"MOST OF THE world is on this diet," says Nathan Pritikin, at age 66 lean as an athlete and perhaps the most soft-spoken conglomerate in all of America. He is speaking to a class of the Downingtown, Pa., Pritikin Center, one of three health centers that carry his name and his part-ownership. His audience, three dozen people, many of whom hope to combat coronary disease, has paid $2,755 each to learn in two weeks how to live more healthfully. Pritikin is reassuring them that this country is coming around to the ideas he started promoting in 1970, his exercise program and diet high in complex carbohydrates, very low in fat (and therefore in cholesterol) and reduced in protein and sodium. "It is the ordinary diet of the underdeveloped population of the world," he declares, and ought to be the diet of the developed populations as well.

"Is there anyone who should not be on this diet?" a center visitor asks.

"I don't know who it would be," Pritikin answers.

"Maybe the president of the American Dairy Association," quips another guest.

In an era when health and diet are as sacred as religion and as profitable as sin, Pritikin's books are best sellers; his "longevity centers" will be expanding, to one within every 500 miles, if his partners have their way. By next year, if all goes according to plan, Pritikin salad dressings, soups, sauces and frozen foods will be on the market. And Pritikin converts are parading their taut bodies and lower cholesterol counts, walking missionaries for healthier hearts.

But the question keeps cropping up, "What's for dinner?" A Chef to the Rescue

That's where Robin Rifkin comes in. Preaching the gospel and instructing converts. Weekday afternoons at 3 o'clock in the Downingtown center, Rifkin, at 28 the age most of her students' children would be, teaches bread baking to cardiac men who may never have been in the kitchen before. She talks about range-fed meat and toxins, shows them the first and only whole egg they will see during their two-week stay, warning them that it is 64 percent fat. And she cautions them that Pritikin's books are filled with wonderful information but terrible recipes.

Pritikin food is changing. No more plain, steamed vegetables on unseasoned brown rice. People who have visited Pritikin's first longevity center, in California, report tedious meals. And Rifkin agrees. The food was very plain, she says, because "nobody knew what to do with the stuff. We were concerned with getting the food out."

As a home economist working for a California spice company, Rifkin grew increasingly interested in vegetarian and French cooking. After reading Pritikin's "Live Longer Now," she decided to figure out a way to work for him, and landed a job as a cook when the Pritikin Center moved to Santa Monica in 1977. Two years later she left to work as the private chef for a man in Tucson who needed to lose 250 pounds. That job lasted a couple of months, followed by a few months of meditating and cooking in California ashrams. Then Rifkin and a new Pritikin center coincidentally wound up in the vicinity of Philadelphia, and Rifkin found herself teaching at the center as well as in her own cooking school called Heart Felt Cookery. Capturing the Audience

Small and spaghetti-slim, with graceful and gentle gestures that show she has been a dancer, Rifkin explains that kneading dough is therapeutic: "It's like meditation." She tosses brand names and cooking hints to her audience, invites people up front to try kneading, chopping. The dough is divided, one part filled with dry-saute'ed onions and caraway seeds, another part braided, a third rolled into pinwheels with cinnamon and raisins. She lines the pans with parchment paper instead of greasing them, and puts them in the oven with a pan of water or sprays them with water as they cook, to increase the crustiness.

Rifkin keeps up a quiet, steady monologue, talking of her grandmother's coffeecake: "I've made it pseudo-Pritikin, with a stick of butter instead of a pound." Getting Down to Basics

Such temptations overcome, she outlines the Pritikin diet: 25 milligrams of cholesterol a day on the regression diet (which Pritikin suggests cardiac patients follow for six months), 100 milligrams on the less-strenuous maintenance diet. The average American, she says, eats 600 milligrams of cholesterol a day. Then she gives the bad news: 3 1/2 ounces of lean fish or chicken averages 40 milligrams, beef 70 milligrams. The maintenance diet allows 3 1/2 ounces lean meat or fish a day, the regression diet 4 to 5 ounces a week.

As for fat, flat fish are very low, but bluefish is 24 percent fat, salmon 60 percent. The Pritikin dieter should cut fat from the typical 40 percent to 10 percent of his daily calories. That means no nuts, olives, avocados, oil, butter or margarine, only limited soybean products and only skim-milk dairy products.

Most foods we think of as high in protein are also high in fat, explains Rifkin. Thus the Pritikin diet de-emphasizes protein. Since the average American eats twice the recommended daily requirement for protein anyway, says Rifkin, that's not a problem. The diet increases the consumption of high-fiber complex carbohydrates, cuts out simple sugars and drastically reduces sodium.

Pritikin knows how strict this diet is and that everyone cheats, she says, but "if you cheat on such a strict diet, you are still okay."

Even so, it is a big adjustment for most Americans. And Rifkin has a lot to teach:

* Nonfat yogurt is not as thick as whole-milk yogurt. In cooking it may need thickening with arrowroot or cornstarch.

* Tortilla chips can be baked rather than fried, first dipping them in diluted tamari or low-sodium soy sauce, then sprinkling with chili powder (read labels, as some chili powder has sodium).

* For sweetening, use unsweetened fruit juices (concentrated frozen apple juice is a favorite) or barley malt syrup, which she warns is highly caloric. She also uses Yinnie's rice syrup.

* Orange juice is acceptable as a sweetener but is too sweet to use as a beverage on the Pritikin diet. It is better to eat a whole orange -- and thereby get the fiber as well -- than to drink orange juice.

* No-salt products are not necessarily low in sodium, particularly meat substitutes such as hydrolized vegetable protein, kelp and autolyzed yeast.

* Low-calorie salad dressings tend to be high in sodium.

Because soybeans are high in fat, bean curd or tofu and tempeh should be used in moderation. Stove-Top Techniques

Two of the most useful Pritikin cooking techniques are stir-frying and poaching. Poaching is obvious, the same method the non-Pritikin world uses, though Rifkin poaches her fish uncovered, along with vegetables, herbs and sherry, turning thick fillets to cook them through. Thus the liquid boils down, concentrating the flavors.

The stir-frying, though, is a Pritikin innovation. Instead of oil, a spoonful or two of broth -- vegetable or defatted chicken broth -- is used to coat the bottom of the pan. This stir-frying is slower than with oil, and the ingredients do not really brown. But they do take on a stir-fried texture and more flavor than from steaming. Rifkin seasons stir-fried dishes with tamari; a tablespoon has less sodium than 1/4 teaspoon salt.

Rifkin also "browns" meat in a little stock or wine, after coating it with Grape Nuts or finely ground bread crumbs. She "fries" onions in a covered iron skillet with nothing added to the pan. And she is constantly inventing: salad dressings, a yogurt-miso combination whose taste is reminiscent of cheese. She speaks yearningly of a chance to teach restaurants how to adapt what they do to a healthier diet. In the meantime, she is sending converts home with packets of recipes for Pritikin adaptations of dishes they never even heard of in their meat-and-potato days.

Here are some of the recipes taught and served at the Pritikin Center. A SIMPLE BUT GOOD VEGETABLE STOCK

When you are cooking during the week, don't throw out the peels or odds and ends from any of your vegetables:

* Use the carrot peels and ends, onion skins, celery leaves, lettuce leftovers, cabbage hearts and leaves, tomato skins, broccoli stalks, cauliflower leaves or whatever else you have on hand.

* Cover them at least double with water, bring to a boil, and then let simmer, covered, for about an hour.

* Pour through a strainer. The stock is ready to use and rich in flavor and vitamins and minerals.

You can freeze this for a later date. One good idea for freezing is to pour some stock into an ice cube tray, and later pop out a cube to saute' your vegetables. A good stock will last a few days in the refrigerator, in a good plastic container.

Additional information on stocks:

* All stocks freeze well and keep well in the refrigerator for three to four days. If you would like to keep it longer in the refrigerator, then bring it to a boil again. Let it cool and return it to the refrigerator. It is also a good idea to boil the stock if you add other "pot liquors" to it. (Pot liquors are waters from steam or cooking other foods.)

* Starchy foods tend to make a stock sour more rapdily.

* When choosing a heavy stock pot, avoid aluminum, as it will affect the clarity of the stock.

* If you have some eggshells, throw them into the stock pot as it cooks. It will help clarify the stock. EGGPLANT ENCHILADAS (Makes 12) 1 large onion, chopped 1 large green pepper, chopped 3 cloves garlic, minced 5 cups peeled eggplant chunks (about 1 large eggplant) 15-ounce can whole tomatoes, drained and chopped 15-ounce can tomato sauce, plus extra as needed 3 tablespoons cider vinegar 1 tablespoon canned diced green chilies 1 tablespoon mild chili powder 1 teaspoon ground cumin 1 teaspoon ground coriander 1/8 teaspoon black mustard seeds 12 corn tortillas

In a skillet, add just enough water to coat the bottom and water-saute' the onion, green pepper and garlic. Stir in the eggplant chunks and tomatoes. Cook over low heat until most of the liquid has evaporated. Add the tomato sauce, vinegar, chilies and spices; stir to mix. Bring to a boil, then cover; turn down the heat, and simmer until the eggplant is tender.

Warm each tortilla briefly in the oven. Using a slotted spoon, fill each tortilla with eggplant mixture. Roll and lay the enchiladas seam side down in a nonstick baking dish. As a topping for the enchiladas, use 1 cup of the sauce remaining in the skillet. (If the sauce in the skillet is not sufficient, add additional tomato sauce to equal 1 cup.) Pour the sauce over the enchiladas and cover the baking pan. If using foil, make a dome to avoid contact with the tomato products. Bake at 350 degrees for 30 minutes. STUFFED SQUASH OR ZUCCHINI (4 to 6 servings) 2 acorn squash, 2 zucchini squash or 3 summer squash 2 cups chopped mushrooms 1 cup chopped onion 2 cloves garlic, crushed 2 tablespoons dry white wine 1/4 cup chopped parsley 1/2 teaspoon basil or thyme Pepper to taste 1 teaspoon tamari soy sauce 1 cup 1 percent or non-fat cottage cheese 3/4 cup cooked brown rice (or millet or bread crumbs)

If you are using acorn squash, pierce it with a fork, then put in a baking dish and bake 1 hour at 350 degrees. If you are using zucchini or summer squash, slice in half lengthwise; cut out center with a knife or spoon, leaving about 1/4 inch skin thickness all around. Steam in a little water for 5 minutes. Saute' mushrooms, onion, garlic in wine in a saute' pan for a few minutes. Add the zucchini or squash insides, if you want. Add parsley, basil, pepper and tamari, saute'ing a few more minutes. Turn off the flame, add cottage cheese and rice and mix well. Let mixture sit a few minutes. Drain mixture through a colander, saving liquid for later. Cut acorn squash in half, scoop out seeds (or use zucchini or summer squash) and fill with vegetable-cheese mixture. Lay squash or zucchini on a baking dish. Bake for 30 minutes at 350 degrees. Heat drained liquid in saucepan. If it needs to be thickened for the sauce, add a little cornstarch or arrowroot to cold water and add to sauce. Cook sauce lightly to thicken, stirring often. Spoon sauce over zucchini or squash and serve. HALIBUT-POTATO CREOLE (4 servings) 1 cup fish stock 2 cups coarsely diced red and/or yellow onions 1 cup coarsely diced green pepper 1 cup diagonally sliced celery 2 cloves garlic, minced 2 shallots, minced 1/2 cup chopped leeks 2 cups potato chunks, cooked 1 cup diced tomatoes 15-ounce can tomato sauce 15-ounce can diced tomatoes in puree, or substitute 3/4 of a 15-ounce can of tomato sauce plus 3/4 of a 1-pound can of whole tomatoes, drained and chopped 1/2 cup tomato paste 1/4 cup white vermouth or dry sherry 2 teaspoons gumbo file' 1/2 teaspoon thyme 1 bay leaf Cayenne pepper or hot pepper sauce to taste 1/2 pound halibut or other firm-fleshed, lean white fish, cut into bite-sized pieces with bones removed 2 tablespoons arrowroot

In a large skillet, bring 1/2 cup of the stock to a boil. Saute' the green pepper, celery, onion, garlic, shallots and leeks in the stock. Add remaining stock and the other ingredients except the fish and arrowroot. Simmer 5 minutes. Add the fish and simmer uncovered for 10 minutes longer, or until the fish is cooked. Mix arrowroot with a little stock and add to fish mixture and thicken. Remove bay leaf. Serve CIOPPINO (6 servings) 2 large onions, diced 2 potatoes, peeled, diced 1 green pepper, chopped 2 cloves garlic, minced 2 cups tomato juice 28-ounce can whole tomatoes 1/2 cup red burgundy wine 1 teaspoon each fresh oregano and minced parsely 1 1/2 teaspoons fresh basil 1/4 teaspoon ground pepper 1 pound halibut, cut in chunks

Put onions, potatoes, green pepper, garlic and tomato juice into large saucepan. Heat to simmer; cook over medium heat 10 minutes, until vegetables are crisp-tender. Add tomatoes, wine, oregano, parsley, basil and pepper. Cook, covered, 10 minutes, until vegetables are fork-tender. Add fish; simmer, covered, 5 to 10 minutes, until fish flakes easily with fork. GAZPACHO SALAD MOLD (6 servings) 2 envelopes unflavored gelatin 1 pint, 2 ounces tomato or V-8 juice 1 tablespoon red wine vinegar or lemon juice Dash hot pepper sauce 2 small tomatoes, peeled and diced 1 medium cucumber, pared and diced 1/2 medium green pepper, diced 1/2 cup finely chopped red onion 1 tablespoon chopped chives 1 diced carrot Watercress for garnish

In a medium saucepan, sprinkle gelatin over 3/4 cup tomato juice to soften. Place over low heat, stirring constantly, until gelatin is dissolved. Remove from heat. Stir in remaining tomato juice, vinegar and several drops of hot pepper sauce. Set in a bowl of ice, stirring occasionally, until mixture is consistency of unbeaten egg white--about 15 minutes. Fold in tomato, cucumber, green pepper, onion, chives and carrot until well combined. Pour into fluted, 1 1/2 quart mold that has been rinsed with cold water. Refrigerate until firm, at least for 6 hours. To unmold, run a small spatula around edge of mold, invert over serving platter, place a hot, damp dishcloth over inverted mold and shake gently to release. Refrigerate. COLESLAW 10 servings) 1 cup shredded red cabbage 3 cups shredded green head cabbage 1/2 cup diced pickles 1/2 cup grated carrots 1/4 cup chopped radishes 1/2 cup chopped scallions 2 cups yogurt 1/4 cup lemon juice 1/4 cup white rice vinegar Pepper to taste 2 tablespoons parsley 1/4 cup concentrated apple juice

Cut all the vegetables to appropriate sizes, mix and set aside. In food processor or blender, pure'e yogurt, lemon juice, vinegar, pepper, parsley and apple juice until well blended. Pour over vegetables; toss to mix well. Let sit 2 hours, at least, to marinate.

Note: The purple cabbage may leave a lavender hue to the dressing. If this is not desired, add purple cabbage just before serving, or replace with additional cup of green cabbage. CARROT SALAD (8 servings) 3 cups grated carrots (7 medium carrots) 1/2 cup raisins 1 cup non-fat yogurt 1/4 cup orange juice 1 teaspoon lemon juice

Combine carrots and raisins. Take 1/2 cup of carrot-raisin mixture and blend with yogurt, orange juice and lemon juice. Combine yogurt mixture with remaining carrot-raisin mixture and mix well. Chill, serve cold. SWEET AND SOUR SAUCE (5 servings) 1 1/2 cups water, or vegetable broth, or chicken broth 1 clove of garlic, minced 1 slice of ginger, minced 1 medium onion, diced 1 medium green pepper, diced 2 tomatoes, quartered 2 tablespoons lemon juice 6 tablespoons of apple juice or pineapple juice concentrate 4 teaspoons arrowroot 1/2 cup diced or chunk pineapple, preferably fresh, or canned without sugar added 1 scallion, chopped 2 teaspoons tamari or soy sauce

Pepper to taste

Heat 1/2 broth in wok; add garlic and ginger. Then add onion, green pepper and tomatoes and cook for 5 minutes. Add lemon juice, fruit juice concentrate and more broth, saving 4 ounces cold broth to mix with arrowroot for thickener. Add thickener to sauce, and cook a few more minutes until starch becomes clear in sauce. Add pineapple, scallion, soy sauce and pepper to taste. Serve hot over vegetables of your choice. ROBIN'S PRITIKIN APPLE TART (8 servings) Crust: 1 cup Grape Nuts 1/4 cup concentrated apple juice 1 tablespoon cinnamon Filling: 4 to 5 apples, peeled and sliced 1/2 cup apple juice concentrate 1/2 teaspoon almond extract 1/2 cup raisins 1 teaspoon cinnamon 1 package gelatin 1 1/2 cups low-fat plain yogurt

Combine crust ingredients in springform pan or pie plate. Bake at 350 degrees for 5 minutes.

In a saucepan, combine apples, 1/4 cup apple juice concentrate, almond extract, raisins and 1/2 teaspoon cinnamon, and simmer until apples are tender, about 5 to 10 minutes. Apples should not be mushy. In another saucepan, add remaining 1/4 cup apple juice concentrate, then sprinkle gelatin over it and let sit a few minutes to soften the gelatin. Heat on a low flame until gelatin dissolves. Add remaining 1/2 teaspoon cinnamon, then add yogurt. Pour this mixture over crust. Add apple-raisin mixture on top of yogurt mixture. Refrigerate until serving.