Pectin is familiar to jellymakers as the substance that helps their preserves thicken. In combination with the sugar and acid in fruit juice, this carbohydrate forms a structure that (upon heating) gels and spreads nicely on english muffins.

Pectin is also an often-overlooked member of the large group of food fibers that, being essentially devoid of nutrients, were traditionally considered only adjuncts to a healthful diet.

Present research, however, shows that dietary fiber may be linked with a lowered risk of degenerative diseases including cancer, high blood pressure, diabetes and heart disease.

Pectin is "much different from any other fiber," says Dr. James Cerda, who has studied pectin for many years at the University of Florida. So far, it is the only fiber that's been found to lower blood cholesterol.

According to Cerda, who directs the human nutrition laboratories at Florida's College of Medicine, pectin may be as effective at lowering cholesterol as some drugs on which physicians now rely. He certainly adds scientific substance to the adage that an apple a day keeps the doctor away.

Include oranges, too, because citrus fruits are next best to apples as a source of pectin. And since the pectin is in the pulp, not the juice, it's important to eat the whole fruit.

In order to take advantage of this fact, then, the following recipe for oranges requires slicing them through the equator and parallel latitudes, rather than vertically to separate each section. This not only includes all the fiber in the servings, it yields a prettier presentation.

To match the Mexican flavor of the orange dish, start the meal with chicken and green chilies, accompanied by a little rice or fried corn tortillas. The whole dinner should require no more than a quick trip through the express lane of the supermarket, provided you have flour, sugar, salt, pepper, butter and/or oil on the shelf at home.

EXPRESS LANE LIST: onion, garlic, chicken, chilies, canned tomatoes, rice, oranges, cinnamon. MEXICAN CHICKEN (4 servings)

This dish is tastier when made with mild green chilies that have been charred under the broiler, then peeled, seeded and chopped. Canned green chilies may be substituted. 2 tablespoons vegetable oil 1 large onion, chopped 2 cloves garlic, chopped 2 1/2- to 3-pound broiler-fryer chicken, cut into serving pieces 3 fresh, mild green chilies (charred, seeded and chopped) or 3 ounces canned, mild green chilies 28 ounces canned tomatoes 2 cups cooked rice

Heat oil in large skillet. Add onion and garlic and cook over medium heat until softened. Add chicken and brown on all sides. Add chopped chilies and tomatoes. Cover and cook 20 minutes. Remove chicken and set aside (keep warm). Boil sauce briskly, stirring, until it reduces and thickens somewhat. Serve chicken and pour sauce over rice. Serve with sliced oranges sprinkled with cinnamon. MEXICAN ORANGES (4 servings) 4 oranges 1/2 to 1 teaspoon cinnamon

Peel oranges. Slice them into 1/4-inch-thick slices across the middle so they end up as disks. Divide among four plates and sprinkle with cinnamon. Serve chilled, if possible.