Vegetarian diets have hit the mainstream now that 1960-era hippie types have turned 1980 professional types. They retain the eating style, are accustomed to it and bequeath it to their children.

People with concerns about weight, arteries and blood pressure have become the new converts to vegetarianism more to maintain family health than express personal politics. Yet for many of these people, vegetarianism remains a mystery.

"I'm sure that many vegetarians wonder if their efforts at combining proteins are sufficient," writes Nancy Cook, of McLean. "When making a meatless meal, are a few nuts, a touch of milk, a pinch of seeds or a handful of beans adequate for the whole family?"

The answer is yes and no. If the whole family consists of adults, there isn't much problem. If there are children or adolescents, vegetarianism should be approached with caution and forethought.

"People have to know a whole lot about what they're doing when they talk about vegetarian diets for infants," says Dr. Mahlon Burnette, executive director for the League for International Food Education. Even after infancy, children experience growth spurts that are diet-related. Thus, rapidly growing vegetarians should be well-versed in the idiosyncrasies of the diet. The problems of raising children on this diet cannot be discussed within the limits of an "Express Lane" column.

But adult vegetarians, says Burnette, "don't have to worry that much" about their protein intake throughout the day. The recommended dietary allowance for protein (for adults) is 65 grams per day, which translates into less than three ounces of pure protein. Further, the RDA allowances include a safety factor, says Burnette, so an adult may actually need as little as two ounces of pure protein.

In terms of real food, that translates into, say, 4 to 6 ounces of dry skim milk powder, or 2 to 3 9-ounce glasses of liquid milk (roughly). You'd be meeting your needs with absolutely no other protein source. But vegetarian adults might include other sources of protein, such as spaghetti with tomato sauce topped with a little parmesan cheese, bean burritos, red beans and rice, stir-fry with tofu and sesame seeds, or even a bowl of oatmeal with a little milk.

Combining certain foods assures adequate protein intake. The rule of thumb (appropriate in a cursory discussion on the subject), suggests that vegetarians combine beans with grains (beans and cornbread), dairy and grains (cereal and milk) and nuts with grains (peanut butter sandwich).

Vegetarians, like anyone else, says Burnette, should eat a wide variety of foods to ensure an adequate diet.

Those interested in pursuing a study of vegetarianism would do well to become acquainted with Francis Moore-Lappe''s book, "Diet for a Small Planet," and "Laurel's Kitchen," by Carol Flinders, Laurel Robertson and Bronwen Godfrey. Both provide helpful, comprehensive nutrition education, albeit in a 1960s approach.

Vegetarianism is practiced successfully all over the world and, as a result, some of the tastiest vegetarian recipes come from other cuisines. Crepes prove a convenient receptacle for all kinds of vegetarian fillings. The following is one alternative. The recipe makes about 20 crepes, which could serve 6 to 10 (2 to 3 crepes per serving). If you have whole-wheat flour on hand, use this in the crepes as called for. Part-whole-wheat crepes are delicious and nutritious.

Julienne unpared zucchini and pared carrots for a colorful sidedish. Cut the vegetables about matchstick size, place in a heavy saucepan with salt, pepper and about a tablespoon of butter, cover and steam over medium heat about 10 minutes. If you have dried dill, add a little of that.

As usual, it is assumed that staples on hand include flour, sugar, salt, pepper and some sort of cooking fat, in this case butter or margarine.

EXPRESS LANE LIST: Eggs, milk, frozen spinach, cottage cheese, parmesan cheese, nutmeg, carrots, zucchini.

SPINACH CREPES (20 crepes)

2 10-ounce packages frozen spinach

1/2 teaspoon salt

Freshly ground pepper

2 cups low-fat cottage cheese

3/4 cups freshly grated parmesan cheese

3 eggs, slightly beaten

1/8 teaspoon grated nutmeg (optional)

20 crepes (recipe follows)

2 tablespoons melted butter

Thaw spinach, or place in covered sauce pan over low heat until it is thawed. Uncover pan and cook over medium heat until liquid has evaporated and spinach is quite dry. Remove from heat, season with salt and pepper and stir in cottage cheese and half the parmesan. Beat eggs slightly, then stir them into spinach mixture. Add a little nutmeg, if desired. Place a little of the mixture (about 3 tablespoons) a little off center on the crepe and roll it up, enclosing the spinach inside the crepe (the ends, however, will be open). Place in an oblong casserole. Repeat with remaining crepes. Brush with melted butter, sprinkle with remaining parmesan cheese and bake in a 350-degree oven for 15 to 20 minutes. Serve with a julienne of carrots and zucchini.

ALL-PURPOSE CREPES (20 to 30 crepes)

3 eggs

1/4 teaspoon salt

2 cups all-purpose flour (substitute 1 cup whole wheat flour, if desired)

2 1/4 cups milk (skim is fine)

1/4 cup melted butter

In a mixing bowl, beat eggs and salt. Gradually add flour alternately with milk, beating with rotary mixer or whisk until smooth. Beat in melted butter. Allow the batter to sit about an hour. Using a 1/4-cup measure (filled 1/2 to 3/4 full), or a large spoon, ladle 2 to 3 tablespoons of crepe mixture into a 6- or 8-inch nonstick skillet, or one that has been lightly brushed with melted butter or cooking oil. Swirl the pan quickly to cover the bottom of the pan. Cook over medium heat until lightly browned (peak under the edge of the crepe to check, it won't bubble like a pancake). Extra crepes freeze well. Stack them, wrap them in heavy foil and freeze. To use again, thaw and separate.