The Dieting Myth That Just Won’t Die

Back in the early 1970s, psychologists at Northwestern University performed an experiment that, on the surface, looked like a child’s fantasy. The researchers gathered 45 college women and asked some of them to drink a milkshake—or two. Then they placed three pints of ice cream in front of each woman and asked her to taste each one. Afterward, they told each participant to “help herself to any of the remaining ice cream, as she wished,” the researchers wrote in the Journal of Personality. Finally—and this was key—each woman completed a survey meant to measure how much she dieted or “restrained” her eating, outside of the treats she had just consumed.

The findings were dramatic. On average, the women who said they didn’t diet or have weight concerns ate less ice cream if they drank at least one milkshake. The first sweet treat satiated their hunger. But for the women who dieted and felt worried about their weight, the milkshake appeared to unleash a hidden hunger. On average, they ate 66 percent more ice cream after the milkshake than they did without it.

From these data, the researchers devised a bold new theory: Dieting and weight concerns make people overeat and gain weight. Dieting remains pervasive in American culture, but the milkshake study, and similar ones that followed, nonetheless reshaped many Americans’ views of dieting and obesity. Experts concluded that all types of eating disorders—including anorexia, binge eating, and bulimia—can be brought on by intentionally trying to reduce the number of calories that you eat. Some scientists believe that dietary restraint causes obesity too.

This line of research inspired treatments for eating disorders, helped launch an anti-diet movement, fueled the trend of so-called intuitive eating, and shifted how many parents raised their kids to think about food. But more recent evidence suggests that attempting to restrict one’s food intake typically doesn’t have such dire consequences after all.

The notion that trying to diet causes eating disorders and obesity makes some sense. “There’s the idea that if you’re finding yourself thinking about food, trying to restrict what you eat or trying not to overeat, then you’re developing an eating-disorder mentality,” Michael Lowe, a psychologist at Drexel University, told me. The theory is also inherently appealing, in that most people don’t like avoiding tasty food; they can easily believe that doing so would be harmful. No wonder, then, that the idea spread far among clinicians and everyday Americans. Social media supercharged the theory, enough that many people now believe that placing any limits on your diet could be dangerous or harmful, Ashley Gearhardt, a psychologist at the University of Michigan, told me. Many parents share the belief that letting kids follow their own appetites will create healthy attitudes toward food; taken to its logical extreme, that way of thinking means that “in a lot of circles now, if you don’t let your kids have unlimited access to ultra-processed foods, it’s a bad thing,” Katherine Balantekin, a registered dietitian at the University at Buffalo, told me.

Such ideas spread even as researchers were uncovering major flaws in early studies on the link between dietary restrictions and eating disorders. Those experiments didn’t use a consistent definition of dietary restraint, and never tested whether it actually caused eating disorders or overeating; they could say only that those behaviors occurred together. Plus, many studies lumped together several types of eating disorders, or didn’t separate participants with obesity from those with low body weights.

[From the May 2023 issue: Nutrition science’s most preposterous result]

Scientists, including the ones who ran the 1975 milkshake study, also relied on self-reports or surveys to quantify how much a participant dieted, assuming that people who said that they greatly restricted their consumption really did take in fewer calories. But decades later, when scientists gave the same surveys to new participants and measured their calorie intake, they found that the surveys simply didn’t correlate with calorie restriction, Eric Stice, a psychologist at Stanford who led some of these measurement studies, told me. People whom such surveys would label “high dieters” may not have been dieting at all, Stice found. In one of his studies, a so-called high dieter ate, on average, 23 calories fewer a day than a low dieter. “That’s like not eating four peanuts each day and saying you’re on a diet,” he said.

By the 2000s, scientists began to run randomized, controlled trials that could accurately test the model proposed back in the ’70s. In one series of studies, people were prescribed personalized diets aimed at reducing calorie consumption, and taught effective ways to adhere to their eating plans. After six months, those volunteers lost about 10 percent of their body weight, on average, compared with the 1 percent that the control group lost. And the increased dieting didn’t exacerbate participants’ eating-disorder symptoms. In fact, it decreased their binge eating, and they felt less concerned about their body size (perhaps, in part, because their body size decreased). In the past decade, psychologists at Yale School of Medicine have run similar randomized, controlled studies on people who had already been diagnosed with binge-eating disorder and obesity. And again, on average, calorie restriction reduced binge eating; participants’ eating-disorder symptoms worsened only occasionally, and no more so than in the control group. In at least one paper, eating-disorder symptoms improved far more among people in the restricted group than in the control group. In another, weight loss led to remission of binge-eating in nearly three-quarters of the participants.

The scientific consensus that has emerged after these and similar studies is much more nuanced than the one proffered 50 years ago—even though that one still has significant traction in American culture today. “Dietary restriction is not necessarily all good or all bad. But different degrees may be helpful or harming to different people,” Sydney Yurkow, a psychologist at Yale School of Medicine who contributed to the recent trials there, told me in an email. For example, she said, cutting back on food would never be recommended for people diagnosed with anorexia nervosa. And even for people without an eating disorder, “extreme restriction that often coincides with self-directed dieting is largely unhealthy and unhelpful,” Yurkow wrote.

[Read: We have no drugs to treat the deadliest eating disorder]

The new generation of experiments has also uncovered a surprising way to prevent future eating disorders in high-risk adolescents and young women: a small amount of effective dietary restraint. One 2021 meta-analysis found that teaching people about healthy eating habits—including how to curb a tendency to overeat—prevents the future development of eating disorders. Altogether, Stice said, the modern experiments suggest two likely pathways for developing eating disorders. The first involves a person who’s dissatisfied with their body and engages in extreme weight-loss behaviors to change it. “But there’s a whole other pathway that many people have been ignoring,” he said—one in which a person overeats or binges, gains weight, and then becomes dissatisfied with their body as a result.

From this perspective, the milkshake study looks quite different. Fifty years ago, psychologists concluded that dietary restraint caused the women to overeat the pints of ice cream. But the modern interpretation suggests that the reverse was likely true for many of the participants: An underlying tendency to overeat drove the women to try to diet.

In hindsight, the timing of the milkshake study is almost prophetic. Around the late 1970s, the food environment in America began to change rapidly, Lowe, the Drexel psychologist, told me. “The availability of fast food, restaurants and high-sugar, high-fat foods began to explode,” he said. Food became much more difficult to resist, even when Americans weren’t hungry. “People suddenly had to restrain their eating much more in order to maintain their weight.” More people began to gain weight, and by the early ’80s, the country had entered the first stage of the current obesity epidemic. Today, the average American consumes about half their calories from ultra-processed foods. The precise bounds of the category generate debate among some scientists, but they generally agree that such foods are highly refined, manufactured in industrial factories, and calorically dense. Studies have found that diets high in these foods can coax people to consume hundreds of extra calories each day, and when people binge eat, they tend to do so with only ultra-processed foods.

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Recently, scientists and government leaders have begun warning Americans about the potential harms of eating too much ultra-processed food, including an elevated risk of diabetes, certain cancers, and depression. Health Secretary Robert F. Kennedy Jr. has declared war on added sugars, and as of this year, the national dietary guidelines advise against eating highly processed foods. In reality, the study of this kind of food is just getting started. But if Americans are ever to really understand just how such foods affect us, now is the time to abandon the misguided lessons of the milkshake study. Maybe then we can explore how we might develop a truly healthy relationship with the tantalizing food around us.