Common examples include:

• Breakfast cereals
• Margarines and spreads
• Packaged snacks
• Hot dogs
• Soft drinks and energy drinks
• Candies and desserts
• Frozen pizzas
• Ready-to-eat meals
• Mass-produced packaged breads and buns

"Over the past decades, in parallel to the rising prevalences of obesity and knee osteoarthritis, the use of natural ingredients in our diets has steadily diminished and been replaced by industrially-processed, artificially flavored, colored and chemically altered food and beverages, which are classified as ultra-processed foods," Dr. Akkaya said.

Why does muscle fat matter for knees?

The thigh muscles provide crucial support around the knee joint. Fatty changes within these muscles may weaken that support and could potentially increase the risk of developing knee osteoarthritis — although the study shows an association, not proof that diet causes the changes.

"Osteoarthritis is an increasingly prevalent and costly global health issue," Dr. Akkaya noted. "It constitutes one of the largest non-cancer-related health care costs in the United States and around the world. It is highly linked to obesity and unhealthy lifestyle choices."

The researchers used widely available, non-enhanced MRI, which requires no contrast agents or advanced technology. "These MRIs do not require advanced or costly technology, which means they can be easily incorporated into standard diagnostic practices," Dr. Akkaya said.

What are the study's limitations?

The findings are preliminary in several respects. The study is observational, so it cannot establish that ultra-processed foods directly cause fat infiltration in muscle. The dietary data relied on participants' recall of what they ate over the previous year, which can be imprecise. And while all participants were at risk for knee osteoarthritis and had no signs of it on imaging at the time of analysis, the study does not confirm they went on to develop the condition.

Still, the results add to growing evidence that diet quality influences body composition in ways that body weight and calorie counts alone do not capture.

What might change for patients?

Standard early treatment for knee osteoarthritis already emphasizes changeable lifestyle factors: maintaining a healthy weight through balanced diet and regular exercise. The new findings suggest the type of food deserves closer attention too.

"This research underscores the vital role of nutrition in muscle quality in the context of knee osteoarthritis," Dr. Akkaya said. "The findings from this research emphasize that dietary quality warrants greater attention, and weight loss regimens should take into account diet quality beyond caloric restriction and exercise."

In practice, that means cutting back on ultra-processed foods could potentially help preserve muscle quality — and, in turn, help reduce some of the burden associated with knee osteoarthritis. Whether that strategy actually prevents the disease will require further research.