Omigosh. How much chicken salad is that guy putting on my sandwich? I know I'm not supposed to eat more than 2,000 calories a day and, according to new government guidelines, no more than 600 should come from fat. Let's see: divide that by 9 to convert fat calories to grams, and that means I should eat only 66 grams of fat a day.

Breakfast had 6 grams. My yogurt and potato chips add 13 grams. My calorie book says that the cheese slice and the croissant add up to 21, putting my total so far at 40.

When I called Mom this morning, she said that dinner tonight would be chicken breast, which -- so long as I don't eat the skin, don't put butter on my potato, don't have any dressing on my salad and skip dessert -- means I can escape with, let's say, 15 grams.

That will put me at 55 for the day, which leaves me 11 for this sandwich. Factor in the mayo and do a quick metric conversion and that allows me just over 3 ounces.

Hey! Easy on the chicken salad. My health's at stake!

With the passage of Congress's new food labeling regulations and the publication last week of the federal government's revised dietary guidelines, the United States has embarked on a grand nutritional experiment.

For the first time in history, Americans will be given consistent, accurate and complete nutritional information on almost all of the food they buy in the supermarket. Misleading terms will be outlawed. The kind of rational planning of fat, cholesterol and salt intake that nutritionists have urged consumers to undertake for years will become possible.

But what is possible and what will happen are not necessarily the same. As the experiment begins, social scientists are not sure whether offering consumers better information on the food they buy will change eating patterns.

Part of the reason is that the new rules don't cover all food, exempting meat and poultry products as well as restaurants, where Americans spend about 42 percent of their food dollar. Diligent consumers still have to fill in the blanks with their own research to find out whether their diet has, say, the government-recommended standard of less than 30 percent of its calories in fat. Few experts expect that many people will go this far.

"If you don't keep track of fat like you keep track of your bankbook, then you will have no idea how much you're eating," said Ron Goor, author of the nutrition book "Choose to Lose." "You have to have some kind of numerical system. Otherwise you're floating around in a vacuum."

Even the notion that shoppers will do the next best thing -- simply using labels to avoid the most calorie- or fat-laden foods -- is open to question.

"The record of information in supermarkets in changing people's behavior has not been outstanding," said Howard Schutz, a professor of consumer science at the University of California at Davis. "You get some changes in attitude and knowledge, but much less in behavior." 'Preaching to the Converted'

Some consumers do use such information. Market researchers say that about 15 to 20 percent of shoppers -- generally better-educated consumers or people who have been instructed by doctors to watch their diet -- read labels assiduously and do the arithmetic needed to make sure they eat well. Another 20 or 30 percent read labels at least some of the time. Both of these groups will benefit from the improved information on the new label.

But what of the remaining 50 percent? Some researchers doubt that people who have not shown any inclination to read labels will start just because labels are improved.

"Policy-makers seem to think that everyone needs the same kind of information and will use it the same way," said James McCollough, chairman of the marketing department at Washington State University. "What we found is that there are actually three or four different market segements . . . one of which not only does not want more nutritional information but is uncomfortable with the amount of information they are now getting."

McCollough says these are largely lower-income shoppers who do not know the principles of good nutrition.

The new food-labeling initiative, McCollough said, is "preaching to the converted. We're providing more and more information to people who are already making informed decisions. But we still don't have a really good format to present information to those who really need it the most."

Cornell University marketing expert Jay Russo said his research suggests that for less-educated consumers, labels are a high-cost means of presenting information, requiring time and effort to read and understand. He prefers lists that rank products by nutrient value, or simple shelf cues pointing out the lowest-fat or lowest-cholesterol brand in a class.

According to some research, even the most dedicated label readers won't use label information to do what nutritionists really want them to -- radically alter their eating preferences. Rather, they select their food based on taste, price and convenience and use the label to choose the best brand.

In other words, good labels won't get people to stop eating TV dinners and start eating bean sprouts. They will only steer people toward the most nutritious TV dinner, a distinctly Pyrrhric victory for general nutrition.

All of this, marketing experts stress, does not mean the package of food label improvements passed by Congress last month is useless. But it is likely to succeed in some indirect ways. Education and Immediate Impact

For instance, Congress ruled that broad nutritional education provisions must accompany the new food labels. Education, nutritionists agree, is the best way to get non-readers of labels to read them, and current label readers to read them more effectively.

At the same time the new labels' immediate impact may be felt not by individual consumers, but by the supermarkets, distributors and companies that dictate the choices available.

Stores can use the information to target the most nutritious products in a class. Restaurant and cafeteria cooks can use the labels to choose the best food for their customers. And food companies can heed the message implicit in a new focus on nutrition.

"Manufacturers who have not had this information on their food products are going to look at their product and say, 'How does this stack up against my competitor?' " said Tim Hammonds, senior vice president of the Food Marketing Institute. "It is important to note that this is not going to revolutionize the supermarket. But over time it will make a big difference."