WHILE THERE IS no evidence that anything we eat or don't eat makes us live longer lives, we certainly can eat in ways that improve our health during those lives. As we age our need for calories decreases, so the nutrient density of the foods we eat becomes ever more important. Fiber becomes, if anything, more vital; calcium and zinc needs remain high. Add to that for some of our aging population a need to reduce salt, take note of having to live on a fixed income, recognize that tastes become fixed over half a century or more of living, not to mention a common disinclination to cook for oneself alone, and it becomes obvious that eating healthfully is a tall order.

Starting with the last problem first, not only is it more efficient to cook several meals at once, but we're also more likely to eat a more balanced meal if we do not have to make each part from scratch. Meat loaf serves the purpose because it can be eaten hot or cold, alone or sandwiched between high-fiber bread or even diced into a soup or spaghetti sauce. Vary the salt content with your diet's restrictions. Melt slices of cheese on top of it for added calcium. Serve a small portion of meat to accompany a hefty serving of summer vegetables, and a potato that bakes along with the meat loaf. And while beef is a source of zinc, the mineral content can be increased by grinding beef liver or heart into the meat. As for the calorie level, buy lean beef and be sure to drain the fat from the meat loaf as soon as you remove it from the oven.

Add to meat loaf's other assets the fact that it is easy to whip up after a quick trip through the express lane, along with salt and pepper--plus butter or oil for the vegetables--from the shelves at home.

EXPRESS LANE LIST: Ground beef, beef liver or heart (optional), bread (preferably whole-wheat), ketchup, onion, egg, green beans, potatoes. MEAT LOAF (5 servings) 1 pound lean ground beef (combine with 1/4 pound ground beef liver or heart if desired) 3 slices whole-wheat bread, broken into pieces and moistened with water 3 tablespoons ketchup 1 medium onion, finely chopped 1 egg 1 teaspoon salt (if desired) or to taste Dash of pepper

Combine ingredients and form into a loaf. Bake at 375 degrees for 1 hour. Remove from oven and drain any fat that has accumulated. Serve hot, or refrigerate and slice for sandwiches.

Bake potatoes along with the meat loaf, and steam green beans for about 10 minutes, until cooked through but still crisp, and season lightly with butter or oil, salt and pepper.