Chris van Tulleken, a doctor, researcher, and father of two, did something most of us would consider absurd: he volunteered to eat like millions already do—on a diet where 80% of his calories came from ultra-processed food (UPF). The month-long experiment was simple in its design yet devastating in its implications. And, for van Tulleken, the effects hit close to home.

UPFs are everywhere. They’re the colorful cereals that snap, crackle, and pop; the frozen pizzas that promise dinner in 10 minutes flat; and the snack bars we toss in our bags for a quick fix. With enticing labels touting “low-fat,” “high-protein,” or “100% natural,” they dominate modern diets. But beneath the shiny packaging lies a cocktail of modified starches, refined oils, emulsifiers, gums, and flavorings—ingredients that, as van Tulleken’s work illustrates, are changing our bodies and brains in ways we’re only beginning to understand.

The Setup: Eating Like One in Five

Van Tulleken’s experiment was designed to mirror reality for many in the UK and US, where UPF makes up 60% of the average diet and, for one in five people, more than 80%. For a month, he committed to eating like the latter group, while a team of researchers tracked his weight, mood, appetite, and even brain activity. The rules were straightforward: he wouldn’t force-feed himself or overindulge. He’d simply eat whenever he was hungry and consume what was available, just like any regular consumer.

First, though, came the control phase: a month of cutting out UPFs entirely. Van Tulleken was surprised to find how pervasive they were in his usual diet. Even foods he thought of as innocuous—like sandwich bread or mayonnaise—were ruled out because of additives and emulsifiers. What was left? Simple meals of sourdough, cheese, butter, vegetables, and meats. During this phase, he lost weight, felt more energized, and noticed a surprising sense of well-being. But as the experiment’s second phase loomed, his cravings for processed foods—microwave meals, snack bars, fast food—intensified.

A Month of Ultra-Processed Living

When the UPF diet began, van Tulleken eagerly reached for the foods he’d spent weeks avoiding. His breakfast kicked off with a nostalgic bowl of Coco Pops. For his young daughter Lyra, the cereal’s cartoon mascot and sugary allure made it irresistible. Watching her wide-eyed excitement turn to a trance-like consumption, van Tulleken noticed something unsettling: the cereal’s allure didn’t diminish. Even as the milk turned to brown sludge, his daughter, like him, kept eating, chasing the elusive joy of the first bite.

This pattern—a rollercoaster of intense pleasure followed by dissatisfaction—was a recurring theme throughout the month. UPFs, engineered to be hyper-palatable, offered fleeting bursts of sensory delight but left van Tulleken hungrier, moodier, and constantly craving more. It was as if the food bypassed the natural stop signals in his brain.

The Science of Cravings and Consequences

Researchers at University College London (UCL) tracked van Tulleken’s biological and psychological responses. The data painted a troubling picture. After just four weeks, his weight jumped by nearly 14 pounds, and his body fat increased by 3%. His hunger hormones were out of sync, pushing him to overeat even when he wasn’t physically hungry.

Perhaps most startling, MRI scans revealed changes in his brain. Key areas associated with reward and impulse control showed heightened activity, mimicking patterns seen in addiction. UPFs, the data suggested, weren’t just tricking his taste buds; they were reprogramming his brain to want more, setting off a vicious cycle of dependency.

It’s Not Just About Weight

Though weight gain was the most visible effect, van Tulleken emphasizes that UPFs’ dangers extend beyond the scale. Research has linked UPF consumption to increased risks of cancer, metabolic diseases, and mental health issues like anxiety and depression. Even for those who maintain a healthy weight, the additives, refined oils, and synthetic components in UPFs wreak havoc on gut health, inflammatory responses, and more.

Moreover, UPFs’ affordability and convenience often come at a societal cost. For families on tight budgets, they’re an easy choice. But as van Tulleken points out, this “cheap” food masks the true price: healthcare systems strained by diet-related illnesses, traditional food cultures eroded by global corporations, and environmental devastation driven by industrialized food production.

A Crisis We Can’t Eat Our Way Out Of

For van Tulleken, the experiment wasn’t about moralizing or shaming people for their choices. As he’s quick to note, UPFs are engineered to be overconsumed. Corporations pour billions into marketing campaigns, deceptive health claims, and addictively perfect flavor profiles. Meanwhile, the broader food environment—from economic inequality to aggressive advertising—makes resisting these foods nearly impossible.

“It’s not about willpower,” van Tulleken writes. “The system is rigged.”

Lessons from the Experiment

By the end of the month, van Tulleken’s disgust for UPFs had reached new heights. He likens the experience to Allen Carr’s approach to quitting smoking: the more you learn about the manipulation behind the product, the easier it becomes to say no. Still, he acknowledges that avoiding UPFs entirely is unrealistic for most people. Instead, he advocates for systemic changes—better labeling, stricter regulations, and more equitable access to whole, minimally processed foods.

For those of us reading his story, the takeaway is clear: the next time you reach for a packaged snack or microwave meal, pause. Read the label. Consider what’s in it and why it’s there. As van Tulleken’s experiment shows, the foods we eat aren’t just shaping our waistlines—they’re shaping our minds, our health, and the world we leave behind.

Book: Ultra-Processed People