EXPRESS LANE LIST: chicken, dijon mustard, shallots, thyme bread (for crumbs), tomatoes and green beans.

The truth may hurt, but after spending our first 20 years developing food habits we have to spend the rest of our lives breaking them.

For the first two decades of life, we grow by leaps and bounds and need both calories and nutrients to fuel that growth. Eating enough to fill our energy needs probably means we're eating enough to meet our nutrient needs.

After our 20th year, however, growth slows and our metabolic rates drop 2 percent each decade we age. While nutrient needs remain more or less the same, caloric needs dwindle.

So, like the dieter or the pregnant woman, people who have passed middle age need a diet that is "nutrient dense." Nutrient density refers to the ratio of vital nutrients to calories that a certain food contributes to the diet.

For example, a glass of whole milk seems fairly caloric at 160 calories a cup. Yet it can be considered "nutrient dense" because it is the main source of calcium in the American diet, contributes a great deal of protein and B vitamins, and will likely contain a large amount of vitamins A and D. Skim milk, which has half the calories but the same nutrients (other than fat), is even more nutrient dense.

Since one ounce of potato chips contains 160 calories, one might be tempted to exchange the milk for the snack. A closer look shows that the chips contain eight times the sodium that milk has and virtually none of the calcium. B vitamins and protein are negligible.

One can easily see, then, that as we age we must be more careful about what we consume, we need to pay as much attention to how our calories are packaged as to how many we're actually consuming.

Fat, alcohol and sugar are all concentrated sources of calories which contribute little else to the diet. For those concerned with getting good nutrition at low calorie expense--especially important as you age--foods with lots of fat and/or sugar in them should be kept to a minimum.

The following recipe is adapted from Julia Child's poulet grille's a la diable in "Mastering the Art of French Cooking Vol. 1." Serve it, as she recommends, with baked, whole tomatoes and steamed green beans.

Every kitchen should be stocked with flour, salt, pepper and, perhaps, a touch of sugar and vegetable oil (for flavoring purposes only, of course).

BROILED CHICKEN WITH MUSTARD (4 servings) 3 pounds (more or less) chicken pieces Vegetable oil for basting 3 tablespoons dijon mustard 1 tablespoon minced shallots 1/2 teaspoon thyme Dash freshly ground pepper 2 cups bread crumbs

Arrange the chicken, skin side down, on the bottom of a lightly oiled broiling pan. Broil the chicken about 5 inches from broiling element for 10 minutes on each side. Brush with oil every few minutes. While chicken cooks, blend mustard with shallots, thyme and pepper. Brush this mixture on the chicken pieces, then roll the pieces in bread crumbs. Place a rack in the broiling pan, place the chicken on the rack and broil until crumbs are toasted.