This study investigated whether ultra-processed foods affect energy intake in 20 weight-stable adults. In a randomized crossover trial, subjects were admitted to a clinical center and provided either ultra-processed or unprocessed diets for two weeks each. Although the diets were matched for calories, sugar, fat, fiber, and macronutrients, participants consumed approximately 500 kcal/day more on the ultra-processed diet. This increased intake led to significant weight gain, whereas the unprocessed diet led to weight loss. The results suggest that limiting ultra-processed foods may be an effective strategy for obesity prevention and treatment.