CHICAGO (AP) _ Food manufacturers who once described their products with slogans like Campbell's ''M'm 3/8 M'm 3/8 Good 3/8'' are singing a new tune, substituting ''healthy'' for tasty as they woo nutrition-minded consumers.

But regulators at the U.S. Food and Drug Administration are finding the growing use of the word ''healthy'' might be enough to give them indigestion, a spokesman says.

The FDA earlier this month proposed detailed new food labels and the first strict definitions of such popular descriptions as ''light,'' ''calorie- free,'' and ''low-cholesterol.'' The agency also has taken action against deceptive use of such words in brand names.

However, the agency's regulators were unable to agree upon definitions of such words as ''natural'' and ''healthy,'' said FDA spokesman Chris Lecos.

In recent months, ConAgra had Healthy Choice Soups, Kraft had new Budget Gourmet Hearty and Healthy Entrees, McCain had Healthy Slices Pizza, and Campbell, which for years has marketed their soup products as ''M'm 3/8 M'm 3/8 Good 3/8,'' now has Healthy Request Soups on the shelves.

''When you describe anything and say it's healthy, it has a positive connotation,'' said Roger W. Spencer, a food analyst with Paine Webber Inc. in Chicago. ''These companies tend to do what works - what sells.''

The new regulations ''do not completely close the door on'' possible deceptive advertising, Lecos said Friday. ''That kind of intramural stuff is going to go on until the dust settles on all these new terms.''

''Healthy'' may be discussed during public hearings on the new FDA regulations, which would take effect late next year, Lecos said. The first hearing is tentatively scheduled for Dec. 12 in Washington.

''Healthy'' poses a definition dilemma not only because it's grammatically incorrect - healthful is the right word - but because virtually all foods can be viewed as having some health benefit. Even sugar is nutritionally useful.

If no rule on the use of healthy is passed, regulators would have to look at each product that uses the word to decide if the implied nutrient content claims actually fit the bill.

Analysts said few companies would risk the wrath of the FDA and, more importantly, their customers by calling their products healthy when they aren't.

''I don't think they're trying to get around the regulations,'' Spencer said. ''Some may be, but they're in effect saying the consumers are stupid.''

He noted that many companies that prominently display the word healthy on their labels back up the contention by listing fat, cholesterol and calorie content.

Consumer interest groups contend that saying a particular food is healthy often is misleading.

''Some of these foods, frozen dinners for example, may have lower cholesterol and calories but be loaded with sodium,'' said Jody Silverman of Public Voice for Food and Health Policy in Washington. ''Does that make it healthy?

''If they can't come up with a very strict definition of what healthy means - and I don't know of one since we haven't examined the problem closely - then they need to get rid of the word altogether.''