Landmark UK study finds ultra-processed foods may undermine the benefits of healthy eating

Key Takeaways: 

• Ultra-processed foods (UPFs) significantly reduce weight loss effectiveness, even when nutritional content aligns with national healthy-eating guidelines.
• Participants lost more than twice as much weight on a minimally processed food (MPF) diet compared to a UPF-based diet, despite identical macronutrient targets.
• The findings underscore the urgent need for UK dietary policy reform, highlighting food processing as an independent factor influencing diet quality and public health.
  

Introduction: Ultra-Processed Foods Under Scrutiny

New research has revealed that diets high in ultra-processed foods (UPFs) may hinder weight loss efforts, even when they comply with national dietary recommendations. The study, published on 4 August in Nature Medicine, was conducted in the United Kingdom and led by Samuel Dicken, an obesity and behaviour researcher at University College London (UCL). According to the researchers, the study provides some of the clearest evidence to date that the way food is processed — not just its nutritional composition — plays a critical role in weight management and overall health.

Defining Ultra-Processed Foods

Ultra-processed foods are industrially manufactured products that combine food extracts with additives, preservatives, and other synthetic ingredients. These products are typically designed for cost efficiency, long shelf-life, and mass appeal, making them highly profitable and widely available. Examples include packaged snacks, soft drinks, instant meals, and many supermarket-ready items.

While their high salt, sugar, and fat content has long been known to contribute to poor health outcomes, this new study suggests that the processing itself — including textural changes, heating methods, and additives — may also play a significant role in how the body responds to these foods.

Study Design: Real-World Eating in a Controlled Trial

This eight-month randomised crossover trial involved 55 adult participants and was designed to reflect real-world conditions as closely as possible. Each participant completed two distinct 8-week diet phases: one based on minimally processed foods (MPFs) and the other on ultra-processed foods (UPFs). The two phases were separated by a 4-week washout period, during which participants returned to their usual diet.

Both diets were constructed in accordance with the UK government’s Eatwell Guide, which provides recommendations on the proportions of food groups and macronutrients (carbohydrates, fats, proteins). Participants received all meals, snacks, and beverages directly to their homes, allowing them full flexibility in terms of portion size and meal timing.

“The motivation behind this study was to address a gap in national dietary recommendations, and to do so in the participants’ everyday environment,” said lead author Samuel Dicken. “It will be the longest-ever trial of a UPF diet to date. It’s the first one to do it in the real world to try and apply all those behavioural influences.”

Findings: Same Nutrients, Very Different Results

The study found that while participants lost weight during both dietary phases, weight loss was significantly greater during the MPF phase. On average, participants lost 1.84 kilograms while consuming minimally processed foods — more than double the 0.88 kilograms lost on the UPF-based diet.

In addition to greater weight loss, the MPF phase led to measurable improvements in body composition, including:

• Reduction in total fat mass
• Decrease in body-fat percentage
• Lower visceral-fat rating
  

Such changes were not observed during the UPF phase.
Participants also reported fewer food cravings while on the MPF diet.

Why the Difference? The Role of Processing

Although the exact mechanism remains uncertain, several hypotheses were proposed by the researchers:

• Texture and structure: UPFs are often softer and easier to chew, which may lead to faster consumption and delayed feelings of fullness.
• Gut microbiome: Additives and heat treatments used in processing could negatively impact gut health and promote inflammation.
• Behavioural cues: Attractive packaging and misleading nutritional claims may encourage overconsumption.
  

“UPFs are engineered to be overconsumed,” noted Carlos Monteiro, an epidemiologist at the University of São Paulo, Brazil, who coined the term ultra-processed food. “These foods are more ‘addictive-like’, energy-dense and tasty, which can lead to higher intake when compared with MPFs.”

Public Health Implications: Beyond Nutrients

The findings challenge existing dietary policies that focus predominantly on macronutrients while paying little attention to how food is processed.

“This trial supports the importance of considering food processing as a distinct and policy-relevant dimension of diet quality,” said Tera Fazzino, a psychologist at the University of Kansas. “Policies that focus solely on reformulating UPFs to meet nutritional standards may overlook the behavioural and physiological effects of how those foods are engineered.”

Fazzino added, “This study strengthens the evidence base needed to inform policy and public-health messaging. By showing that processing matters even when nutritional quality is controlled, it challenges the current focus on nutrients alone and may help catalyse broader discussion and reform.”

A Socio-Economic Dimension

In the UK, over 50% of the average person’s daily energy intake comes from ultra-processed foods — a figure echoed across much of Europe and North America. Dicken warned that the ubiquity and affordability of UPFs pose a particular risk for individuals from low-income communities, where access to fresh, minimally processed foods is limited.

“Issues around diet and obesity related to these [UPFs] are very much related to socio-economic inequalities,” he said. “People might have unhealthy diets not because they aren’t trying to eat healthily, but because our food environment is just setting us up to fail.”

Conclusion: A Call to Re-Evaluate National Guidelines

This landmark trial provides compelling evidence that ultra-processed foods can substantially blunt the effects of otherwise healthy diets. It also highlights a pressing need to broaden the scope of national dietary guidelines to include food processing as a factor in health outcomes.

According to Dicken, “This isn’t just about calories or nutrients — it’s about the way our food is made, marketed, and made available to us. If we want meaningful change, we need policy that reflects that complexity.”

CCH Insight:

This is an excellent study and the results provide confirmation of what many of us have suspected for a long time; that these foods have health implications beyond just their nutrient content – the food has been altered in a way which has a wider impact on health. But the challenge is how to reverse the increasing levels of consumption of these foods, especially amongst deprived communities who are most vulnerable to obesity and associated chronic health issues. One option is taxation, in the hope this will not drive up prices, but will motivate the food industry to re-formulate foods (as they re-formulated sugary drinks) to reduce the degree of processing.