It costs about $27.83 a week to feed 16-year-old Justina M. Johnson of Arlington.

The vivacious cheerleader at Wakefield High School recently volunteered to keep a record of the food she ate -- each sip of soda and handful of nuts. And although it is difficult to define a "typical" teen, the resulting record indicates what many area high school students are eating.

One of three daughters of Mr. and Mrs. Paxton B. Johnson, Justina is 5 ft. 2 in. tall and weighs 96 pounds. She has an active and varied social life -- including athletics, choir, Latin Club and three honor societies. She spends leisure hours with friends, whose access to cars makes high-calorie, sugar-laden convenience foods much more available.

Wednesday is a typical day.

Breakfast

4 slices toasted French bread, 1 tablespoon butter, 8 ounces milk with 1 teaspoon chocolate mix, 2 ounces Cran-grape, 1 Vitamin C tablet.

Lunch

Peanut butter and jelly sandwich on 2 slices of bread, 1 bite apple, 2 sugar cookies.

Dinner

1 (2 ounce) lamb chop, 1/2 cup string beans, 1 small boiled potato, 1 teaspoon butter, 1 small slice French bread, butter.

Snacks

1 cup rasberry sherbet, 2 anisette toast cookies with 1 teaspoon butter, 2 cinnamon graham crackers, 1/2 orange.

Justina's average daily intake of 2,350 calories is not at all unusual for a growing teen with such an active schedule. But at least 40 percent of those calories came from foods such as doughnuts, candy, cake and fruit drinks. In contrast, dietary goals for the United States recently published by a Senate committee recommended that the ideal American diet contain not more the ideal American diet contain not more than 15 percent of calories as sugar.

Justina normally eats within the guidelines of the Department of Agriculture's "Basic Four" food groups, and her diet meets most of the National Academy of Science's Recommended Dietary Allowances (RDA), including those for vitamins. The wide variety precludes almost any chance of deficiency disease.

But, according to Montgomery County nutritionist Sue Boysen, "Just because she meets the Basic Four and the RDAs does not mean she chooses a healthy diet. Excesses in fats, sugar and salts may be just as harmful as deficiencies." Boysen warned that meat, nuts and "the hidden fats derived from baked products" can add to fat intake without seeming to do so.

Like many women, Justina doesn't get enough iron. On some days she doesn't meet the RDA for calcium either. (Scientists have only recently suggested that a woman's ability to bear healthy offspring is in part determined by good nutrition during the teen-age period of rapid growth when nutritional requirements are very high.)

Gloria Johnson, Justina's mother, believes in a good breakfast. Fortified cereals such as Raisin Bran, corn flakes, toasted cracked wheat bread or doughnuts are mainstays, as are juice, milk and occasionally eggs.

Mrs. Johnson, a reading specialist at Kenmore Intermediate School, is concerned about nutrition-related health issues. (She stopped buying bacon last year. For more elaborate weekend-morning meals, nitrite-free sausage serves as a substitute. And she feeds her family whole-grain breads and raw sugar.)

Although Mrs. Johnson is suspicious of synthetic foods, she alternates Cran-Grape drink with orange juice because Justina drinks it instead of soda.

After eating her own meal, the mother assembles sack lunches for Justina and her father, a program specialist with the International Communications Agency.

Before Justina leaves, especially if she's feeling tired, she will take a vitamin pill, usually vitamin C. "I guess it must work," she says, "I haven't had a cold in a long time."

"Our doctor says we don't need extra vitamins if we eat right," Mrs. Johnson added, "but I'm not always so sure Justina eats right."

To date there are conflicting claims about the relationship between large doses of vitamin C and the common cold. However, research has shown that some people's diets are lacking even small amounts of vitamin C necessary for good health. In addition there is evidence that certain people may require more vitamin C than others: those who smoke, have undergone extended illnesses or engage in heavy labor. Justina, though, gets plenty from food.

"I am a nagging mother," Mrs. Johnson admits with a cheerful, but determined grin. "I buy the food and cook it, so I am in control of what my family eats. I work hard at it.

"I've made all my girls very figure-conscious and fortunately none of us has a weight problem.

(Nutritionist Boysen commented: "Just because they don't have a weight problem doesn't mean they are healthy. What's going to happen when Justina is 35, has carried these food habits through life and has reduced her activities?) Thus fortified by vitamins on top of, say, a 425-calorie breakfast featuring 6 ounces of milk, a bowl of corn flakes with half a banana and a sprinkling of raw sugar, two (very small) cinnamon doughnuts and a sip of juice, Justina sets out for school.

Not all her peers arrive so well-fed. A school-run snack bar does thriving early-morning trade in fruit juice or drink, milk, doughnuts. Justina rarely indulges, plowing through a heavy schedule of morning classes with no further sustenance until lunch.

Wakefield has an "open campus" policy allowing its students to go anywhere they wish during the 50-minute lunch period.

According to Maxine Dunn, cafeteria manager, Justina is among a dwindling number of students who bring lunch from home. Five hundred of 1,700 students avail themselves of the Department of Agriculture's typical hot lunch program. Others buy sandwiches, fries, fruit, juice, milk, shakes, doughnuts, ice cream, potato chips or cookies at the snack bar. Still others go off-campus to fast-food establishments.

"Those places are jammed at noon," Mrs. Dunn says. "The snack bar has to offer similar fare in order to compete." Soda, coffee, tea and hot chocolate are also available from vending machines. Student organizations hold bake sales regularly.

Surprisingly, these factors play no part in Justina's diet. Because her extra-curricular activities interfere with homework, she spends lunch hour in the library, studying, talking with friends and nibbling on sandwiches, fruit and cookies from home. One Wednesday, for example, the brown bag featured a thick slice of leftover meatloaf on buttered, cracked wheat bread, a tangerine and two commercially-baked cookies.

"I don't like the things they serve in the cafeteria anyway," she said.

Cheerleading practice lasts until 4:30 p.m. Famished from exertion, the teen embarks on an afternoon of snacking: a couple of cookies, vanilla cupcake with chocolate icing, a slug of Cran-grape. These sugary foods -- plus or minus a candy bar -- will tide her over until dinner.

There may be steak with mozzarella cheese on top, milk, string beans, blackeyed peas, grits. For dessert, a few bites of apple. "I don't like vegetables much but Mom makes me eat them," Justina admits.

Other dinner menus feature combinations of meat, vegetables, carbohydrates and fruit. Mrs. Johnson often serves sweet potatoes, salads, greens and spinach, all unpopular, yet important sources of vitamins and minerals such as iron.

She believes a large amount of her $70-a-week food budget goes toward fresh fruits and vegetables. Meat is also a major expense, but she saves money by serving two-to four-ounce portions.

Since her mother refuses to buy candy, Justina chooses an evening's worth of snacks from a constantly-changing array. A handful of almonds, cookies and M and M's are enough for one night. Other current favorites are peanut butter cups, lollipops and "Lik-A-Stick," a brightly colored, synthetic-flavored straw full of sugar.

Dieticians believe that poor eating habits exist because, once established, they are hard to change. Yet people learn to eat new foods every day. And although nutritionists disapprove of "junk" foods, most say that between-meal snacks are important for teens, furnishing necessary nutrients as well as extra energy.

In fact, Justina's snacking habits during the sample week could have made up for the deficits in her diet. Some snack foods that would have provided more minerals: Milk, cheese, plain yogurt, raisins, dark green and leafy vegetables, sandwiches on whole grain bread, whole grain or fortified cereal. Specific supplements or pills could fill in the gaps, too, but they wouldn't provide accompanying trace nutrients found in food.

So -- this is what Justina M. Johnson eats. What about her judgment on its nutritional value?

"I sure have a sweet tooth," Justina said. "But I'm lucky. Some of my friends have to diet. I don't worry about my weight. I don't even have many cavities. When I just eat junk I feel like I'm not taking very good care of myself, but I feel I can enjoy candy as long as I eat the nutritious foods first."