A new study using MRI imaging has found a meaningful connection between eating high amounts of ultra-processed foods and poorer muscle quality in adults who are at risk for knee osteoarthritis. The findings, published in the journal Radiology, add a new dimension to an already growing body of concern about how modern eating habits are affecting the body at a structural level, well beneath what the scale can measure.
Researchers analyzed MRI data from more than 600 participants and found that those who consumed more ultra-processed foods tended to have greater fat infiltration in their thigh muscles. The association held up even after accounting for body weight, and it became even more pronounced when researchers factored in abdominal fat distribution rather than overall body mass. That detail is significant because it suggests the relationship between diet quality and muscle composition goes beyond simply being overweight.
What fat infiltration in muscles actually means
Muscle fat infiltration is exactly what it sounds like: fat that accumulates within muscle tissue rather than beneath the skin. It is a marker of poorer muscle quality, and it is associated with reduced strength, diminished physical function, and a higher risk of mobility problems as people age. The condition is not visible from the outside, which is part of what makes it difficult to detect without imaging.
The study focused specifically on thigh muscles because of their central role in knee function and their relevance to osteoarthritis progression. Researchers graded fat infiltration across ten individual muscle groups in both legs using an established classification system that ranges from no visible fat to more than half of the muscle tissue being replaced by fat. Among the muscle groups studied, the flexors showed the highest levels of infiltration overall, while the adductors demonstrated the strongest association with ultra-processed food consumption.
Why ultra-processed foods may be to blame
Ultra-processed foods, the packaged, shelf-stable, heavily modified products that now make up a substantial portion of what many people eat every day, are typically high in salt, sugar, fat, and chemical additives. They tend to displace more nutritious options from the diet, leaving people low in the protein and key micronutrients that muscle tissue needs to stay healthy. Over time, that nutritional gap appears to take a physical toll.
The study population had an average age of 60 and was predominantly female. Ultra-processed foods accounted for roughly 41 percent of their total daily caloric intake, a figure that mirrors broader patterns in Western diets. Abdominal obesity was widespread in the group, affecting the large majority of women and nearly half of the men, which may help explain why abdominal fat distribution turned out to be a particularly relevant variable in the analysis.
What the findings do and do not tell us
It is important to note what this research does not claim. Because the study was cross-sectional, meaning it captured a single point in time rather than tracking participants over years, it cannot establish that ultra-processed foods directly cause fat infiltration in muscles. What it does show is a consistent and statistically meaningful association, one that held up across both sexes and across multiple methods of analysis. Sensitivity tests confirmed the results were not driven by total dietary fat intake alone.
The study also found that the association was stronger in participants with early signs of knee deterioration than in those with no detectable changes at all, suggesting that people already on a trajectory toward joint problems may be particularly vulnerable to the effects of poor diet quality on muscle health.
What this means going forward
Researchers are now calling for longer-term studies that can trace how diet quality influences muscle composition and joint health over time, and whether changing eating patterns can slow or reverse the process. For now, the findings offer a compelling argument for paying closer attention to what is on the plate, not just for the sake of weight management but for the sake of what the muscles underneath look like and how well they will hold up with age.
