"Less fat, less sugar, less alcohol, less salt and more fiber" is what Dr. Dennis Burkitt, 69, prescribes for a long and healthy life. Nicknamed "the bran man" by the press (because bran, the outer crust of a kernel of grain, is nature's perfect fiber), the English physician is the author of scores of scientific papers on the importance of a high-fiber diet. (His latest book is Eat Right, Arco, $4.95.) Until recently, says Burkitt, "fiber has been overlooked. It is not digested; therefore it was thought to be irrelevant to the diet. But it serves a purpose by protecting against the most common Western diseases." Among them Burkitt lists constipation, diverticulitis, cancer of the colon and gallstones.

A leader in the move to reintroduce fiber into modern diets, Burkitt came to nutrition after 20 years as a surgeon in Uganda. In the medical world, he is best known for his discovery of Burkitt's lymphoma, a kind of cancer in children that can often be cured by chemotherapy. ("If I hadn't been given recognition for the lymphoma work I did, Burkitt would be another food crank and nobody would listen," he chuckles.)

As a surgeon in the operating room in Africa, he recalls, "I cut straight through the skin into muscle" without encountering the layers of fat his patients in England have. Back home, while making hospital rounds, Burkitt also became "acutely conscious that a high proportion of the beds in any Western hospital are filled with patients suffering from diseases which are rare or unknown in the rest of the world."

The reason is diet, Burkitt says. Westerners refine the fiber out of their carbohydrate food (such as white flour) and, partly as a result, 40 percent of them are overweight. In Uganda and other Third World countries, the people consume two and a half times more fiber than in the West by eating starchy staples like brown rice, potatoes, maize and casaba.

Fiber, the indigestible part of food, comes in many forms. Wheat bran is the richest source, followed by whole grains such as rye, barley and rice. Legumes (peas, nuts, beans) and root vegetables (potatoes, carrots, turnips) are also good. Burkitt is against the use of fiber pills as a substitute for natural fiber. "We should eat the food before the fiber is extracted," he says. Since fiber can absorb many times its weight in fluids, eating high-fiber foods increases stool size and softness and speeds elimination, which Burkitt says is critical to good health. Rapid elimination of carcinogens, which may be present in the colon, can reduce the risk of cancer.

A high-fiber diet can also be a major factor in weight control. "It provides bulk without energy," Burkitt points out. "It gives you the feeling of being full without the calories." He thinks the American diet should become more like those of underdeveloped countries. "In primitive countries, obesity is rare, and weight usually falls during middle age, whereas in Western countries it goes up," he observes.

Rating popular American diets for fiber content, he gives Pritikin (very low fat, high carbohydrates) good marks but calls it too strict for most. "He has demonstrated that disease can be arrested by diet and exercise, but I couldn't stay on his diet myself," says Burkitt. "He won't allow me to have margarine, butter or jam. That's going too far." Burkitt dismisses the high-protein (steak and egg) diets of Atkins and Stillman—"even lean meat is 40 percent fat"—and generally thinks Americans eat too much meat ("better a small steak and a large potato than the other way around"). He foresees the time, he says, "when for economical reasons in a starving world, meat will be used as a condiment, for flavor."

Some of Burkitt's advice:

•Eat brown or preferably whole-grain bread.

•Choose fiber-rich breakfast cereals, most of which have "bran" in the title.

•Eat boiled or baked potatoes in their skins.

•Try more leafy vegetables (spinach, broccoli, cauliflower, lettuce). Go easy on the salad dressing. It contains fat.

•Use more lentils, beans and peas.

Burkitt lives with his wife of 36 years, Olive, in a rustic cottage in the Cotswolds, 100 miles from London, and stokes up on fiber every day. He eats at least five slices of homemade whole-grain bread; for breakfast he has a packaged bran cereal in the summer and oatmeal with a spoonful of bran in the winter. "But I'm not such a crank," he says with a smile, "that I don't put a little milk and sugar on it."