Eve Tilley-Coulson, a Los Angeles lawyer and a millennial, made a TikTok video last year pointing out that Generation Z will never know the horror of the 100-calorie snack pack, in which stale brownie thins cosplayed as an indulgent dessert.

“It’s such a throwback, like the old days of Lean Cuisines,” she said in an interview. “It was only 200 calories, but you need three to feel full. The math was not mathing.”

And for many Americans, the calorie is no longer calorie-ing. For more than a century, it stood as the nation’s primary measure of nutrition and weight management, both an aspirational carrot and a tyrannical stick. But its glory days may be over.

The 100-calorie pack has been tossed onto the nostalgia heap as the multimillion-dollar glow of low-cal products fades. The advent of GLP-1 medications, which can sharply curb appetites, has made counting calories irrelevant for many people. Influencers who preach calorie reduction on “SkinnyTok” have been increasingly accused of promoting disordered eating.

Even WeightWatchers, born in 1963 and long considered the grandmother of calorie-counting programs, filed for bankruptcy in May. The company still relies on a system of calculating “points,” but in January began dispensing Wegovy in pill form.

“The stranglehold that calories have had on our culture is loosening,” said Helen Zoe Veit, a food historian at Michigan State University whose new book, “Picky: How American Children Became the Fussiest Eaters in History,” will be published in February. “They are just not a great way for most people to relate to eating, and counting them is a fundamentally unsustainable way for people to have a healthy or joyful relationship with food.”

A new wave of interest in nutrition and a more sophisticated understanding of the physiology of hunger have made relying solely on caloric intake to lose weight seem as simplistic as telling teenagers to “Just Say No” to drugs.

Health organizations have refocused. The Obesity Medicine Association recently changed language on its website to make it clear that obesity is much more than just the result of eating too many calories.

Dietary guidelines from the federal departments of Agriculture and Health and Human Services didn’t overtly suggest calorie limits until 2005, when they introduced an average goal of 2,000 calories a day.That benchmark hasn’t changed, but how to get there has. The revised dietary guidelines announced this month dropped advice to eat more low-fat forms of dairy and whole grains and instead suggest full-fat dairy and more meat — advice some nutritionists said could make limiting calories a challenge.

For the first time, the guidelines warn against ultraprocessed foods. Despite having questionable nutritional value, they make up more than half of the calories Americans eat at home, according to a 2024 study by the Johns Hopkins Bloomberg School of Public Health.

“We’re at that point where we are now considering nutrition composition. It’s not calorie counting, it’s which calories count,” said Keith Albright, the senior consumer insight manager for the agricultural and food giant Cargill. “That doesn’t mean calorie counting is completely dead, but the consumer right now is asking, “What is the value of that calorie and what do I want my food to do for me?’ ”

A War Against Weight

Calorie numbers still dominate the nutrition labels on food, in large, bold type, a change the Food and Drug Administration made in 2016 at the height of Michelle Obama’s “Let’s Move” initiative. Since 2018, a federal mandate has required all chain restaurants to list them on menus.

But such measures don’t appear to have curbed calorie consumption. A study last year of similar efforts across several countries showed that the average reduction in people’s caloric intake was the dietary equivalent of a few potato chips.

The calorie wasn’t born to drive weight loss. A French scientist began using the term as a measure of heat in the 1820s as he searched for a way to explain the power of a steam engine. Wilbur Atwater, a chemistry professor at Wesleyan University applied the calorie to food in the 1880s, using it in part to help poor families get the most nutritional bang for their buck. Some food scientists still use a modern version of that era’s essential equipment, the calorimeter.

The novel idea that food fueled bodies in the quantifiable way that gasoline fueled the newfangled automobile caught on in a time when new methods of measurement like statistics and box scores were an obsession.

During World War I, the U.S. government asked the public to eat less of calorie-dense foods that were then considered healthful, like flour, sugar and red meat, because American soldiers and their allies needed the calories. Restaurants added calorie counts to menus so diners could monitor their consumption and be reassured that they were still getting plenty of calories even if they were patriotically eating substitute foods.

“The 1910s were a strange decade, where calories could relate pretty equally to efforts to gain weight, lose weight or maintain weight in popular culture,” said Dr. Veit, who explored the period in her book “Modern Food, Moral Food: Self-Control, Science, and the Rise of Modern American Eating in the Early Twentieth Century.”

Dr. Lulu Hunt Peters, a pathologist from California who served with the Red Cross in the Balkans, picked up the patriotic message and ran with it, writing about her personal struggles with weight loss in a series of syndicated columns and diet guides. Calorie restriction as a popular pursuit was born.

Dr. Peters was an unabashed fat shamer. “A disgrace to be fat!” became a catchphrase in her column. She embraced the war effort, calling dieters “soldiers of the kitchen” and developing a prototype of the modern weight loss support group that she called Watch Your Weight Anti-Kaiser Classes. Her 1918 book. “Diet and Health: With Key to the Calories” was a best seller. In it, she outlined her own version of the 100-calorie measure for dieting.

“Hereafter,” she told readers, “you are going to eat calories of food. Instead of saying one slice of bread, or a piece of pie, you will say 100 calories of bread, 350 calories of pie.”

Thinness, once considered a sign that you were too poor to eat well, became a status symbol. The exuberant flappers of the 1920s celebrated it. For decades to come, the calorie would inspire countless diner diet plates, cabbage soup diets and low-cal marketing campaigns.

The Oprah Effect

No one has embodied America’s weight-loss struggles more than Oprah Winfrey, who has publicly gained and lost hundreds of pounds, cycling through one diet strategy after another. In the 1980s, she went on a liquid diet and, on her show, pulled a wagon filled with 67 pounds of animal fat — the amount she had lost — across the stage. The weight came back.

She jumped on the low-fat, high-carb bandwagon of the 1990s, had a personal trainer on call and ran a marathon. She dabbled in the low-carb, high-fat diets of the early 2000s, then fell for WeightWatchers, losing 40 pounds on the company’s system of calorie restriction and peer support before joining its board in 2015.

She announced she was taking GLP-1 medication in 2023, confessing that she wrestled with the shame of not being able to lose weight through sheer willpower. She resigned from the WeightWatchers board. The company’s stock tumbled.

Now she has aligned herself with doctors and researchers who see managing weight and treating obesity as a nuanced pursuit that involves brain biology, hormones and a food environment that promotes weight gain.

She outlines her revelations in a book, published last week, that she wrote with Dr. Ania Jastreboff: “Enough: Your Health, Your Weight, and What It’s Like to Be Free.” It redefines obesity not as a personal failure but as a neurometabolic disease made worse by ultraprocessed food, lack of sleep, little exercise and stress. Health, not a number on a scale, is the goal.

“Calories arejust a piece of information,” Dr. Jastreboff said. “They are a way one can measure energy within a given nutrient but they don’t tell us about the nutrient content itself.”

Telling people to simply eat less is as ineffective as telling an alcoholic to drink less, said Dr. Jason Fung, a doctor and author widely credited with popularizing intermittent fasting. His book, “The Hunger Code: Resetting Your Body’s Fat Thermometer in the Age of Processed Food,” will be published in March.

“Calories are such a small part of the actual human physiology and human psychology of eating, and people believed it to be the be-all and end-all,” he said. “I’m just saying the quiet part out loud. The emperor has no clothes. Calorie restriction alone doesn’t work.”

Number’s Up

Not everyone believes the calorie’s time is done.

The celebrity health and fitness influencer Jillian Michaels, who rose to fame as a trainer on “The Biggest Loser,” contends that the only reason people lose weight on GLP-1s is that “they are enforcing calories-in, calories-out at a cost financially and with side effects.” The drugs, she said, should be used only in cases when there is a significant amount of weight to lose and under a doctor’s care.

“I am always going to tell you to try to eat less on your own,” she said. “The best advice is to shut your mouth, make good food choices, move more often, focus on sleep, drink water and it’ll all work out, I promise you.”

Marion Nestle, the nutrition professor and author, has been researching calories and portion sizes for decades, even when it was more popular to deploy varying amounts of fat or carbohydrates as a way to lose weight.

“Calories count,” she said. “You just shouldn’t be counting them because you can’t do it accurately.” She recalled a 1997 experiment conducted by Marian Burros, a New York Times food reporter, who took Ms. Nestle and three other dietitians and nutritionists to dinner and asked them to estimate how many calories they consumed.

“We were off by hundreds and hundreds of calories,” Ms. Nestle said. “It was so embarrassing.”

As calorie counting fades, new ways to evaluate food continue to emerge. Easy access to a dizzying amount of nutrition advice has created an army of armchair experts. Many people are abandoning the complicated math required to decipher nutrition labels, with their often-deceptive serving sizes and mystifying daily-value percentages, and instead counting grams of protein and fiber.

Sam Citro Alexander, a former Estée Lauder executive who grew up eating 100-calorie snacks in her school lunches, spent the past five years developing the FoodHealth Score. Her company’s algorithm rates foods on a scale of 1 to 100, based on the quality of ingredients and their nutrient density. Already, big retailers like Kroger are adding the scores to items on their websites, with suggestions on healthier swaps.

“We don’t need that calorie,” she said.

Or do we?

“Americans have done nothing but zigzag in modern times when it comes to food,” said Dr. Veit, the food historian. “For all I know, by the 2030s we’ll all be obsessed with calories again in some new way.”

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