The current dietary advice for diabetics has two emphases. One is to lower dietary fat, the other is to eat plenty of complex carbohydrates: slow-burning, high-fiber foods like potatoes, rice, pasta and whole grain cereals (legumes too), fresh vegetables of all kinds and fruit, too, in moderation.

Clearly then a cereal-based vegetarian diet that uses animal products sparingly should be right on the money. And it is. However, the dietary exchange system used to guide daily food choices is not well adapted to the vegetarian diet. The "meat" group, from which one would supposedly be getting the bulk of one's protein, lists how much of which meats, eggs or cheese should be eaten at a meal. If you do not eat meat, you are left with some rather high-fat options.

Since it is common knowledge now that combinations of legumes, grains, nuts and seeds provide the protein equivalent of meat, why not plug these combinations into the meat slot? That way, you'd be getting more fiber and starch and, provided you concentrated on legumes and grains rather than the fattier nuts and seeds, fewer calories.

Well, this is exactly what Patricia M. Mozzer did when her husband was diagnosed as diabetic. Cooking to meet her husband's new dietary requirements, she developed a well rounded collection of tasty and satisfying dishes. They are available in her new book, "Vegetarian Cooking for Diabetics" (The Farm Publishing Co., 1987, $8.95). The book also contains complete instructions on adapting the standard exchange system to a vegetarian exchange system.

Ironically, the one real flaw I find in the book is inseparable from its chief strength and selling point. That is, the recipes are haunted by theories of protein complementarity that even their most ardent and initial popularizer, Frances Moore Lappe, has modified dramatically in recent years.

Legumes and grains do not actually have to be eaten at the same meal for their amino acids to find and fulfill one another. Furthermore, protein needs, in general, just aren't as difficult to meet as we once thought.

Given the limitations of the exchange system itself, however, this may be the best that can be done for now. One guesses that in the future the whole exchange system might get the reappraisal that all food grouping systems are under nowadays, and that a much less rigid approach to planning vegetarian diets for diabetics will emerge.

Meanwhile, we have a fine start here with "Vegetarian Cooking for Diabetics." Here are a couple of its very appetizing and, typically, hearty dishes.

CREAMY GARBANZO SOUP (4 servings)

1 1/2 cups uncooked garbanzo beans

2 cups chopped onion

1/2 cup chopped celery

2 cups diced carrots

2 cups lowfat milk

1/4 cup milk powder

2 tablespoons chopped fresh parsley

Dash pepper

1 teaspoon kelp or to taste

Cook beans until soft (3 hours or so), adding onion, celery and 1/2 cup of the carrots for last hour. Check occasionally to be sure beans are covered with water.

Pure'e in blender, then return to the pot. Stir in milk and powdered milk. Steam remaining carrots meanwhile, and add to soup. Season to taste with parsley, pepper and kelp.

MUSHROOM SUPPER CAKE (4 servings)

1 cup chopped mushrooms

1/2 cup chopped bell peppers

1/2 cup chopped onions

1 clove garlic, minced

1 teaspoon butter

1 tablespoon water

4 slices whole-wheat bread, crumbled

2 eggs

3 tablespoons parmesan cheese

3/4 cup lowfat cottage cheese

2 ounces swiss cheese, grated

Simmer mushrooms, chopped peppers and onions, garlic, butter and water in a covered frying pan for 10 minutes.

Remove from heat and mix in crumbled bread. Allow bread to soak up flavors for 10 minutes.

Beat in eggs, parmesan cheese, and cottage cheese. Turn out into an ungreased shallow baking dish, and bake at 350 degrees for 15 minutes. Sprinkle on swiss cheese and return to the oven for 5 more minutes.