NIH trial · 20 people · 4 weeks

Same sugar, same fat, same fibre. One diet made them eat more.

People say: “A calorie is a calorie” We read the study.

What they did

Randomised controlled trial (RCT). 20 people. 28 days as inpatients: 2 weeks on each diet, no washout.

A trial. Chance decided who did what.

Who: 20 weight-stable adults (10 men, 10 women), mean age 31, mean BMI 27, living at the NIH Clinical Center metabolic ward (Bethesda, USA); aged 18-50, no diabetes, eating disorders or special diets

What they found
  • Calories eaten a day+508ultra-processed vs unprocessed
  • Weight in 2 weeks+0.9 kgunprocessed: −0.9 kg
  • Eating speedfaster+17 kcal per minute

Ultra-processed diet: +508 calories eaten per day vs unprocessed, with meals matched on the plate

The catch

They didn't even rate it tastier.

More to know, from our read of the paper:

  • Very small (n=20) and short (2 weeks per diet); weight changes were under 1 kg
  • Inpatient metabolic ward: free food, no choice of meals, hard to generalise to real life (stated by the authors)
  • No run-in or washout period; the authors partly address this by showing the gap persisted in the final week (459 kcal/day)
  • Energy intake on the ultra-processed diet fell over the 2 weeks (-25.5 kcal/day per day), so the gap might shrink over longer periods (raised in published comment 'Pitfalls of extrapolation from short studies')
  • Study was not designed to identify WHY people ate more; protein leverage, eating speed and energy density are proposed explanations
The verdict

Same plate. 500 more calories a day.

One thing to try
  1. Swap in unprocessed mealsOn unprocessed food, people ate less and lost weight.
  2. Slow downThey ate faster on ultra-processed food (a possible cause, not proven).
  3. Don't trust your hungerHunger and fullness felt the same on both diets.
In short

20 adults lived at the US National Institutes of Health for 4 weeks and ate as much as they liked: 2 weeks of ultra-processed food and 2 weeks of unprocessed food, in random order, with meals matched for calories offered, sugar, fat, salt and fibre. On the ultra-processed diet they ate about 508 more calories a day and gained 0.9 kg in 2 weeks; on the unprocessed diet they lost 0.9 kg. They did not rate the ultra-processed meals as tastier, but they ate faster. A small, short, tightly controlled trial of 20 people.

The paper

Hall et al. (2019). Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain: An Inpatient Randomized Controlled Trial of Ad Libitum Food Intake. Cell Metabolism.

Design: Randomised controlled trial (RCT) · randomized controlled crossover trial (inpatient, 2 weeks per diet)

DOI: 10.1016/j.cmet.2019.05.008

Goals:

Eat better
How we checked

We read the abstract and the full text of this paper and checked every number on this page against it (18 facts, checked on 2026-10-01). The study design is named above. Cohort and other observational studies are described as linked, not proven. Found an error? Email theis@onestudyaday.com.

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