"A whole wheat donut, I suppose, is better than what we have, but it still has too much sugar and too much fat," said the doctor most responsible for America's fiber craze as he munched on a glazed donut and sipped his instant coffee sweetened with saccharin.

Earlier, Dr. Denis Burkitt had been regaling a notoriously straitlaced audience with scatological tales about his research in Third World countries and theories about the relationship between dietary fiber-what grandma used to call roughage-and the diseases of the Western World.

He told scientists at the Department of Agriculture symposia on human nutrition last week that the most important thing they could do is "get cereal fiber back in the diet. It would," he said, "be the end of the laxative industry."

The Scottish-born Burkitt, associated with the Medical Research Council in London, pulls no punches. And no one sleeps in the darkened auditorium as he lectures with slides that are as direct as his speech: "Disease depends on the relationship between causative factors and protective factors. I am suggesting that fiber may be protective."

Then he lists nine diseases or abnormal states which may be related to the lack of fiber in the diet: coronary artery disease, diverticulitis, appendicitis, cancer of the bowel and colon, hiatus hernia, varicose veins and hemorrhoids, diabetes, obesity and constipation. The last, almost all scientists agree, is definitely attributable to low-fiber diet and according to several there is evidence linking diabetes to lack of fiber. Evidence about the others is still hypothetical and not everyone is as convinced as Burkitt.

The Institute of Food Technologists says "any generalized statements about the use of dietary fiber as a drug to cure specific diseases should e looked at with reservations . . . the feeling of urgency about fiber in nutrition should not . . . cause the public to make drastic changes in their diets . . ."

But that hasn't prevented a number of people from capitalizing on the fiber fad in self-help books and cookbooks. Fiber or bulk-the indigestible part of foods-is needed for satisfactory peristalsis, the wave-like contractions that send intestinal contents on their way.

Sales of bran cereals have risen about 20 percent each year since 1975 and now account for about 15 percent of the ready-to-eat cereal market. The sale of whole-grain breads is up. Cookbooks featuring high-fiber recipes were temporarily outpacing even low-cholesterol cookbooks.

According to Burkitt, Americans should consume 11/2 ounces of bran per day.

What that works out to, says Dr. Peter Van Soest, professor of animal nutrition at Cornell University, is: over 2 ounces of All Bran cereal; 51/2 ounces of Wheaties; 61/2 ounces of Grape Nuts; or over 91/2 ounces of Cherrios. In vegetables terms, it means: more than a pound of potatoes in their skins; 2 pounds of green beans; over 4 pounds of peeled potatoes, cauliflower, or carrots; over 5 pounds of Romaine lettuce; or over 6 pounds of celery.

Burkitt is pleased with the number of breakfast cereals featuring bran that have been marketed in the last two years. He says he asked McDonald's to make a whole wheat hamburger bun, and that "they weren't interested." But he is ambivalent about the current fiber fad and some of its promoters, like Dr. David Reuben, best known as the author of "Everything You Always Wanted to Know About Sex and Were Afraid To Ask," who later wrote another very popular "Everything" book about fiber.

It is estimated that Americans eat only about one-fifth as much fiber as their ancestors did before the rolling mill supplanted the stone-grinding mill around 1870.

Burkitt would like to see affluent nations "retrace our footsteps to 1870 and eat a diet rich in complex carbohydrates," adding fiber to the modern diet, high in refined sugars and refined grains and high in fat, "is a compromise.

"You have to marry idealism with reality. Scientific work remains sterile until it is applied," Burkitt said, as he took another bite of his donut and noted that he eats everything put before him.

"The kind of attitude which really annoys me is the scientists and doctors who say, 'I wouldn't dream of making any recommendations without controlled scientific experiments.' That is total rubbish. This idea that we do nothing until the case is proved it totally immoral."

Despite his strong language, many of Burkitt's peers admire his style. The chairman of USDA's Nutrition Institute, Dr. Walter Mertz, said that "even if he overstates his case a little, what he says makes sense."

What he was saying at the conference was what others were saying in different ways: In all probability, many Americans are eating themselves to death. Eating habits should be changed to follow a diet similar to that recommended by the Senate Nutrition Committee's Dietary Goals for the United States-reduction in consumption of fat, sugar and salt, coupled with increased consumption of complex carbohydrates, whole grains, fruits and vegetables.

"Fat and fiber are reciprocally related," Burkitt said. "As fat consumption rises, carbohydrates fall." He was specific about what complex carbohydrates should be eaten. "The last way to get fiber is through salad. The United States is the largest salad-eating nation, yet it's the most constipated," Burkitt told his audience. "Fiber intake has fallen in the last 200 years and more recently cereal fiber has been replaced by vegetable fiber.

"Soon," he said, "we won't talk about fiber but about components of fiber." There have been some efforts already to list dietary fiber rather than crude fiber on foods. Crude fiber, the standard measurement for fiber until recently, bears little relationship to dietary fiber and has little meaning for humans. When dietary fiber is broken down, there are several different kinds and they perform different functions, many of which have not yet been sorted out. Apples have pectins and cellulose; whole grains have lignins and hemicellulose. Lignins and some cellulose do not break down and thus help speed food through the alimentary canal.

Van Soest, one of the leading authorities on dietary fiber, has developed a well-accepted method for measuring dietary fiber. In studies conducted at Cornell, Van Soest has discovered that the transition rate is affected not only by the kind of fiber but by the size of the particles. "Coarse grade of wheat bran is the most effective. We took the same bran and made it flour size and it lost its ability to do the same thing."

"The amount of stool you pass," Burkitt said, "is a better indicator of the kind of health and life you will lead than your plasma cholesterol level. Whatever causes colon cancer," he explains, "it would be less dangerous if diluted and if it was swept out of the bowel in 12 hours instead of a week. If people would eat one heaping tablespoon of miller's bran, they would be way ahead."

Dr. Guy Newell, deputy director of the National Cancer Institute, says there is "no known direct cause-and-effect relationship between dietary fiber intake and colon cancer. But he said there is "early suggestive data" and "generalized increase in dietary fiber has no known disadvantages for almost all normal individuals."

Burkitt said it's ridiculous to "take out vitamins, minerals and fiber from food," and even more ridiculous that "your FDA won't allow you to add anything in food which has one-in-a-million chance of giving you a sore nose, but it's okay to take out protective things."

The last slide that Bukitt flashed on the screen was a cartoon of several men trying to mop up a floor as a sink continued to overflow from an open faucet."That vast amount of health care," Burkitt said, "goes to mopping up the floor instead of turning off the faucet." CAPTION: Illustration 1, no caption, By Cameron Gerlach for The Washington Post; Illuatration 2, no caption