Why most diets don’t work—and what to try instead

PopSci is spending September relearning how to eat. As intuitive as our love of chowing down is, a lot stands between us and optimal eating. This month, we’ll break down diet myths, unlock delicious kitchen hacks, and explore our most common misconceptions about our grub.

Nearly a century ago, one of the first fad diets, known as the Hollywood or grapefruit diet, suggested that eating the tart fruit with every meal was the secret to staying thin. 

It wasn’t. But that didn’t stop diets from ballooning into a multi-billion dollar industry in the United States.

You wouldn’t know it from reading the promotional materials of all the apps, guidebooks, and influencers promising to help you shed pounds, but there’s a lot that researchers and physicians still don’t understand about weight loss. The one thing experts are fairly certain of is that dieting rarely works in the long run. Science suggests that there are many reasons for this, ranging from genetics to the kinds of foods available to us, as well as how the body reacts when we suddenly change how we eat.

“We’re oversimplifying the effect of ‘calories in and calories out’ without considering all the complexities of the physiology of the body’s natural desire to want to regain weight,” says Holly F. Lofton, director of the Medical Weight Management Program at NYU Langone. 

Here’s what we know about how fad diets set us up for failure—and why emphasizing goals other than weight loss may be the key to improving your health. 

Investigating diets

What exactly is a diet? 

At its most basic, the term refers to nutritional and lifestyle changes that a person makes to improve their health, Lofton says. Not all “diets” are bad or misguided; certain diets are essential for managing medical conditions, such as avoiding gluten if you have celiac disease. 

Often, though, when people use the word “diet,” they’re thinking of weight loss. Countless trendy plans have emerged in recent decades, from Atkins to Zone. Some are designed to restrict calories, while others limit fat or carbs, or cut out certain foods such as sugar and legumes. 

Whatever the approach, diets tend to follow a similar pattern: most people lose weight for several months, and then begin to gain back some or all of what they’ve lost, if not even more. Scientists aren’t fully certain why this is, says Kevin D. Hall, the section chief of integrative physiology at the National Institute of Diabetes and Digestive and Kidney Diseases. 

[Related: In 2021, as ever, the best diets are simple]

Part of the problem is that dieting is rather tricky to study. For the experiments that Hall and his colleagues run, participants stay at a clinic for several weeks and eat a tightly controlled meal plan. The goal is to explore how nutritional changes influence a person’s physiology. Earlier this year, the team reported that participants following a low-fat diet consumed fewer calories than those eating ketogenic (high-fat and low-carb), but had higher blood sugar and insulin levels, However, these studies are expensive, short, and don’t necessarily reflect how people behave outside the lab. 

More common are real-world studies in which people are randomly assigned to one diet or another and tracked over time. In one such clinical trial nicknamed DIETFITS, Stanford University researchers followed more than 600 adults for a year and found no significant difference in weight loss between those told to adopt a low-fat diet compared with a low-carb one. 

These kinds of studies also have drawbacks. It’s rare for the trials to last longer than a year or two, Hall says, and researchers have little way of knowing for sure how well people are sticking to their assigned diet. “We’re not lab rats, and so we go out and live our lives,” he says. “If I go to somebody’s birthday party, I might have a slice of birthday cake whether or not I’m on a low-carb diet.”

So it’s not easy to disentangle whether people regain weight because they aren’t following the diet as strictly as they did in the beginning, Hall says, or if there’s some other underlying physiological reason. Also puzzling is the small minority of participants in diet studies who actually do manage to lose weight and keep it off. 

“We don’t know whether or not [a diet] was effective for that person because of some sort of biological reason, or whether or not it was more about the social supports and being in the right place and time in their life to make a sustained change,” Hall says. Another conundrum, he says, is whether these people would have been equally successful if they’d been assigned to a different diet.

Weight is complicated

What is clear, however, is that dieting isn’t just a matter of willpower, says A. Janet Tomiyama, director of the UCLA Dieting, Stress, and Health Laboratory.

“Something society doesn’t quite grasp yet is that weight is really, really hard to control,” she says. “When somebody gains weight or their diet fails, they blame themselves rather than the thousands of forces that are conspiring to keep that weight on and to make you gain more weight.”

A big part of the explanation lies in genetics; some people’s bodies store fat more readily than others, says Lofton. But there are plenty of other variables, including their activity level, how much sleep a person is getting, and what medications they’re taking. 

There’s evidence, for example, that a poor night’s sleep disrupts levels of hunger-controlling hormones, leading to an increased appetite. And people who regularly work night shifts tend to gain more weight over time than daytime workers. “That can be really confusing for the body and people’s eating habits, if they’re working the overnight shift and sleeping poorly,” says Linda Antinoro, a registered dietician in the Nutrition and Wellness Service at Brigham and Women’s Hospital in Boston.

A person’s job is just one part of their food environment—the context in which people make decisions about what to eat. For many Americans, what’s most readily available are so-called ultra-processed foods. Researchers are still trying to figure out whether the additives and preservatives in these foods can affect our metabolisms, Lofton says.  

[Related: How to eat sustainably without sacrificing your favorite foods]

Hall and his colleagues are investigating the role that a person’s food environment plays in weight gain. In one experiment, participants who were offered ultra-processed foods such as Honey Nut Cheerios and margarine wound up eating about 500 calories per day more than people given whole foods such as apple slices and olive oil. This effect is particularly striking since the meals in each diet had the same amounts of fat, sugar, and other nutrients. 

However, Hall acknowledges, cutting out ultra-processed food isn’t exactly simple; not everybody has the time, money, or easy access to fresh produce. “One of the reasons why ultra-processed foods are so popular is because they’re so darn convenient,” he says. “They are typically engineered to be quite tasty, they have a long shelf life, they don’t require a lot of skill or equipment to prepare, and they don’t take very much time to prepare.”

Why your body resists weight loss

On top of all of this, dieting itself triggers changes in the body that fight against weight loss. When a person slashes their calorie intake, Tomiyama says, their body reacts as if it were enduring a famine. “Your body evolutionarily thinks, ‘Oh no, we’re in starvation mode…we have to get really efficient,’” she says. In other words: the dieter’s metabolism slows down, and they wind up burning fewer calories.

Hall and his colleagues have seen this firsthand with people who participated in the extreme weight-loss competition “The Biggest Loser.” While competing, the participants shed more than 120 pounds on average. Six years later, though, most had regained a fair amount of weight. Yet their resting metabolisms still remained sluggish. The people who’d kept the most weight off were those who’d increased their activity level by the greatest amount. In a surprising twist, though, they were also the participants whose metabolisms had slowed the most.

“It seems like in those folks, the lifestyle interventions that they had continued in order to keep the weight off were still being met by this continual resistance by the body,” Hall says.

[Related: What are calories?]

Dieting also causes a person’s appetite to increase. “You start to notice food more, and so it’s not just your body but also your brain that’s working toward getting you to eat more whenever you start depriving your body of calories,” Tomiyama says.

She and her team have also seen that dieting is just plain stressful. People who’d reduced their calorie intake had higher levels of the stress hormone cortisol than those who hadn’t. Being stressed only makes it harder to lose weight. “One of cortisol’s jobs is to signal your body to deposit energy as fat, especially in the belly region,” Tomiyama says. 

And fat-shaming just compounds the problem, she adds. “When you’re treated unfairly or experience discrimination because of your size, that’s ironically triggering these processes in your body that make you gain even more weight that’s going to put you at even more risk for fat-shaming and weight stigma,” she says. 

What you should do instead of dieting

Realistically speaking, Antinoro says, fad diets are rarely sustainable over the long haul. “We ask the question, ‘Is that doable for your lifestyle? Could you see yourself never having carbs for the rest of your foreseeable life?’” she says.

Still, it’s not surprising that diets remain entrenched in American culture. 

“Socially there’s pressure to lose weight, physicians are pushing you to lose weight, and so it makes sense why people would want to do something about it,” Tomiyama says. “Everybody’s screaming at them to.” 

Layered on top of this are the compliments dieters tend to receive as soon as they begin to lose weight. And with almost any fad diet, Hall says, there will be compelling success stories. For many people, though, dieting is ultimately a discouraging experience. 

“I do want people to get away from that term of ‘diet,’” Antinoro says. “It implies you’re on or off, you’re good or bad, it’s black or white.” Instead, she suggests, think about health-oriented steps you can take in your daily life—even if they never change your pant size. 

“It doesn’t always change the on-the-scale weight, but you’re seeing other markers that change in a healthy way,” Antinoro says. “Maybe it’s, ‘My blood pressure is better and I don’t need these three medicines,’ or, ‘I can play with my kids more and I’m not tired and am sleeping better.’”

Tomiyama recommends focusing your efforts on four key areas: stress management, getting good sleep, moving more, and adding more unprocessed foods such as fresh fruits and vegetables to your diet when possible. 

“You’ll notice I’m not talking about taking away the Snickers; I’m saying get more kale and strawberries,” Tomiyama says. “As soon as you start thinking of it as depriving yourself, that’s going to set off the stress processes.”

Start small, she advises. Pick an action you can do in the space of 10 minutes, whether that means going for a quick walk, eating an apple, stretching before bed, or reading a chapter of a book to destress. “Let’s just forget about the number on the scale,” Tomiyama says. “These are things that are going to help your health regardless of the number on the scale.”

One approach that’s gaining momentum is called Health at Every Size, which is aimed at building healthy eating and exercise habits and cultivating respect for people of all weights. More studies will be needed to assess the effectiveness of HAES for different groups. However, Tomiyama says, emerging research indicates that these interventions can be beneficial for health even if—or perhaps exactly because—the focus isn’t placed on weight loss.

Changing one’s relationship with food isn’t always easy, and some aspects of what and how we eat or move or sleep aren’t entirely within our control. The COVID-19 pandemic has only underscored this reality.

“All the indicators show that our diets have become worse during the pandemic, people are engaging in less physical activity, stress levels are through the roof,” Tomiyama says. “This is such a tough time for so many people, and it’s going to be all the more important to have compassion for yourself and not get into this blame-and-shame cycle that can result from dieting.”