Clinically reviewed by Associate Professor Christopher O’Donnell, Nurse Practitioner, FACN · Founder and Clinical Director · AHPRA NMW0001453715 · Last reviewed
In a tightly controlled trial where diets were matched for calories, sugar, fat, fibre and salt, participants ate around 500 more calories per day on the ultra processed diet and gained weight, while losing weight on the unprocessed diet. Processing itself appears to affect intake independently of nutrients.
What ultra processed actually means
The NOVA classification separates foods into four groups by degree of processing. Ultra processed foods are industrial formulations typically containing ingredients not used in home cooking, such as protein isolates, modified starches, emulsifiers, colours and flavour enhancers.
The category is broader than people expect. It includes packaged breads, breakfast cereals, flavoured yoghurts, most snack bars and many ready meals, not only obvious confectionery and soft drinks.
The trial that changed the conversation
Most nutrition evidence is observational, which cannot separate the food from the lifestyle around it. One inpatient study addressed that directly.
Participants lived in a research facility and spent two weeks on an ultra processed diet and two weeks on an unprocessed diet, in randomised order. The diets were matched for total calories presented, sugar, fat, fibre, sodium and macronutrients. Participants ate as much or as little as they wanted.
On the ultra processed diet they ate around 500 more calories per day and gained roughly a kilogram. On the unprocessed diet they lost about the same. The difference came from the food, not from the surrounding circumstances.
Why processing changes how much you eat
Several mechanisms have been proposed and partly demonstrated.
- Eating rate. Ultra processed foods are softer and require less chewing, so they are eaten faster, often before satiety signals arrive
- Energy density. More calories per gram means more energy consumed before stomach stretch registers
- Lower protein density. If the body seeks a protein target, lower protein foods may drive greater total intake to reach it
- Reward engineering. Combinations of fat, sugar, salt and texture rarely found together in whole foods are highly palatable and easy to overconsume
- Reduced fibre and altered food structure, which affects both satiety and absorption rate
Keeping it proportionate
A useful caution: the category is very broad, and lumping wholemeal bread and tinned legumes together with confectionery is not helpful. Some ultra processed foods are nutritionally reasonable and practically valuable.
The evidence is strongest for reducing the most energy dense, lowest fibre, lowest protein items. Treating every packaged food as equivalent tends to produce a restrictive approach that is hard to sustain and can worsen the relationship with food.
Practical priorities
Rather than eliminating a category, target the changes with the largest effect.
- Reduce sugar sweetened drinks first, since liquid calories produce minimal satiety
- Prioritise protein at each meal from whole food sources
- Increase fibre from vegetables, legumes, fruit and wholegrains
- Slow eating rate, which partly offsets the softer texture effect
- Keep convenience options that are reasonable, such as tinned fish, frozen vegetables, plain yoghurt and tinned legumes
Frequently asked questions
Does ultra processed food cause weight gain?
A controlled inpatient trial found participants ate around 500 more calories per day on an ultra processed diet matched for nutrients, and gained weight, while losing weight on the unprocessed diet.
What counts as ultra processed food?
Industrial formulations containing ingredients not used in home cooking, such as protein isolates, modified starches, emulsifiers and flavour enhancers. The category includes many packaged breads and cereals.
Are all ultra processed foods equally bad?
No. The category is very broad. The evidence is strongest against the most energy dense, low fibre, low protein items rather than every packaged food.
Why do processed foods lead to eating more?
They are softer and eaten faster, more energy dense, often lower in protein and fibre, and engineered for palatability, all of which delay or weaken satiety signalling.
What is the single most useful change?
Reducing sugar sweetened drinks, because liquid calories produce very little satiety and are not compensated for at later meals.