Ultra-Processed Food
A landmark 2019 randomised controlled trial at the NIH — the first to test ultra-processed food in a controlled inpatient setting — found that people ate significantly more calories when given ultra-processed food, even when the meals were matched for calories, protein, fat, carbohydrates, and fibre.
Quick read · 4 min
- •Want to understand why certain foods make overeating easier
- •Are trying to improve your diet quality alongside a caloric deficit
- •Have heard about ultra-processed food and want to know what the science actually shows
- •In the first proper RCT on this, people given ultra-processed food ate 508 more calories per day — and gained 0.9 kg in just 2 weeks
- •The meals were matched for calories, protein, fat, carbs, and fibre — so the food format itself drove the difference
- •Ultra-processed food appears to bypass normal fullness signals, making it easier to overeat
- •This was a small trial (20 people, 4 weeks) — but it is the strongest causal evidence on the topic to date
Based on clinical trials · No rankings · Every claim linked to source
Last reviewed: March 2026
The key trial — Hall et al. 2019 (NIH)
Participants were admitted to a research ward and given access to either an ultra-processed food diet or an unprocessed food diet for two weeks each (crossover design). Meals were carefully matched to present equal calories, macronutrients, sugar, sodium, and fibre. Participants ate ad libitum (as much as they wanted).
Weight changes correlated strongly with energy intake (r=0.8, p<0.0001), confirming that the extra calories — not some other factor — drove the weight difference.
Why ultra-processed food may cause overconsumption
The Hall trial did not definitively identify the mechanism, but several factors are thought to contribute:
- Faster eating rateUPF is typically softer and less chewy — participants in the trial ate significantly faster on the UPF diet, spending less time eating per calorie consumed. Faster eating reduces the gut's time to signal satiety.
- Reduced satiety signallingWhole foods trigger stronger release of GLP-1, PYY, and other satiety hormones. Highly processed textures and compositions may blunt these responses.
- Higher palatabilityUPF is engineered for palatability — combinations of fat, salt, and sugar that stimulate appetite and reward systems, encouraging continued eating past satiety.
- Lower satiation per calorieUPF often has lower water content and fibre density per calorie — providing less physical bulk to trigger stretch receptors in the stomach.
What counts as ultra-processed food?
The NOVA classification system, used in most research, defines UPF as industrial formulations containing ingredients rarely found in domestic kitchens: emulsifiers, stabilisers, flavour enhancers, artificial colours, preservatives, and hydrolysed proteins.
- • Packaged ready meals
- • Reconstituted meat products (nuggets, sausages)
- • Mass-produced bread and pastries
- • Soft drinks and flavoured drinks
- • Breakfast cereals with additives
- • Instant noodles and soups
- • Flavoured crisps and snacks
- • Fresh and frozen fruit and vegetables
- • Eggs
- • Meat, fish (plain)
- • Legumes (dried or canned, no additives)
- • Plain yogurt, milk, cheese
- • Rice, oats, pasta (plain)
- • Nuts and seeds
Limitations of the evidence
The Hall 2019 trial had only 20 participants and lasted 4 weeks (2 weeks per diet). It is a landmark study precisely because it is the first RCT to show causality — but larger, longer trials are needed. Observational evidence from population studies consistently shows associations between UPF consumption and obesity, but this trial provides the most direct causal evidence to date.