The breakfast had the look of tradition and the stamp of science: "cholesterol-free egg substitute" (scrambled), strips of substitute bacon, muffins filled with "artificial blueberry-flavored nuggets," orange drink -- not juice -- topped off with a cup of coffee with nondairy creamer and substitute sweetener.

Since margarine came on the market in 1873, food manufacturers have worked to develop imitation foods that would lower production costs, last longer on the shelf -- and increase the companies' profits.

But over the past decade, imitation products have taken over more and more space in the supermarket -- in the frozen-food section, next to the fresh meats, with the canned goods and on the soft-drink aisle.

Manufacturers maintain that the "fake foods" are as safe and nutritious as the originals, but nutrition activists don't necessarily agree.

Our substitute bacon "is all meat," said an official of Oscar Mayer, which makes Lean 'n Tasty. "We grind it. Extrude it. Shape it. Smoke it (to give it color ) and fully cook it. Then we slice it and lay it out on a board. It's not bacon. It is a substitute for bacon."

"Just because they (substitute foods ) are not the same doesn't mean they are not as good or maybe even better and more nutritious (than the traditional foods )," said Kent N. Mittelberg, chairman of the Food Protein Council, a trade group that promotes the use of soybean derivatives in food products. He said substitutes, particularly those made from soybeans, can even represent "an improvement in the food system," because the imitation product may have fewer calories and less cholesterol.

But nutrition activists generally oppose substitutes.

"We don't even know everything about real food, so the idea that we are ready to fabricate equivalents and keep humans healthy is absurd," said Dr. Joan Gussow, author of two books on health and nutrition and chairman of the nutrition program at Columbia University's Teachers College.

For instance, some questions have been raised about the nutritional value of imitation meats made from soy products because although they have less cholesterol than regular meat, some of the soy "meats" tend to be higher in sodium.

The discriminating consumer who wants to compare traditional foods and their substitutes may be hindered because of confusing labels and, in some cases, lax enforcement of labeling requirements. Some imitation foods, for example, don't say "imitation." Instead, they may have a fanciful name that implies that they are the traditional food they are imitating. Further fuzzing the difference between imitation and real foods is the way many stores place substitute foods next to the original food.

Complaints to the Food and Drug Administration, which enforces labeling rules for nonmeat products, may go un-answered because its budget for fighting labeling violations has been cut 75 percent over the past two years.

"The companies know that and they take advantage of it," said Gene Newberry, chief of food labeling enforcement. "We have had an increase in the complaints about labels."

Under the FDA rules, a food producer doesn't need to get its labels approved in advance. If someone charges that the label fails to meet disclosure rules, such as listing ingredients properly, the FDA is supposed to investigate and, if necessary, order that the label be corrected. But with the current budget crunch, that happens only in extreme cases.

At the U.S. Department of Agriculture (USDA), which has had authority over meat-product labeling since 1914, the manufacturer must have prior approval of his label and his meat-processing system before the product can be sold.

In fiscal 1980, the eight-person label section staff processed 104,826 labels, more than 95 percent of them within a week after their submission, according to Bob Hibbert, director of the meat and poultry standards and labeling division. "Some companies have representatives who make appointments to come in and they can get approval on the spot," he said. The immediate approvals may involve some simple change in an existing label, such as a change in the color, he said. Applications for labels for new processed products are referred to another section of USDA for review and approval, Hibbert said.

Though manufacturers point out that the imitation foods are usually cheaper than their traditional counterparts, that isn't always the case. For example, a 12-ounce package of Swift's Sizzlean, a substitute bacon, sold for $1.79 last week at Safeway. Allowing for less shrinkage, that was less than name-brand bacon, priced at $2.29 a pound, but more than the Safeway house-brand bacon, prices at $1.29 a pound.

Nutritionist Gussow said that consumers seeking foods with less fat, less cholesterol or a lower price should skip the substitutes and look for an alternative among the traditional supermarket products. "If bacon fat is the problem, then they might try fish . . . or some lean beef that has been sliced thin," she said. "And the lean beef may even cost less than the bacon," she said.

Consumers can save themselves from surprises if they are generally aware of the types of simulated foods available in the stores.Here is a summary of the most common kinds:

Imitation products. When there is a government standard for a product, such as cheese or mayonnaise, a food producer must follow that standard in making the product. If he doesn't, then he has two choices: He can label the procuct as an imitation or name the product something other than the food it pretends to be. In case of a cheese product, the item could be called "imitation cheese" or it might be given a name such as "Smooth Sandwich Spread."

Substitute products. The word "substitute" may describe nearly any simulated product, whether it is fabricated from real food or synthetics. Two common ones are the substitute bacon, made from all-meat products, and sweeteners, made from synthetics.

Synthetic products. Fruit drinks, nondairy creamers and sweeteners typically are synthetic products from non-food sources.

Analogs. This generally includes simulated bacon bits, bacon or pork links made from textured vegetable protein, such as the soybean.

Extended and restructured products. Fresh or frozen beef patties made from beef, an extender such as soy, and added fat is one example. Some frozen convenience dinners contain an extended meat product, such a Salisbury steak. Meat rolls made from meat shreds, water, soy flour and usually an extender are shaped and then chopped and used in an assortment of canned products, including chicken-noodle soup.

Although no one is quite sure how much fabricated food the average consumer now buys, there is general agreement that the amount is increasing because of the availability of new products and the changes being made in traditional foods.

"Like chicken-noodle soup," said David Shenkenberg, senior staff officer of standards for USDA.

He said that soup makers increasingly use a chicken roll formed from shredded chicken, water, flavorings and in most cases an extender. Formed into a roll, the chicken can be diced and used in chicken soup, in compliance with standards that call for a minimum of 2 percent chicken meat in soup bearing the label "chicken-noodle soup."

Without the chicken roll, the manufacturer would be unable to use some parts of the chicken, such as the chicken neck meat, because it wouldn't be in acceptable form for soup, Shenkenberg said.

He said the chicken and meat rolls now are commonly used in meat pot pies, frozen dinners and institutional meals.

"Uniformity is one reason for the rolls," he said. "And another reason is the economics: to keep costs down and improve profitability."