Eating lots of ultra-processed foods appears linked to a buildup of fat in the thighs, which in turn raises a person's odds for arthritis in the knees, new research shows.

“In an adult population at risk for but without knee or hip osteoarthritis, consuming ultra-processed foods is linked to increased fat within the thigh muscles,” said study lead author and radiologist Dr. Zehra Akkaya, who conducted the research while at the University of California, San Francisco (UCSF).

Her team presented the findings Wednesday in Chicago at the annual meeting of the Radiological Society of North America (RSNA).

Ultra-processed foods are made mostly from substances extracted from whole foods, like saturated fats, starches and added sugars. They also contain a wide variety of additives to make them more tasty, attractive and shelf-stable, including colors, emulsifiers, flavors and stabilizers.

Examples include packaged baked goods, sugary cereals, ready-to-eat or ready-to-heat products and deli cold cuts.  

As Akkaya's team explained, it's long been understood that the health of the thigh's musculature plays a role in the development of arthritis in the knee. A buildup of fat within the thigh has been linked to a higher odds for knee arthritis.

A decline in thigh muscles "is potentially associated with onset and progression of knee osteoarthritis," Akkaya said in an RSNA news release, and "on MRI images, this decline can be seen as fatty degeneration of the muscle, where streaks of fat replace muscle fibers.”

To see what role diet might play in all of this, the UCSF team used medical imaging to gauge levels of fat within the thighs of 666 people. Participants averaged 60 years of age and were not yet affected by osteoarthritis when they entered the study.

On average, about 40% of the participants' diets were made up of ultra-processed foods.

The researchers found that as rates of consumption of ultra-processed food went up, so too did levels of intramuscular fat within their thighs.

That relationship held true regardless of whether or not the person was obese, how many calories they consumed per day, how much they exercised or their income and education.

Letting folks know that a high intake of ultra-processed foods may encourage arthritis could be valuable, Akkaya said.

"Osteoarthritis is an increasingly prevalent and costly global health issue. It is the largest contributor to non-cancer related health care costs in the U.S. and around the world,” she noted.

Lifestyle changes — including cutting back on ultra-processed foods — could help prevent the debilitating disease, her team said.

Because these findings were presented at a medical meeting, they should be considered preliminary until published in a peer-reviewed journal.