Michael Donahue, co-founder of Lyfe Kitchen, wants to convert customers “who have always thought healthy food has to taste like straw.” Photograph by Andrew B. Myers for The New Yorker

Like many of their millennial peers, Kathleen Davis and Andrea Nguyen eat out a lot. “Nothing fancy,’’ Davis told me one recent evening, as she took a sidewalk table next to mine at Sweetgreen in Nolita. “We want what we eat to be healthy and tasty,’’ Davis said. “Decent prices matter, too.” The women were working their way through one of the restaurant’s seasonal specialties—the “wastED” salad, which consists almost entirely of carrot peels, broccoli stalks, roasted bread heels, cabbage cores, and other ingredients that are usually tossed out.

Ten years ago, no American would have regarded a bowl of vegetable scraps dressed with lime-cilantro or spicy pesto vinaigrette as fast food. Many people wouldn’t have considered it food at all. But millions of diners, fuelled by concerns about their health and the state of the environment—and propelled by a general distaste for industrially produced and highly processed food—have begun to shun the ubiquitous chains that have long shaped the American culinary character. Sweetgreen and places like Lyfe Kitchen, Chipotle, Smashburger, Five Guys, Shake Shack, and Dig Inn now occupy the rapidly expanding middle ground between restaurants with tablecloths and the giant fast-food chains. The category, referred to broadly as fast casual dining, is growing more quickly than any other segment of the market.

For more than fifty years, eating at fast-food restaurants has been an almost clinically impersonal experience: the food is rapidly prepared, remarkably cheap, utterly uniform, and served immediately. The cheeseburger you get at a McDonald’s in Orlando is exactly the same as the one you get at a McDonald’s in San Francisco, Montreal, or Little Rock. Each month, more than two hundred million people eat at least one meal at one of the hundred and sixty thousand fast-food restaurants in the United States. McDonald’s alone serves twenty-six million people every day at its fourteen thousand American outlets—more than the population of Australia. Millions more visit Burger King, Wendy’s, Subway, Pizza Hut, Dunkin’ Donuts, In-N-Out Burger, as well as the other chains that occupy virtually every highway, strip mall, and town center in the nation.

Almost seventy per cent of customers at places like McDonald’s, which are known in the trade as quick-service restaurants, get their food at a drive-through—a process that, according to last year’s Drive-Thru Performance Study, conducted by QSR, an industry magazine, takes an average of 219.97 seconds and costs most people about five dollars. I asked the women at Sweetgreen if they ever patronized McDonald’s or similar restaurants. Davis shuddered and said nothing. After a brief silence, Nguyen owned up to eating at McDonald’s once or twice a month, but not for a Big Mac or French fries. “They have some surprisingly good food these days,’’ she said in a confessional whisper. “But I would never be seen walking down the street with a McDonald’s bag in my hand.’’ I asked why. “Shame,” she replied. “I don’t know anyone who would feel differently.’’

Hers is a commonly voiced sentiment. Speed and convenience matter as much as ever to American diners. But increasingly people also demand the information that places like Sweetgreen offer. They want to know what they are eating and how it was made; they prefer to watch as their food is prepared, see the ingredients, and have a sense of where it all came from. And they are willing to pay more for what they perceive to be healthier fare. Most of these restaurants, where meals generally cost between eight and fifteen dollars, rely on a few ingredients, stress the quality of their food, and often treat the farms that supply their vegetables with the kind of reverence once reserved for fine wineries.

The rise of the healthy fast-food chain has been aided by the easing recession, but it comes largely at the expense of traditional competitors. None have struggled more than McDonald’s, one of the world’s most recognizable brands. In March, the company replaced its chief executive with one of his deputies. Two months later, it ended its long-established practice of issuing monthly reports on individual store sales. And this year, for the first time since 1970, McDonald’s will close more locations in the U.S. than it opens. When I asked Dan Coudreaut, the company’s executive chef and vice-president of culinary innovation, what mattered most to McDonald’s, taste, price, or efficiency, he sighed. “Our main job is to create value for our shareholders, for our company, for our restaurateurs,’’ he said. “We are not a nonprofit organization and we are not married to any one area. We are married to being a successful business. Society is shifting in a major direction, so guess what—McDonald’s is going to shift, too.’’

The company is trying everything it can to win back deserters. Last month, in keeping with prevailing desires and current nutritional wisdom, McDonald’s abandoned margarine for butter. The company announced recently that it would stop selling chickens that have been raised with antibiotics that could affect human health, and milk from cows that had been treated with growth hormones. They introduced low-calorie “artisan grilled chicken” sandwiches and, this month, began serving breakfast all day—fulfilling a request that the Egg McMuffin crowd has been making for years. McDonald’s has also jumped on the seasonal-food bandwagon, having sold about thirty-seven million Cuties, the brand of clementines that come with Happy Meals. The company has even begun to introduce restaurants with digital kiosks, where customers can build their own dishes on a touch screen, then grab a G.P.S. locator, find a seat, and wait until the freshly made product is delivered by a server who has homed in on the signal.

McDonald’s describes all these changes as an attempt to “reassert” itself as “a modern, progressive burger company.’’ Nonetheless, daunting questions hover over its ambitious agenda, and over the entire industry. Can traditional restaurant chains, indelibly branded as places to eat cheap food fast, switch to healthier fare and stay profitable? And to what degree can companies like Sweetgreen thrive by offering a fresher, more nutritious alternative?

Fast food has become a synonym for bad food. Yet, the industrial farm system that has made it possible for McDonald’s and many other chains to sell cheeseburgers for a dollar has also enabled Americans to spend a smaller percentage of their income on food than people do in any other country. At the start of the First World War, food purchases consumed half the average paycheck; today the figure is six per cent. According to federal statistics, an American in 1919 had to work for two and a half hours to earn enough money to buy a chicken; these days it would take less than fifteen minutes of labor.

Fast food has transformed America, but we hardly invented it. The most impoverished citizens of ancient Rome, unable to store supplies or afford cooking oil, often ate at booths that served what we would now call pizza. According to a twelfth-century account of life in London, travellers who were unwilling “to wait fasting till fresh food is bought and cooked’’ could “hasten to the river bank, and there all things desirable are already to their hand.’’ And in 1802 Thomas Jefferson, the nation’s third President and its first foodie, asked the White House chef, Honoré Julien, to prepare “potatoes served in the French manner.” The potatoes, “deep-fried while raw, in small cuttings,” have been on our dinner tables ever since. It wasn’t until the middle of the twentieth century, however, that, beguiled by the open road, and the drive-through window, the United States truly became what Eric Schlosser described in his 2001 book: a “fast-food nation.’’