Groundbreaking study reveals that ultra-processed foods make us overeat and gain weight

December 16, 2024

Dr. Kevin Hall conducted the first randomized controlled trial to find causation between ultra-processed foods (UPFs) and overeating and weight gain.

You’ve probably heard about ultra-processed foods (UPFs)—the boxed, bagged, or bottled convenience meals that make up a huge chunk of our diets. UPFs account for nearly 60% of the average American adult’s calorie intake and a whopping 70% for kids. These numbers are even higher in lower-income communities and among underrepresented minorities, where fresh, whole foods are often less accessible.

These cheap, convenient, and long-lasting products have long been linked to health issues like obesity, type 2 diabetes, cancer, and a higher risk of death. Until recently, though, most of this research was based on large-scale observational studies, which couldn’t definitively answer a crucial question: Are UPFs truly the problem, or are there other factors at play? Enter Dr. Kevin Hall and his team at the National Institutes of Health (NIH), who conducted the first randomized controlled trial (RCT) on UPFs, revealing a critical insight: people with ultra-processed diets consumed more calories and gained more weight, even when the nutritional content was matched with unprocessed diets.

This pioneering study, published in Cell Metabolism, doesn’t just add to the conversation – it changes it. The findings suggest there’s something inherent in UPFs that drives overeating, independent of nutrients like sugar, sodium, and fiber. While the mechanism behind why UPFs have this effect is still a mystery, Hall’s study underscores that UPFs actively drive us to consume more than we need. As more research unfolds, one thing is clear: tackling the UPF problem isn’t just about promoting individual health – it’s about reshaping the food system itself.

How did the study work?

Over the course of a month, 20 healthy adults – 10 men and 10 women – were randomly assigned to two distinct diets for two weeks each. One group started with an ultra-processed diet, while the other began with minimally processed meals. After two weeks, the groups switched. The unique setup allowed researchers to directly compare the effects of UPFs versus unprocessed foods on the same individuals.

The diets were carefully matched for calories, sugar, fat, fiber, fat, carbohydrates and macronutrients. An example of an ultra-processed breakfast would include a cheese, egg, and turkey bacon sandwich using English muffins with tater tots, ketchup, and orange juice on the side, while its unprocessed counterpart consisted of oatmeal with blueberries, almonds, and 2% milk. Despite their nutritional similarities, the participants were free to eat as much or as little as they wanted. Both diets consisted of 3 meals and snacks per day, containing almost 4,000 calories.

To ensure unbiased results, participants wore loose clothing to mask any visible weight changes, and every bite they ate was meticulously tracked. This level of control made Hall’s study the first of its kind, offering clear evidence that UPFs can lead to overeating and weight gain, independent of their nutritional content.

What exactly were the study’s findings and results?

The results of the study were eye-opening: participants consistently ate more and gained weight on the ultra-processed diet. On average, those eating UPFs consumed 500 extra calories per day, leading to a weight gain of about two pounds in just two weeks. Meanwhile, the same participants lost an equivalent amount of weight when they switched to the minimally processed diet.

Interestingly, the increased calorie intake wasn’t driven by taste or satisfaction – participants rated both diets as equally enjoyable and filling. Instead, the study pointed to a subtle, subconscious shift in behavior triggered by UPFs. People on the UPF diet ate faster, likely because these foods are softer and easier to chew, which may have delayed satiety signals and encouraged overeating. These findings suggest that there’s something inherent to UPFs, beyond their nutrient content, that promotes overconsumption and weight gain.

What were the study’s shortcomings?

While groundbreaking, Dr. Hall’s study has its limitations, primarily due to its small sample size and short duration. With only 20 participants over a single month, the study provides a snapshot rather than a full picture of UPF consumption’s long-term impact. This brevity stems partly from the intensive nature of controlled feeding trials, which require participants to live in a confined setting under constant monitoring—a setup that’s both resource-heavy and challenging to sustain for ethical and practical reasons.

Critics argue that the results might not hold over a longer period. Some experts believe that weight gain observed on the ultra-processed diet could stabilize over time as the body adjusts. Additionally, while the study revealed clear differences in calorie intake between the diets, those differences slightly diminished as the trial progressed. This raises questions about whether the effects of UPFs are as pronounced in the long term.

The study also has limited real-world applicability. In the trial, participants didn’t face the usual trade-offs of cost and convenience, as all meals were provided free of charge and prepared for them. Since UPFs are typically cheaper and more accessible than unprocessed foods, the study didn’t account for how these factors influence people’s choices in everyday life. The controlled environment of the NIH Clinical Center makes it difficult to generalize findings to free-living conditions, where people are exposed to a wider range of food options and eating habits.

What’s next?

While this study is a significant step forward towards better understanding UPFs, it’s just the beginning. Hall emphasizes that the next challenge is pinpointing exactly what it is about UPFs that drives people to eat more. Is it the softer textures? The higher calorie density per bite? Or maybe the speed at which these foods can be consumed? These are the questions Hall’s team is now focused on answering.

To dig deeper, Hall is already running a new study with a slightly larger group and is set to conclude in 2025. This time, he’s tweaking the ultra-processed diet, adding dishes like stews and gumbos to test if slowing down the eating process can reduce calorie intake. If we can figure out the specific triggers within UPFs, it might be possible to re-engineer these foods to be healthier while still retaining their convenience and affordability.

But Hall is quick to point out that individual action isn’t enough. It’s not just about telling people to “eat healthier” – the reality is that UPFs are cheaper, more accessible, and heavily marketed. Systemic changes are needed, including policies that make healthier foods more affordable and available to all. Tackling the dominance of UPFs in our diets will require collaboration across society – from governments to the food industry and beyond.

As Hall’s work continues, so does the conversation. With more studies in the pipeline, the hope is that we’ll soon have clearer answers and better tools to combat overeating and the health issues linked to ultra-processed diets. For now, the takeaway is clear: reducing our intake of UPFs, where possible, is a step toward better health.

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