Two weeks of a 95% ultra-processed diet did not break the body's fullness signals

A randomised crossover trial analysed in the European Journal of Nutrition on 24 September 2026 tested a popular idea: that ultra-processed foods make people overeat because they blunt the body's fullness signals. 32 healthy young adults (average age 27, average BMI 23.6) who usually ate little ultra-processed food completed two 14-day diets in which about 95% of energy came from ultra-processed foods, with a 14-day break in between. The two diets differed in texture, which changes how fast food is eaten.

Before and after each diet, participants ate a fixed-calorie meal, and the researchers tracked appetite ratings and four gut hormones for three hours: GLP-1, ghrelin, pancreatic polypeptide and peptide YY. The hormone responses did not differ after two weeks of ultra-processed eating. Hunger and fullness ratings, added up over the test, shifted slightly but significantly towards more hunger and less fullness, yet no single time point differed, and morning appetite stayed steady through each diet.

The authors conclude that their results do not support the idea that the fullness response is disrupted after two weeks on such a diet. That does not make ultra-processed food harmless. The trial was short, in lean young adults, and it tested one mechanism. Other explanations, such as eating speed, energy density and taste, remain open, and large observational studies continue to link these foods with poorer health.