Industrial formulations made mostly from food extracts and additives. Strongly associated with worse health outcomes — but 'processed' isn't one thing.
The nuance
Frozen vegetables, canned beans and yogurt are processed and nutritious. Classification is debated and the mechanisms are still being studied.
Evidence & sources
In a tightly controlled inpatient trial, people offered ultra-processed diets ate about 500 kcal/day more than on unprocessed diets matched for presented nutrients. Evidence: limited. Known: Randomized crossover trial, 20 adults. Unknown: Small, short; mechanisms unclear. Source: Hall et al., Cell Metabolism, Ultra-Processed Diets Cause Excess Calorie Intake and Weight Gain (2019).
Greater ultra-processed food exposure was associated with higher risk of adverse health outcomes across many meta-analyses. Evidence: moderate. Known: Umbrella review. Unknown: Mostly observational; confounding possible. Source: Lane et al., BMJ, Ultra-processed food exposure and adverse health outcomes: umbrella review (2024).