FIVE years ago, low-sodium foods were supermarket wallflowers, set in quiet corners and largely ignored.

All that's changed, according to Kenneth Carlson, vice president of the A.C. Nielsen Company (famous for Nielsen television ratings). He told a recent Baltimore meeting of food technologists and writers that consumers are asking "most clearly for products with less salt."

"This is a trend we saw begin in 1982, that we're sure will develop further," he said Fewer than five low-sodium products entered the national market in each of the four years before 1982, said Carlson. But last year, major companies such as Libby's, Ralston (with Chicken of the Sea Tuna) and Kellogg's contributed to the more than 35 national brands that were introduced, not including store brands, local products and generic labels. Carlson said 1983 promises even more proliferation--with at least 18 new low-sodium products recorded so far.

The significance of new products lies as much in who's making them as in their appearance. A familiar brand name reinforces a trend.

Decaffeinated coffee is also off and running, with Hills Brothers, an established coffee packer, joining the ranks. While the caffeine-free race, which involves not only coffee but soft drinks and tea, is too early to call, Carlson said "there's certainly activity out there."

These are just individual examples of a "good-for-you or at-least-better-for-you-than-what's-already-out-there trend," said Carlson, with manufacturers capitalizing on perceived health benefits. Variations on the theme include new styles of yogurt, granola bars, whole-grain cereal, natural fruit juices and low-calorie convenience foods. "Actually," said Carlson, "with concern for sugar in the diet continuing, and the concern for salt and caffeine developing, we could lump together a vast array of products . . . into one large healthful group . . ." While prototypes of these have been available for a long time, the profusion of similar products shows the trend becoming entrenched. Fad becomes a way of life.

Low-sodium products are just one example of the 5,000 new grocery products developed each year to entice the American consumer away from one brand and toward another. There's a constant struggle for a bigger piece of a pie that never changes, says Dr. James Albrecht, vice president of McCormick & Co.

Albrecht explained that, figuring for inflation and population growth, demand for food products remains fairly stable. So every food company maintains a constant effort to steal bits of the market away from its competitors. Thus, Americans get to choose among products such as canned puddings, pressed potato chips and processed cheese slices. Of the thousands of new products introduced, about 3 percent succeed. To succeed, says Albrecht, the product must enjoy $15 million in sales in one year. That's a lot of Pringles.

Most of the thousands of new products fail, but may do so in stages. The ready-to-eat cereal category "was shaken up" in the 1970s with the release of "natural"-type cereals, said Alan Miller, vice president of Selling Area-Marketing Inc. (SAMI). "Then the whole thing caved in," Miller said, because of bad press concerning the cereals' expense and dubious nutritional value.

Next the cereal was pressed into a bar. "They took what turned out to be a disaster, thought it out a little better and came up with something," said Miller. Of the granola-bar market, he added, "The end is not in sight."

The inescapable conclusion is that success is not entirely predictable. One might have thought America was waiting for a low-cost, convenient, relatively nutritious breakfast meat, said Albrecht, and thus a company made one out of turkey. But the product failed.

What are succeeding these days, according to Nielsen's Carlson, are Mexican foods and processed tomato products. The trends may actually reinforce each other.

Tomato-based sauces "were always out there, but now there are a lot more variations on a theme," he said. "I think there are spicier and hotter variations on pizza sauce and catsup. I don't know why this has suddenly started but I can only assume it's because people's tastes have changed toward the hotter Mexican food thing." Hot sauces and spaghetti sauces lead tomato-based sauce sales.

In addition, he guessed that producers might have improved the quality of their sauces, creating stiffer competition. If one product is thicker and spicier and it succeeds, there's a good chance its competitors will become thicker and spicier.

This tomato-based sauce surge may arise from the Mexican food craze. Mexican fast-food restaurants have created mainstream demand for these spicy foods and the processors, from Del Monte to McCormick, are following suit.

What Albrecht sees next is a revival of Chinese, and after that, "maybe Greek?" While the choices are presented by the manufacturers, Carlson maintains the public retains crucial veto power: "Marketers, for all their creativity and cleverness, cannot perform miracles," he said. ". . . they cannot persuade consumers to buy a product that they neither need nor want."