Why ultra-processed foods (UPFs) make us overeat, and what it means for your brain

Diets high in ultra-processed foods (UPFs) are consistently linked to overeating and long-term health risks. Scientific evidence is growing around how processed foods, beyond calories and nutrients, can shape appetite, metabolism, and even brain health. Here, Hussein Yassine, MD, director of USC’s Center for Personalized Brain Health, interprets the science and provides you with meaningful steps to take.

Evidence from a randomized controlled trial

A randomized controlled trial led by researcher Kevin D. Hall at the National Institutes of Health (NIH) directly tested the impact of UPFs. Twenty healthy adults were divided into two groups, following different diets for 14 days: one ultra-processed, the other unprocessed. All meals were designed to be matched for calories, energy density, macronutrients, sugar, sodium, and fiber. The only variable was the degree of processing. Participants could eat as much as they wanted during each phase.

The results were striking:

• On the ultra-processed diet:
  • Participants consumed 508 extra calories per day.
  • They gained about 2 pounds in just two weeks.
  • They ate faster—17 versus 11 calories per minute.
  • The excess intake came primarily from carbohydrates and fat, while protein intake stayed stable.
• On the unprocessed diet, participants lost 2 pounds on average.

This trial is one example within a larger body of evidence suggesting that processing itself—not just nutrient content—can influence appetite and weight regulation. When foods are nutritionally identical on paper but produce drastically different consumption patterns, it suggests that the physical structure and processing of food, not just its nutrient content, directly influences our eating behavior and weight regulation. The study provides the first correlative evidence that ultra-processed foods actively promote overconsumption through mechanisms we’re still working to understand. This isn’t about willpower or personal discipline. It’s about how these foods interact with our brain’s appetite control systems.

What makes a food “ultra-processed”?

Not all processing is harmful. Freezing vegetables or fermenting yogurt improves storage and digestibility. Ultra-processing, however, is a distinct category defined by the NOVA food classification system:

• Group 1: Unprocessed or minimally processed foods – Fruits, vegetables, eggs, milk, or meat that are cleaned, frozen, or fermented but remain close to their natural state
• Group 2: Processed culinary ingredients – Oils, butter, sugar, and salt extracted from whole foods for cooking
• Group 3: Processed foods – Simple combinations of Groups 1 and 2, such as canned fish, cheese, or fresh bread
• Group 4: Ultra-processed foods (UPFs) – Industrial formulations with ingredients rarely used in home

Research shows that UPFs disrupt satiety signals, encourage overeating, and are associated with higher risk of obesity, metabolic disorders, and dementia. Their effects go beyond calories, reflecting the way their engineered structure interacts with appetite regulation.

The “Plant-based = healthy” misconception

Plant-based doesn’t automatically mean healthy. Studies show that vegetarians and vegans often consume more UPF products than meat-eaters, including highly processed meat substitutes, protein bars, and packaged snacks.

A plant-based burger with 20 industrial ingredients can affect appetite regulation much like any other UPF. By contrast, whole, minimally processed foods—whether plant or animal based—are more nutrient-dense and metabolically supportive. Examples include grilled salmon, eggs, plain yogurt, beans, and fresh vegetables. The key isn’t whether food comes from plants or animals—it’s the degree of industrial processing it has undergone.

“It’s really hard for people to change their habits,” said Yassine. “Take it one step at a time.”

Even gradual dietary shifts away from ultra-processed products can help reset your natural hunger cues, reduce systemic inflammation, and support long-term metabolic and cognitive health.

Practical brain health strategies

Yassine recommends a step-by-step approach in reducing ultra-processed foods intake:

• Start by reducing sugary drinks: Replace soda or sugary juices with water or unsweetened (or lightly sweetened) iced tea.
• Swap processed red meats for cleaner proteins: Shift toward fish, chicken, lentils, beans, and nuts.
• Increase fiber-rich whole foods: Add more fruits, vegetables, legumes, and whole grains to support gut and brain health.
• Cook mostly from basic ingredients—simple one-pan meals, batch-cooked grains/beans, and frozen veggies keep it easy.
• Build plates around a whole-food trio like protein (eggs, fish, beans), fiber/starch (potatoes, oats, brown rice), and healthy fat (olive oil, nuts).
• When buying packaged foods, favor less than 5 ingredients and check labels for sodium, added sugars, and emulsifiers.