A new study suggests that diets high in ultra-processed foods may be doing more than expanding waistlines. Researchers now say these foods could be quietly changing muscle quality in ways that may worsen joint problems over time.
The study, published in the journal Radiology, found that people who consume more ultra-processed foods tend to have higher levels of fat embedded within their thigh muscles. This type of fat buildup, known as muscle fat infiltration, is considered a sign of poorer muscle quality and has been linked to reduced strength and function.
Researchers analyzed MRI scans from 615 participants who were at risk for knee osteoarthritis but had not yet developed clear signs of the condition. They found a consistent pattern. Higher intake of ultra-processed foods was associated with greater fat accumulation inside thigh muscles. This link remained even after accounting for body weight.
The connection became even stronger when researchers adjusted for abdominal fat. That suggests central fat distribution may play an important role alongside diet quality.
Ultra-processed foods are a major part of modern diets. They often contain high levels of sugar, salt, unhealthy fats, and additives. At the same time, they tend to replace more nutritious options that provide protein and essential nutrients needed for muscle health.
The findings help explain why poor diet quality has been linked to worse outcomes in people at risk of joint disease. Fat buildup inside muscles can reduce their ability to support joints properly. Over time, that may increase strain on the knees and contribute to pain or reduced mobility.
The study also found that this relationship was consistent across men and women. Among different muscle groups, the strongest link appeared in the inner thigh muscles, known as adductors.
Participants in the study consumed a significant amount of ultra-processed food. On average, these foods made up about 41 percent of their daily calorie intake.
Knee osteoarthritis is a common condition, especially in older adults. It affects not just the joint itself but also surrounding tissues like muscles and fat. Obesity is already known to be a major risk factor. This research suggests that what people eat may also influence how muscles change before the disease fully develops.
Still, researchers caution that the study does not prove cause and effect. It was observational, meaning it shows an association but cannot confirm that ultra-processed foods directly cause muscle fat buildup.
Even so, the results add to growing concerns about the long-term health impact of ultra-processed diets. The researchers say more studies are needed to determine whether improving diet quality could help preserve muscle health and slow joint decline.
For now, the findings point to a simple takeaway. Diet choices may affect not just weight, but also how muscles function and how joints feel over time.