This isn't a survey study — it is a randomised trial conducted inside a hospital with complete oversight of every bite. And its result explains a great deal of what happens to us.
The trial
At the US National Institutes of Health (NIH), participants were admitted as inpatients and randomly assigned to an ultra-processed or unprocessed diet for two weeks, then crossed over. Meals were matched for presented calories, energy density, macronutrients, sugar, sodium and fibre — and participants were told to eat as much or as little as they wished[1],[2].
The result
On the ultra-processed diet, participants consumed 508 extra calories a day (increases in carbohydrate and fat, not protein) and gained 0.9 kg over two weeks — while losing 0.9 kg on the unprocessed diet over the same period[1],[3].
ultra-processed food
two weeks
— with the same calories offered
Why does this matter so much?
Because it dismantles "a calorie is a calorie" at the behavioural level: the type of food changes how much you eat, not just its calorie content. Ultra-processed food is eaten faster, satiates less, and is easier to overconsume.
What counts as "ultra-processed" in our market?
Crisps, biscuits and packaged cakes, sweetened drinks, ready-to-heat meals, sausages and processed meats, and sweetened breakfast cereals. It does not mean "everything in a bag" — milk, yoghurt, packaged dates and simple canned goods are not in this category.
Realistic steps
- Don't aim for perfection: replacing a third of your shopping with food close to its original form is an enormous change.
- The golden shopping rule: what spoils quickly is usually better for you.
- Cook once, eat twice: an extra portion from tonight's cooking covers tomorrow's meal and displaces the ready-made option.