The nation's fast-food chains, long dismissed as peddlers of junk food, are serving up a new item in their ongoing marketing war: nutrition.

Yesterday, Hardee's, the nation's third-largest burgermaker, said it would be the first chain to introduce a low-fat hamburger, an announcement that had competitors scrambling to defend the healthfulness of their fare.

Responding to what it said was consumer demand for lower-fat products, North Carolina-based Hardee's said it would begin marketing the Lean 1 burger this fall throughout its 3,300 outlets, including the 648 Roy Rogers restaurants it bought from Marriott Corp. earlier this year. The quarter-pound burger contains 18 grams of fat, which Hardee's said is about 10 percent leaner than McDonald's Quarter Pounder and about 50 percent leaner than Burger King's Whopper.

Wendy's International, No. 4 in the burger wars, said its regular quarter-pounder contains only 15 grams of fat. And a Burger King spokeswoman hinted that the Miami-based chain, second to McDonald's in sales, is considering introducing a reduced-fat burger. "We listen to our consumers. Nutrition is very much a concern for us," said Cori Zywotow of Burger King.

The big chains have targeted weight- and health-conscious patrons for the past two years by adding salads and relatively low-calorie chicken sandwiches to their menus, as well as by switching from high-fat beef tallow cooking oils to all-vegetable oils. Last week, McDonald's said it would post nutritional data about its food in its 11,000 outlets.

The latest fast-food infighting drew the grudging praise yesterday of nutrition advocates, who otherwise aren't big fans of the $50 billion industry. "Just the idea that the junk-food purveyors of the world would pay attention to nutrition is interesting," said Michael Jacobson of the Center for Science in the Public Interest. "People will still eat greaseburgers, but now at least they're getting some choices."

The American Heart Association, however, cautioned that a claim of reduced fat is not enough. "They're saying they've reduced the amount of fat, but they're not telling you a hamburger is still a relatively high-fat food," said spokesman Joe Marx.