A new scientific study is sounding the alarm on what millions of people eat every day, revealing that ultra-processed foods may be quietly damaging muscle health in ways that increase the risk of knee osteoarthritis, even when calorie intake and physical activity are taken into account.

Researchers have discovered a concerning link between high consumption of ultra-processed foods and increased fat buildup inside thigh muscles, a condition that could weaken mobility and accelerate joint degeneration over time. The findings were published in Radiology and focus on a population already considered at risk for knee osteoarthritis.

What counts as ultra-processed food

Ultra-processed foods are engineered for convenience, long shelf life, and intense flavor. They are typically packed with combinations of sugar, fat, salt, and refined carbohydrates designed to stimulate the brain’s reward system and encourage overconsumption.

These products include breakfast cereals, margarines and spreads, packaged snacks, hot dogs, soft drinks, energy drinks, candies, desserts, frozen pizzas, ready-to-eat meals, and mass-produced packaged breads and buns, often made with synthesized ingredients.

"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prevalences of obesity and knee osteoarthritis, the use of natural ingredients in our diets has steadily diminished and been replaced by industrially-processed, artificially flavored, colored and chemically altered food and beverages, which are classified as ultra-processed foods," said the study's lead author, Zehra Akkaya, M.D., researcher and consultant for the Clinical & Translational Musculoskeletal Imaging group at University of California, San Francisco, Department of Radiology and Biomedical Imaging.

Dr. Akkaya and her team set out to determine whether ultra-processed food consumption was directly linked to changes in muscle composition, particularly fat accumulation inside thigh muscles.

How the study was conducted

Researchers analyzed data from 615 participants enrolled in the Osteoarthritis Initiative, a nationwide research program funded by the National Institutes of Health aimed at understanding how to prevent and treat knee osteoarthritis.

Participants were free of osteoarthritis at the time of imaging analysis. The group included 275 men and 340 women, with an average age of 60. On average, participants were overweight, with a body mass index of 27. Nearly 41 percent of their total food intake over the previous year consisted of ultra-processed foods.

"Osteoarthritis is an increasingly prevalent and costly global health issue," Dr. Akkaya said. "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."

What the MRIs revealed about muscles

The imaging results showed a clear pattern. The more ultra-processed foods participants consumed, the higher the amount of fat found inside their thigh muscles, regardless of total calorie intake.

On MRI scans, this appears as fatty degeneration, where streaks of fat gradually replace healthy muscle fibers, potentially weakening strength and function.

"In addition to investigating the quality of our modern diet in relationship to thigh muscle composition, in this study, we used widely available, non-enhanced MRI, making our approach accessible and practical for routine clinical use and future studies," Dr. Akkaya said. "These MRIs do not require advanced or costly technology, which means they can be easily incorporated into standard diagnostic practices."

The findings suggest that diet quality, not just calorie balance, plays a critical role in how muscles age and function.

Why diet quality matters for joints

Researchers say the study highlights an often-overlooked factor in joint health: the composition of the diet itself, not just weight loss or exercise.

"This research underscores the vital role of nutrition in muscle quality in the context of knee osteoarthritis," Dr. Akkaya said.

"Addressing obesity is a primary objective and frontline treatment for knee osteoarthritis, yet 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."

Experts emphasize that lifestyle changes remain central to preventing and managing knee osteoarthritis, especially through balanced nutrition and regular physical activity.

Reducing ultra-processed food intake may help preserve muscle integrity, potentially easing stress on joints and lowering long-term disease burden.

"In recent years, several researchers have shown detrimental impacts of ultra-processed food on various health outcomes but data on the relationship of ultra-processed food and body composition in the context of knee osteoarthritis is limited," Dr. Akkaya said.

"This is the first study assessing ultra-processed food's impact on thigh muscle composition using MRI. By exploring how ultra-processed food consumption impacts muscle composition, this study provides valuable insights into dietary influences on muscle health."