High UPF diets linked to higher fat inside thigh muscles, experts flag

A diet high in ultra-processed foods (UPFs) are correlated with higher amounts of fat stored inside thigh muscles, regardless of the amount of calories consumed or level of physical activity, according to new research. It suggests that more intramuscular thigh fat can also increase the risk for knee osteoarthritis. 

These findings were presented at the ongoing annual meeting of the Radiological Society of North America in the US (December 1–5).

“The novelty of this study is that it investigates the impact of diet quality, specifically the role of UPF in relation to intramuscular fat in the thigh muscles assessed by MRI,” says author Zehra Akkaya, M.D., researcher and former Fulbright Scholar in the Department of Radiology and Biomedical Imaging at the University of California, San Francisco.

“This is the first imaging study looking into the relationship between MRI-based skeletal muscle quality and quality of diet. Understanding this relationship could have important clinical implications, as it offers a new perspective on how diet quality affects musculoskeletal health.”

The researchers of the study stress that the use of natural and minimally processed ingredients in many modern diets has decreased, more often being replaced with ingredients that have been industrially processed, artificially flavored, colored or chemically altered.

Linking processed foods to thigh fat

Foods such as breakfast cereals, margarines or 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 include synthesized ingredients and are highly processed.

These UPFs usually have longer shelf lives and often contain a combination of sugar, fat, salt and carbohydrates which affect the brain’s reward system, making it hard to stop eating.

For the study, researchers analyzed data from 666 individuals who participated in the Osteoarthritis Initiative and were not yet affected by osteoarthritis, based on imaging.

The Osteoarthritis Initiative is a nationwide research study, sponsored by the US National Institutes of Health, which helps researchers better understand how to prevent and treat knee osteoarthritis.

“Research from our group and others has previously shown that quantitative and functional decline in thigh muscles is potentially associated with onset and progression of knee osteoarthritis,” says Dr. Akkaya.

“On MRI images, this decline can be seen as fatty degeneration of the muscle, where streaks of fat replace muscle fibers.”

Link to knee osteoarthritis

Of the 666 individuals, (455 men, 211 women) the average age was 60 years. On average, participants were overweight with a body mass index (BMI) of 27. Approximately 40% of the foods that they ate in the past year were UPFs.

The researchers found that the more UPFs people consumed, the more intramuscular fat they had in their thigh muscles, regardless of energy (caloric) intake.

The researchers found that the more UPFs people consumed, the more intramuscular fat they had in their thigh muscles, regardless of caloric intake.“In an adult population at risk for but without knee or hip osteoarthritis, consuming UPFs is linked to increased fat within the thigh muscles,” says Dr. Akkaya.

“These findings held true regardless of dietary energy content, BMI, sociodemographic factors or physical activity levels.”

Targeting modifiable lifestyle factors — mainly prevention of obesity via a healthy, balanced diet and adequate exercise — has been the mainstay of initial management for knee osteoarthritis, Dr. Akkaya notes.

“Osteoarthritis is an increasingly prevalent and costly global health issue. It is the largest contributor to non-cancer related health care costs in the US and around the world,” Dr. Akkaya continues.

“Since this condition is highly linked to obesity and unhealthy lifestyle choices, there are potential avenues for lifestyle modification and disease management.”

Debate surrounding impact

There is an ongoing debate among experts regarding these foods’ health impact, as research suggests not all UPFs impact health risks to the same degree. Some imply that certain chocolates and candies contribute fewer calories and added sugars to US diets than other indulgent foods. 

Meanwhile, a WHO-backed study revealed that ultra-processed bread and cereals and plant-based alternatives lower chronic disease risk.

In the UK, the government is cracking down on highly processed junk food advertising in a new law, which will ban these promotions on TV before the 9 pm watershed from October 2025. This also includes paid online junk food adverts, with aims to reduce children’s excessive exposure to many foods high in fat, sugar or salt.