America's love affair with fast food has hit a rocky patch.

There is litigation in the air. Nutritionists warn us about trans-fats and super-sizing our way to obesity. Something is not right in the drive-thru, our transport to a half-century of low-cost, high-calorie, turned-on-a-dime comfort food.

As Americans get fatter, and critics look for culprits among peddlers of cheeseburgers and chicken fingers, the nation may be about to engage in an epic culinary transformation.

"People who eat fast food tend to have the high-fat and low-fiber diets we associate with obesity," says Gail Woodward-Lopez, associate director of the Center for Weight and Health at the University of California at Berkeley. "We're not asking anyone in the fast-food industry to go out of business. But serving nutritious foods must be given a priority in our culture, just like providing safe buildings."

Saddled with slumping sales and worried by the prospect of obesity lawsuits and more government regulation, the industry suddenly finds itself on the defensive. With its dismal record of developing healthy alternatives to burgers and fries - remember the McLean Burger and Taco Bell's low-calorie Border Lights line? - the large chains wonder: Do we stick with the Big Macs and Whoppers that got us where we are, or do we beef up our menus with healthier selections our core customers may not even want?

"Americans feel they can make their own decisions on what they should be eating, and they don't want the federal government or anyone else telling them what to eat," says Steven Anderson, president of the 300,000-member National Restaurant Association, which often serves as the mouthpiece of the fast-food chains. "We are driven by customer demand. In 2003, we'll serve more than 53 billion meals at all restaurants, which means there is no industry that has its finger on the pulse of 280 million Americans like we do. We see them every day and we know what they want."

So what will Americans want to see on the McMenu of the future? Most new items are in development for as much as two years, so any real change will be gradual. And no one is predicting the demise of fast food, which food writer Claire Hope Cummings calls an American "addiction, built deep into our psyche and lifestyles." Still, there are signs that a modest makeover may be on its way.

Last year, Burger King unveiled the BK Veggie Burger, the first major non-meat offering in the company's history. There's even brown rice in the patty. McDonald's is switching to cooking oil that reduces saturated fats and trans-fatty acids, it says, without sacrificing taste. And Wendy's has rolled out a new line of salads like the Mandarin Chicken and the Chicken BLT, each with packets of optional ingredients to give the customer the ability to custom-fit the amount of fat and calories.

For now, "let the customer decide" remains the fast-food mantra. And even though they vow to keep experimenting with new items, healthy menu items are a hard sell. Take Burger King. Spokeswoman Kim Miller says the company first market-tested veggie burgers in the late 1980s, only to abandon the effort after they bombed at the counter. In the early 1990s, they tried teaming up with Weight Watchers to offer frozen meals, "but they didn't do well. Consumers told us, 'We're coming to Burger King to eat burgers,' so we ended up pulling that out of our restaurants, too."

But times have changed. More and more households have things like packaged veggie burgers in their freezers. Some trends suggest a restlessness with the holy trinity of burger, fries and soda that has long been the industry's mainstay.

"Six or seven years ago, I went to a conference, and McDonald's was test-marketing those little packages of carrots," said Alice Ammerman, associate professor at University of North Carolina's department of nutrition. "My assumption is that the carrots bombed, but that was before they had become popular in stores. Something like that, reintroduced, may now stand a better chance of succeeding.

At Burger King, more people are ordering their Whoppers without buns. Women in particular were substituting BBQ sauce for mayonnaise. "So we did focus groups on a bunch of items and asked, 'What would you buy?'" Miller said. "They said they wanted more choices beyond sandwiches. So we rolled out chili last year for the first time. At 400 calories, it's relatively low-calorie, and it's a huge hit."

Even if fast-food chains do offer healthier items, they still must deal with the critics' top complaint - super-sizing, or offering a lot more of an item for a minimal increase in price. Since industry's biggest cost is labor, that extra few cents is nearly pure profit. So while doing things like using healthier oils is a step in the right direction, it doesn't solve the problem of what nutritionist Ammerman calls "the hugeness of everything" in today's fast food.

Ammerman says super-sizing takes advantage of "low-income people because it's presented in a way that makes it hard to resist. You get much more food for a little money, but you also get huge amounts of fat in the process."

She said one of her students estimated that by super-sizing three meals a week, just the additional calories would potentially lead to a weight gain of 16 pounds a year.

The big challenge, as marketing futurist Faith Popcorn put it, "is trying to provide a good diet that is cool and hip as well as healthy.

"The fast-food industry can take ownership of the problem and get ahead of the curve," Popcorn says. "Get parents and kids and nutritionists together in a room and find ways to become known as the 'good fast-food company."'

Harry Balzar, a national expert on Americans' eating habits, predicts a smaller burger designed specifically for women, as well as more fruit on the fast-food menu. Others say flat breads and healthier chicken items will pop up, too. Meanwhile, fast-food chains will continue to manipulate the menus they have already by simply adding new and perhaps healthier condiments to slather on your cheeseburger.

For Lanette Kovachi, Subway's chief nutritionist, the future looks like this: "We'll continue to offer lower-calorie condiments and more gourmet breads, as well as so-called functional foods, or foods that have health benefits outside of basic nutrition. Fiber, for example, decreases cholesterol, and tomatoes are rich in lycopene, which reduces the risk of prostate cancer."

Wendy's, which introduced salad bars at its outlets in 1979, prides itself on keeping pace with consumers looking for fresh alternatives to deep-fried and salty foods. "We realize people today are more well-traveled and enjoy foods that blend different cultures," says spokesman Bob Bertini. "They're willing to experiment with new ingredients, and they're seeking taste excitement, new flavors and textures. Today we're offering things like Asiago cheese, which you wouldn't have heard of 10 years ago at a fast-food restaurant."

Healthier fast food is even coming from McDonald's, which claims 42 percent of the market, although not necessarily through the golden arches. Four years ago, the chain became a major investor in Denver-based chain Chipotle Mexican Grill, known for its fresh handmade burritos. Chipotle serves free-range pork from Niman Ranches, which supplies America's top restaurants with quality meats raised on family farms. McDonald's' involvement with Chipotle, says Bill Niman, is an indication that fast food will gradually reflect America's healthier eating habits.

"McDonald's' own brand is stale and the market is saturated and they can't expand their franchise," says Niman, who says cheeseburgers and fries are simply the tip of the iceberg in a sprawling and environmentally disastrous system of mechanized, mass-produced food. "We're excited to be working with a huge organization like McDonald's," Niman, says "because without them signing on, there is no hope of ever changing the system."