Even on a Recommended Diet, Ultra-Processed Foods Halve Weight Loss: 8-Week Crossover Trial
Of two diets that followed the same nutrition guidance, the minimally processed diet produced about twice the weight loss of the ultra-processed diet.
A randomized controlled trial found that even when people follow national dietary guidelines, weight change can differ depending on how heavily the food is processed. Researchers at University College London (UCL) in the United Kingdom compared two diets that both matched the UK dietary guideline, the Eatwell Guide: a minimally processed food (MPF) diet and an ultra-processed food (UPF) diet. The same people received each diet in turn.
Design and results
Participants were 55 adults in England with a body mass index (BMI) of 25 or higher who usually got more than half of their calories from ultra-processed foods. It was a 2×2 crossover design in which each diet was provided for 8 weeks, to be eaten as much as participants wanted, and 50 people were included in the analysis. Weight fell on both diets, but the rate of loss was 2.06% on the MPF diet and 1.05% on the UPF diet, and the difference of 1.01 percentage points was statistically significant. Mild gastrointestinal adverse events were common on both diets.
The researchers interpreted the results to mean that, in addition to existing guidelines that address only nutrient targets, guidance on how much food is processed is needed. However, the sample was small and the period of 8 weeks was short, so changes in long-term weight and cardiometabolic markers are unknown. Another limitation is that, given the nature of food, it was hard to keep participants blind to which diet they were on. The difference in weight loss itself is also only about 1 percentage point.
- 55 adults with overweight or higher, assigned to each diet for 8 weeks in a crossover design (50 analyzed)
- Weight loss 2.06% on MPF vs 1.05% on UPF; the 1.01-percentage-point difference was significant
- Raises the need for guidance on degree of processing in addition to nutrition guidelines
Limitations This was a small, short trial. Long-term health outcomes were not assessed, and the effect size is small, at about 1 percentage point.
Original Research
- Dicken SJ, et al. · University College London. Ultraprocessed or minimally processed diets following healthy dietary guidelines on weight and cardiometabolic health: a randomized, crossover trial. Nature Medicine. DOI: 10.1038/s41591-025-03842-0
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