SAN FRANCISCO, CALIFORNIA — Researchers at the University of California San Francisco published a study in the journal Radiology linking diets high in ultraprocessed foods to increased fat deposits between thigh muscles, regardless of body mass index. The study analyzed data from 615 people with an average age of 60 who were enrolled in the National Institute of Arthritis and Musculoskeletal and Skin Diseases' Osteoarthritis Initiative.

Participants who consumed diets high in ultraprocessed foods had more intermuscular fat regardless of their BMI, caloric intake, and exercise levels, the study found. Researchers analyzed participants' body mass index, dietary intake, and thigh MRI scans to measure muscle fat storage. Approximately 65% of study participants were overweight and about 24% had obesity.

"We thought maybe it was just obesity or belly fat, which you get with high caloric intake." said Dr. Thomas Link, chief of the musculoskeletal imaging section at UCSF. "What is not so well-known is that diet also has a notable impact on musculoskeletal health," he said.

All participants enrolled in the Osteoarthritis Initiative because they were at risk of knee osteoarthritis due to obesity, overweight, or knee injuries. Higher levels of intermuscular fat are a risk factor for knee osteoarthritis. The buildup of fat in muscles changes muscular structure and alters how muscles exert force, and changes in how the knees absorb force from thigh muscles can break down cartilage and lead to arthritis.

Muscles store fat in two ways: intermuscular fat, which sits between healthy muscles, and intramuscular fat, which is stored in droplets within muscle fibers. All individuals, regardless of weight or physical ability, have some of both types, though extreme athletes typically do not have thick streaks of intermuscular fat. In athletes, intramuscular fat droplets serve as an energy reserve used during high exertion. When those reserves are not used, or in individuals with metabolic diseases such as Type 2 diabetes, fat particularly accumulates as intermuscular fat. "Everyone had a little bit of fat between their muscles, but any expansion is going to not be good," said Christopher Fry, co-director of the Center for Muscle Biology at the University of Kentucky.

Ultraprocessed foods — which include salty snacks, sugary drinks, and fast food — are often high in calories and low in vitamins, minerals, and nutrients needed to maintain muscle health. Such foods make up the majority of calories in Americans' diets and have been associated with negative effects on heart health and increased diabetes risk. Further research is needed to establish causality between high consumption of ultraprocessed foods and increased intermuscular fat.