The Milkshake Study Was Wrong. Now What?

For fifty years, a single milkshake convinced millions of people that trying to eat less would only make them eat more. What if the experiment was wrong from the start?

The Study That Rewired How America Thinks About Food

In 1975, psychologists at Northwestern University ran what seemed like a dream experiment: they recruited 45 college women, gave some of them milkshakes, then placed three pints of ice cream in front of each participant and told them to eat as much as they liked. The results looked striking. Women who said they didn't diet ate less ice cream after drinking a milkshake—they were simply full. But women who reported restricting their food intake did the opposite. After the milkshake, they ate 66 percent more ice cream than they did without it.

From this, the researchers built a sweeping theory: dieting and weight concerns cause people to overeat and gain weight. The idea spread fast. Clinicians began arguing that all forms of intentional calorie restriction—not just extreme fasting—could trigger eating disorders including anorexia, bulimia, and binge eating. Some scientists went further, claiming dietary restraint was itself a driver of obesity. The research inspired new treatments for eating disorders, fueled the intuitive eating movement, and changed how an entire generation of parents taught their children to think about food.

The theory had obvious appeal. Most people don't enjoy skipping dessert. Being told that the effort is not only unpleasant but actively harmful is, frankly, a relief to hear.

The Problem: The Science Didn't Hold Up

By the 2000s, cracks had appeared—and they were substantial. Early studies on dietary restraint used no consistent definition of what "dieting" actually meant. More critically, they could only show that restriction and overeating appeared together; they couldn't prove one caused the other. And they relied entirely on self-reported surveys to measure how much participants were actually restricting their food intake.

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That last flaw turned out to be fatal. Eric Stice, a psychologist at Stanford University, gave those same surveys to new participants while simultaneously measuring their actual calorie intake. The two numbers barely correlated. People the surveys classified as "high dieters" consumed, on average, just 23 fewer calories per day than so-called low dieters. As Stice put it: "That's like not eating four peanuts each day and saying you're on a diet."

When randomized controlled trials—the gold standard of medical research—finally tested the original theory, the results contradicted it. Participants placed on structured, calorie-restricted diets lost an average of 10 percent of their body weight over six months, compared to just 1 percent in control groups. More importantly, their binge-eating symptoms didn't worsen. They improved. Researchers at Yale School of Medicine ran similar trials on people already diagnosed with binge-eating disorder and obesity; again, calorie restriction reduced binge eating on average, and in at least one study, nearly three-quarters of participants experienced remission of their binge-eating disorder after losing weight.

The scientific picture that has emerged is more nuanced than the one offered in 1975. Sydney Yurkow, a psychologist at Yale who contributed to the recent trials, put it plainly: "Dietary restriction is not necessarily all good or all bad. But different degrees may be helpful or harming to different people." Cutting calories would never be recommended for someone with anorexia nervosa. And extreme self-directed restriction remains genuinely problematic. But moderate, well-supported calorie reduction? The evidence suggests it's far less dangerous than the milkshake study implied—and may, in many cases, be protective.

Why the Wrong Idea Lasted So Long

This is where the story gets culturally interesting. The milkshake theory didn't just survive its scientific challenges—it thrived. Ashley Gearhardt, a psychologist at the University of Michigan, points to social media as an amplifier. The intuitive eating movement, body positivity, and anti-diet content spread rapidly online, hardening a fringe scientific claim into something that felt like common sense. The belief reached into parenting culture too. Katherine Balantekin, a registered dietitian at the University at Buffalo, noted that in many circles today, not giving children unlimited access to ultra-processed foods is treated as a parenting failure.

The timing of the original experiment, in retrospect, was almost ironic. Around the late 1970s—just as the milkshake study was reshaping clinical thinking—America's food environment began to transform. Fast food proliferated. High-sugar, high-fat products flooded supermarket shelves. The first wave of the modern obesity epidemic arrived. Today, the average American gets roughly half their calories from ultra-processed foods, which research suggests can push people to consume hundreds of extra calories daily without feeling proportionally satisfied.

Michael Lowe, a psychologist at Drexel University, argues this context reframes the milkshake study entirely. In a food environment engineered to override satiety signals, more people found themselves fighting urges to overeat—and turned to dieting as a response. The original researchers saw people who dieted and binged, and concluded that dieting caused the binging. The more plausible reading, Lowe and others now suggest, is the reverse: a pre-existing tendency to overeat drove people toward dietary restraint in the first place. The cause and effect were backwards.

This matters beyond academic debate. Robert F. Kennedy Jr., now U.S. Health Secretary, has declared a public campaign against added sugars. The latest national dietary guidelines explicitly advise against highly processed foods. Public health messaging is, slowly, shifting its focus from how people eat to what they eat. If that shift is to be grounded in evidence rather than cultural comfort, the lessons of the milkshake study—still widely cited, still shaping treatment protocols, still shared on social media as established fact—may need to be revisited more openly than they have been.