Introduction
Ultra-processed foods are constantly under the microscope in nutrition circles – and for good reason. They’re cheap, convenient, hyper-palatable, and heavily marketed. But the question remains: do ultra-processed foods actually cause weight gain?
To explore this, researchers Hall et al. (2019) conducted one of the most tightly controlled studies to date on the topic. Their goal? To isolate the effect of food processing on how much people eat—and what happens to their body weight as a result.
This article breaks down what they found, what it means, and—most importantly—what coaches can take away to better support clients who are navigating the modern food environment.
What Are Ultra-Processed Foods?
Before diving into the study results, let’s define what exactly we mean by ultra-processed foods.
According to Monteiro et al. (2018), who developed the NOVA food classification system, ultra-processed foods are “formulations mostly of cheap industrial sources of dietary energy and nutrients plus additives, using a series of processes.”
In plain terms, these are foods that have been heavily modified from their original state and often contain ingredients not found in a home kitchen—think stabilizers, flavour enhancers, colourings, emulsifiers, and preservatives.
Examples include:
Packaged snacks (crisps, cookies, protein bars)
Sugary breakfast cereals
Soft drinks and energy drinks
Instant noodles and ready meals
Processed meats (hot dogs, chicken nuggets)
Mass-produced breads and buns
These foods are often energy-dense, low in fibre, highly palatable, and very easy to overconsume—making them particularly relevant when coaching clients around fat loss, satiety, and eating behaviours.
Inside the Study: Design and Methodology
The Hall et al. (2019) study used a randomised crossover design, often considered a gold standard in nutrition research. Here’s what that looked like:
20 adult participants (average BMI ≈ 27, overweight but weight-stable)
Each participant spent 28 consecutive days living in a metabolic ward, an ultra-controlled environment
The study was split into two 14-day phases:
One phase on a diet of unprocessed whole foods
One phase on a diet high in ultra-processed foods
Crucially, all meals were ad libitum—participants could eat as much or as little as they liked
Each diet was carefully matched for total calories, macronutrient ratio, sodium, sugar, and fibre per meal, as closely as possible.
The only significant difference? The degree of processing.
Participants ate three main meals per day (with optional snacks), each meal lasting 60 minutes. Because the same people experienced both diets, this crossover format meant that individual differences were eliminated – essentially, each person was their own control.
This allowed the researchers to isolate the impact of food processing on appetite, calorie intake, and body composition.
What Did the Researchers Find?
The results were both striking and simple: when participants were on the ultra-processed diet, they ate significantly more and gained weight as a result.
Key findings:
Participants consumed ~508 kcal more per day on the ultra-processed diet (± 106 kcal, p = 0.0001)
This excess came almost entirely from carbohydrates (+280 kcal/day) and fats (+230 kcal/day), despite protein intake remaining the same
Over the 14-day period, participants gained an average of 0.9 kg on the ultra-processed diet
Conversely, they lost 0.9 kg on the unprocessed diet, even though both diets were matched for macronutrient breakdown
Fat mass increased by 0.4 kg on the ultra-processed diet and decreased by 0.3 kg on the unprocessed one
These are net changes in just two weeks, under conditions where the only meaningful variable was the type of food – processed vs. unprocessed.
So what caused the difference in energy intake if hunger levels and food availability were similar?
The researchers looked into that too.
Interpreting the Findings: What Does It Really Mean?
On the surface, it might be tempting to say “processed food causes weight gain”, but the truth is more nuanced. The study doesn’t prove causation in a vacuum. What it does show is that ultra-processed foods significantly increase the likelihood of overeating, even when calories and macronutrients are matched.
So, what’s driving the extra intake?
1. Energy Density
Ultra-processed foods tend to be more energy-dense, meaning they pack more calories into smaller volumes. Participants consumed more calories without necessarily feeling fuller, simply because they were eating foods with a high calorie-per-gram ratio.
2. Fibre Content
Despite the researchers’ efforts to match fibre content, natural fibre from whole foods behaves differently in the gut than added fibre. It slows digestion, enhances fullness, and contributes to satiety hormones more effectively than isolated fibres added to processed products.
3. Eating Rate
One of the standout findings was that participants ate faster on the ultra-processed diet (measured in kcals/min). Fast eating gives the brain less time to register fullness, leading to passive overconsumption.
4. Palatability ≠ Overeating
Interestingly, the study found no significant differences between the two diets in terms of pleasantness, hunger, or fullness ratings. This challenges the assumption that ultra-processed foods drive overeating purely because they taste better. Instead, structural properties like energy density and texture may be more influential than flavour alone.
So while ultra-processed foods aren’t inherently “fattening,” they create the perfect storm for easy overeating, especially in an environment of abundance and low movement.
What Coaches Should Take Away
This study offers more than a headline – it delivers practical insight into how food structure, not just food content, influences behaviour. For coaches working with clients on weight management, here are the key lessons:
1. Ultra-processed foods aren’t evil, but they’re easy to overeat
They’re calorie-dense, often low in natural fibre, and eaten quickly. This makes them ideal candidates for unintentional overeating, especially in clients who struggle with appetite regulation or eat while distracted.
2. Portion size and eating environment matter
Encourage clients to slow down meals, avoid screens while eating, and become more mindful of how quickly and how much they eat -particularly when ultra-processed foods are involved.
3. Whole foods promote natural appetite regulation
Unprocessed meals tend to have higher food volume, better fibre integrity, and lower energy density. These factors help regulate hunger and fullness signals more effectively. That’s why a “whole-foods first” approach isn’t just a cliché – it’s a strategy grounded in physiology.
4. Balance over restriction
Demonising processed foods isn’t helpful. Instead, educate clients on why certain foods are easier to overeat, and how to enjoy them with structure and awareness—rather than guilt or rigid avoidance.
Ultimately, coaching isn’t about food rules. It’s about helping clients make informed, sustainable decisions that support their long-term goals.
Conclusion
The Hall et al. study doesn’t claim that ultra-processed foods are inherently fattening, but it does highlight their potential to drive passive overconsumption in a way that unprocessed foods don’t.
In a world where ultra-processed foods are everywhere – cheap, convenient, and designed for speed – it’s no surprise that managing weight is harder than ever. This study reinforces what many coaches already observe in practice: the structure and processing of food influence how much we eat, not just what we eat.
Key takeaways for coaches:
Ultra-processed foods lead to significantly higher energy intake—even when matched for macros
Factors like energy density, fibre quality, and eating speed play a major role in this
Clients benefit from a whole-foods-first approach, but flexibility is key—education beats restriction
Helping clients slow down, add volume with whole foods, and be mindful of portion size can make a big difference
Your job as a coach isn’t to remove processed foods—it’s to remove the confusion around them and help clients navigate a food environment that doesn’t make regulation easy.
