the epidemiologic evidence and the result of carcinogenesis studies in animals strongly suggest that dietary fiber exerts some type of protective effect against colonic cancer."

But bending over backward to be conservative, the scientists, in a recent report to the FDA, said the relationship should be "regarded cautiously."

Dr. Peter Van Soest, an expert in fiber research at Cornell University, thinks it's about time the FASEB group acknowledged the connection between a high-fiber diet and prevention of colon cancer. "There has been a lot of evidence for a long time," he said.

Van Soest's research includes measuring the dietary fiber content of foods. Until recently only the crude fiber content has been measured. Van Soest said that information is useless for humans. "Dietary fiber is more meaningful."

Some companies have begun to list the dietary fiber content of their foods, but the public doesn't yet make the distinction between the kinds of fiber foods contain and what the different fibers do.

Fiber is the cell walls of plants; it is made up of different components. The fiber in cereal grains is not the same as the fiber in fruits and vegetables. Cereal grain fiber is more woody and it much higher in substances called lignins and hemicellulose. They add bulk to the diet and are not digested. They speed up the time it takes food to leave the digestive track.

Fruits and vegetables, on the other hand, are very rich in fermentable pectins and cellulose. That means the bacteria in the digestive tract ferment (digest) these fibers, which then permits them to act as detoxifying agents -- as some scientists, including Van Soest, believe. According to Van Soest, "They probably act as detoxifiers of organisms that may be hazardous, organisms such as nitrosamines."

Since classes of fibers act differently, it is important to choose food containing all of them to get the various kinds of protection they offer. Now that you know that much, there is another crucial aspect to fibers that Van Soest says is more important than the difference between the fibers.

"Coarse particle size is more important," he says, "than other considerations. If you take wheat bran and mill it to flour fineness, it loses its capacity to speed up passage of food through the intestinal tract.If you grind the bran flour finely, you actually increase the problems of constipation."

That means that wood pulp added to certain breads to increase the fiber content "has zero effect." Because it is so finely ground "it doesn't do any of the things that it claims," Van Soest said. "Fiber pills are worse than useless. They could cause constipation and they are terribly expensive."

If coarse grind leaves more fiber intact, then the claim for the higher nutritional quality of stone-ground flours is true. According to Van Soest, stone-ground flours, whether wheat, oats, barley or rye, are better because they are "flaky flours. Coarseness is crucial," he explained.

If, on the other hand, you insist on eating white bread and white bread only, you ought to toast it and you ought to eat the crust, Van Soest said. There is little reliable research on toasting effects yet, but the effect of toasting "can't be overlooked and it may be large." Toasting white bread may increase its fiber (in this case lignin) content "fourfold," according to Van Soest.

The same action that increases the fiber content of toasted bread also is in operation with certain methods for cooking vegetables, but not boiling or steaming.

"Boiling the daylights out of them dissolves the pectins. Steaming removes less pectin than boiling," Van Soest explained. "But eating fruits and vegetables raw doesn't necessarily mean the fiber contents is highest. In something like frying or stir-frying, where you are 'burning' the edges, you may actually be increasing the fiber."

Van Soest said the increase in fiber content in deep-fried foods is not a reason to recommend that method of cooking. "Deep frying will give you a hell of a lot more fat. Potato chips are the epitome of deep-fried material. The lignin content goes up significantly but so does the fat content." The number of fat calories you take in, Van Soest said, are not worth it. For the same reason, "salads are not a good fiber source if you put a lot of dressing."

On the other hand, the amount of fiber decreases when fruits and vegetables are peeled. Pureeing them reduces it further; turning them into juice leaves almost no fiber.

Seeds and nuts are like cereals in their fiber content, and like deep-fried foods they are high in fat calories.

The increased interest in fiber has resulted in a dramatically different look at the cereal and bread shelves in the last few years. At last count there were almost a dozen bran cereals on the market. Van Soest thinks that most of these cereals are useful because the bran in them is fairly coarse.

At last count there were more than a dozen breads with some form of whole grains in them. Whether they are useful depends on how coarsely the flour has been milled.

Generally foods naturally high in fiber, which means they are high in carbohydrates, are lower in calories than foods with little or no fiber content. There is some thought that because foods high in fiber give you a feeling of fullness, they cut down on your caloric intake and help you lose weight or keep it off.

Just how curative fiber is remains a subject of considerable debate because of the lack of solid evidence. In addition to what is known about a high-fiber diet in relation to constipation and colon cancer, there is also some evidence that foods high in fiber and carbohydrates but low in fat and sugar help diabetics control their blood sugar levels.

The kind of fiber found in fruits and vegetables also may reduce blood cholesterol levels, but in one study the greatest reduction in cholesterol levels was achieved by simultaneously eating a diet lower in fats and cholesterol.

Although we are not sure if fiber prevents the formation of gallstones, hemorrhoids, varicose veins, appendicitis, heart disease, spastic colon and diverticulitis, its value is already such that it should be an important part of everyone's diet.