Those who think nutrition education should be taught to preschool children, especially boys, may have an ulterior motive. As one mother of a 5-year-old boy explained: "When he's 21, if he knows these things, he won't just find a woman who cooks and marry her!"

Beyond that compelling, but ordinarily unspoken, motive is the more practical desire to improve the food habits of very young children. And one California project has made encouraging, if limited, progress.

Fifty children who took part in the two-year Children's Nutrition Education Project at California State University in Los Angeles reportedly increased their comsumption of fruits and vegetables by 25 percent, and of milk by 21 per cent. They correspondingly decreased their intake of bread by 18 percent, and dessert 11 percent, while the consumption of meat stayed the same.

Like most attempts to change the diet of children, the experiment in Cal State's on-campus day-care center began against terrific odds. Whatever nutrition education children learn faces constant undermining by peers who are not exposed to the same information, television commercials for sugary foods, displays in the supermarket and even parents who follow the dictum, "Do what I say, and what I do," as they snack on candy and soft drinks while watching television.

According to Jackie Bigby, one of the nutrition education specialists, what the project found in the kids' lunch bags in the beginning was "kinda bad: Fritos, pop, bologna sandwiches - generally on white bread - cookies, cakes, cupcakes, Twinkies. Very few had vegetables. The kids didn't necessarily eat their lunches. They'd trade off.

"Parents," Bigby explained, "have a hard time trying to figure out what to make for lunch. A lot of parents are working and they just throw together the quickest thing."

The parents weren't the only problem. The nursery-school's own snacks for the children consisted of Hi-C, a fruit flavored drink which is high on sugar and low on fruit juice, and crackers.

So the project coordinators began by serving hot lunches. One of the first meals, according to director, Dr. Jane Lewis, was macaroni and cheese, a usual chilldhood favorite. "It didn't go over very well the first time," said Lewis, a professor or nutrition at Cal State. "But it improved over time."

As if to reinforce Lewis, Steven Gota, 5 1/2, one of two participating children who had come with their mothers to tell a reporter what they learned from their experience with nutrition education, said his favorite meal is "macaroni and cheese."

Tommy Camous, 5, on the other hand, said his favorite foods were "ice cream, candy bars, spinach and Twinkies," in that order. Which goes to prove you can only teach kids so much.

California provided Cal State with $33,000 to fund the project. The children, from varying ethnic and economic backgrounds, came from the surroundings community.

Their daily activities centered around nutrition. The children did a lot of tasting, touching and looking. They made granola - felt the sesame seeds and smelled the cinnamon that went into it. They crushed the almonds with a hammer. They used their hands to stir the mixture.

Carrots were not introduced for lunch until the children had participated in the dramatization of how carrots grow. Then they sold carrots in a play store, compared their color with other foods.

They explored the inside of green peppers; smelled onions and cried; planted seeds and watched them grow.

They learned to brush their teeth and discovered (by eating them) that certain foods help to clean the teeth. "Steven," his mother said, "doesn't like carrots but eats them because he knows they clean his teeth"

They were taught that food is eaten for many reasons - not just because it tastes good, but because the body needs it to grow; and eating foods with friends is an important social occasion.

The children developed small motor skills by preparing foods - scraping, grating, slicing and stirring.

They tried foods from cultures other than their own: Irish stew of St. Patrick's Day, a Chinese meal for Chinese New Year.

According to one of the aides on the project, the teachers pulled no punches. "When we talked about junk food with children, we called it junk food." Asked to describe junk food, Steven mentioned "candy, cookies and certain cereals we have."

The children were never coerced or bribed to eat particular foods. Sometimes, ordinary peer pressure did the job.

In order to make sure that whatever was learned in school was not unlearned at home, a parents' advisory council was formed. Nutrition workshops were held. A newsletter called "Nutrition Nuggets," sent home to the parents, described what the children were learning in school and offered basic nutrition information. The effort was also meant to encourage parents to upgrade their own eating habits, but test results at the end of the program showed that the parents did not improve their intake of the six major food groups.

Although the children did, a year from now the results might be different. "The children," Mrs. Gota said, "need constant reinforcement. It's diluted and negated by television. TV creates a lot of conflicts. Steven knows if things have a lot of suger. He'll still ask for them, but he knows my answer.

"Steven is in a school now where no junk food is allowed. Peer pressure is enough. They can't trade food until after lunch."

His lunch is some combination of soup, cereal, string cheese, dried fruits, turkey, chicken or peanut butter sandwich.

Mrs. Gota, who has a masters degree in nurition, is "totally against Saturday morning television, totally against commercials. You go down the supermarket cereal row and my goodness, there's always a child crying and mother gives in. I used to avoid it, but I don't anymore."

"If Steven had ice cream, candy and pop in front of him, he would eat it," she says. "But he's just as happy with an apple. It's my responsibility to see that the right foods are there."

Mary Camous, a home economist, doesn't buy sugar-coated cereals either. "Unfortunately, stores put them where kids can see them, at the 3-foot level. This nutrition program," she said, "made it a lot easier to deal with. Tommy's a better eater than the older child and maybe that's why."

According to Dr. Lewis, that's what the program was all about. "The children learn what is good and what is bad, but what it's hard for them to do is resist. It's better to remove the temptation. It's better to them to know why they are being denied something, by saying, 'We don't have that at our house because it's junk food."

Offering them alternatives leaves them open to temptation, Dr. Lewis admits. She described what happened when the children were offered a choice between potato chips and carrots. "Just as many kids might take the chips. One kid said: 'I know this is good and I know this is junk food, but I'd rather have the chips.'" CAPTION: Illustration, no caption, by Richard Bangham for The Washington Post