Researchers used MRI scans to measure fat inside the thigh muscles of 615 adults at elevated risk for knee osteoarthritis. People who reported eating more ultra-processed foods tended to have greater intramuscular thigh fat—hidden fatty deposits that are not visible from the outside and may affect how the knee joint functions.
Importantly, the association held even after accounting for calorie intake and physical activity. That suggests the extra muscle fat was not explained simply by eating more energy or moving less, pointing to a possible independent relationship between food processing and fat accumulation in muscle tissue.
Because this was an observational study, it cannot prove that ultra-processed foods cause fat to build up in thigh muscles. Still, the findings add to growing concern about diets heavy in industrially formulated foods, especially for people already at risk of knee osteoarthritis.