CARBOHYDRATES. The word resounds with magic of promise: Victory made possible through the loading of complex sugars.

It seems to be true, what they say about carbohydrates helping push back the wall -- that ethereal obstacle which becomes so real to joggers who have hit it after running 18 to 20 miles of a marathon.

Runners interested in improving their performance in Sunday's Marine Corps Marathon should know that increased consumption of carbohydrates can help push the wall back, postponing it until mile 23 or 24 or perhaps the entire race.

What happens when a jogger "hits the wall," explains Dr. Mel Williams, an exercise physiologist at Old Dominion University in Norfolk, is that the muscles and the liver run out of ready fuel (glycogen).

Limited carbohydrate loading can raise the level of glycogen from 1.5 milligrams per 100 grams of tissue to 3 or 4 or sometimes even 5 milligrams. This means that the body can continue to draw from a ready supply of sugar to postpone the SMACK if not the insidious foot and muscle pains.

This modified loading represents a prescription change from the old days when the experts recommended a long, hard run seven days before the Big Race and a high protein diet during days six through four. The theory behind this regimen was to starve the muscles and liver so, with a new dose of carbohydrates, they would overcompensate by absorbing lots of sugar, boosting the stores to well over normal. This new supply of sugar would come from a diet made up exclusively of carbohydrates for days three through one before the race.

Modified loading skips the high protein part and just concentrates on a diet heavy in carbohydrates for the last three days before the race. Williams, director of the Human Performance Laboratory at Old Dominion, says that the meal the night before is especially important: He'll be eating fettuccine, "a nice salad" and bread. Right before bed, he usually fills up on Archway cookies with dates.

Right now, nobody is exactly sure what causes the wall. It could erupt from hypoglycemia -- with fatigue and dizziness setting in as a result of low blood sugar. On the other hand, it may be that when the body runs out of glycogen and resorts to using fatty acids as fuel, the runner must slow down to accommodate this less efficient means of getting energy. Perhaps even dehydration could be causing enzymes within the cells to be working improperly.

"The wall is there," assures Williams, after speculating about its origins. "Pacing is important. The person who goes out too fast is the person who will hit the wall first." Starting out slowly, he explains, helps preserve glycogen.

So will coffee. Or at least studies show that a little caffeine may stimulate the excretion of epinephrine (a hormone), which liberates free fatty acids (small molecules of fat) so that they may be used (in addition to simple sugars, which prevent any possible fatigue) as fuel early in the race. As a result, glycogen is conserved and the wall moves back.

The coffee thing can backfire, though. If you are unaccustomed to coffee, or don't usually drink it before a race, it can have adverse effects. Some runners report increased intestinal activity in the midst of the race after drinking coffee.

If the day is too warm, cautions Williams, the temperature combined with the natural diuretic effect of caffeine may cause you to become dehydrated (Williams, a Marine Corps Marathon veteran, says that the weather is usually perfect on marathon day). In addition, if coffee is taken too far in advance of the race, the diuretic effect may occur in the middle of the run, and that means losing a few minutes for a stop.

"Water is probably the most important nutrient to the marathoner," says Williams. Dehydration is a concern for someone running long distances on hot days. He advises the marathoner to drink a pint of water 30 minutes before the race.

Other than that, there is little evidence that specific foods will enhance the runner's ability.

Irene Diamond, runner and wife of a marthoner, recommends that runners eat "a good basic diet."

Because her background in home economics is coupled with an interest in running, the Potomac Valley Seniors Track Club has asked her to speak about food to runners at various marathon clinics.

"In order for a runner to reach maximum efficiency," she says, "you must fulfill your nutritional needs. There is no one food -- no tablet or pill -- that will make you a super runner."

It turns out that a diet based on the basic four food groups fills most of the runner's needs -- exceptions, of course, are for the aforementioned liquid and for extra calories, if they are warranted.

But anything else can be found in the basic four, says Diamond. Her family gets most of their protein (for building muscle) from skinless poultry, fish and legumes (peas and beans) and rarely from red meat. She says she doesn't worry too much about eggs -- that they are a good source of protein and that the Diamonds sometimes indulge in two a week.

She advises runners to cut down on their intake of saturated fat -- found in red meats but also hidden in hot dogs, sausage, bacon, solid shortening and things like frozen whipped topping and some coffee lighteners.

She calls the candy bar's alleged contribution of quick energy "propaganda." Brown rice, rye and wheat breads, pasta, potatoes, corn, peas and fresh fruits are not only excellent sources of highly sought carbohydrates, but contribute vitamins and minerals to the diet as well.

She counsels joggers to consume citrus fruit (as much for the potassium as for the Vitamin C). If muscle cramps become a problem, nuts, beans, whole grains and dairy products will contribute the dietary magnesium that may be lacking.

The list from there sounds like the kindergartener's familiar recitation of the alphabet. The necessary vitamins (A through K) and minerals can all come from a well-balanced diet.

It's the same old song and dance. Health, for anyone including the runner, comes from a well-balanced, varied diet heavy in complex carbohydrates (lots of vegetables and fruits), light on fat and including sources of protein and calcium.

Beer is optional.

The following recipes are for joggers interested in heavy duty carbohydrate loading before the marathon. They can also help the runner stay in shape between competitions.

JOGGER'S SPAGHETTI (4 to 6 servings) 28 ounces canned tomatoes 2 tablespoons olive oil 3 small yellow onions 5 cloves garlic 1 medium eggplant 1 teaspoon basil 1 bay leaf 3 small zucchini, sliced 6-ounce can tomato paste 1/2 teaspoons ground pepper 1 pound spaghetti

Put the canned tomatoes in a small saucepan over medium heat (uncovered) and let them simmer while you prepare the other ingredients. In a large dutch oven, heat the olive oil over medium heat. Peel and slice the onions about 1/4-inch thick and add to olive oil. Peel garlic cloves, smash them with the flat side of a knife and add to dutch oven. Peel eggplant and cut in 1/2-inch cubes. Add to dutch oven with basil and bay leaf. Slice the ends off the zucchini and, if you haven't found small ones, peel them. Slice 1/4-inch thick. Over high heat, stir the eggplant in the dutch oven until it begins to brown and give off some moisture. Add sliced zucchini and continue to stir about 2 minutes. Add simmering tomatoes and reduce heat to medium. Simmer, uncovered, about 15 minutes. Add tomato paste and pepper and continue to simmer while you prepare pasta. Serve with green salad and warmed Italian bread.

BAKED PEARS WITH GINGER (6 serving) 6 tablespoons butter 6 large, ripe but firm pears 1 cup brown sugar 1/4 cup brandy 1/2 cup crystallized ginger Juice of 1 lemon 1 cup plain yogurt or sour cream

Using a little of the butter, grease a shallow baking dish. Peel, halve and core the pears. Place them flat side down in the baking dish. Combine the brown sugar, brandy and crystallized ginger. Sprinkle over pears. Melt remaining butter and combine with lemon juice. Pour the mixture over the pears. Bake for 30 minutes in a 350-degree oven, basting the pears occasionally. Serve hot, topped with yogurt and sprinkled with additional ginger, if desired.

MUSHROOM POTATO SALAD (4 to 6 servings) 1/2 pound fresh mushrooms (small ones, or large ones halved or quartered) 2 tablespoons lemon juice 1/2 pound red potatoes, cooked and cubed 1/4 pound green beans, cooked briefly in boiling water 1/2 cup sliced celery 2 tablespoons green onions, sliced 1/2 cup salad dressing 1/4 cup parsley, chopped

Combine mushrooms with lemon juice. Add potatoes, green beans, celery and green onions to mushroom mixture. Mix vegetables with dressing. Sprinkle with chopped parsley, Marinate several hours, if possible.