Researchers at the University of California, San Francisco used MRI to look inside the thigh muscles of 615 middle-aged adults.
The more ultra-processed foods a person ate, the more fat streaks had settled between their muscle fibers.
The link held up even after the team accounted for how many calories people ate, how much they exercised, and their body weight, according to the study published in the journal Radiology in April 2026.
Ultra-processed foods are the industrially made items that dominate most grocery aisles.
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Packaged snacks, sugary drinks, ready meals, breakfast cereals, deli meats, and sweetened dairy products all fall into this group. On average, participants got about 41% of their food from these products over the year before their scan.
Fat streaks show up on the MRI
The scans looked for fat that grows inside a muscle, not the layer sitting under the skin.
Radiologists call this fatty infiltration, and it shows up as pale streaks running through what should be lean, dark muscle tissue.
When muscle fibers get replaced by fat in this way, the muscle becomes weaker and less able to protect the joint it moves.
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The thigh is the key muscle group for the knee. Strong quadriceps at the front of the thigh act like a shock absorber every time you walk, climb stairs, or stand up from a chair.
When those muscles are marbled with fat, the joint takes more of the load, and cartilage wears down faster.
That link between fatty thigh muscles and knee problems is why the team focused on people who were still free of osteoarthritis but at higher risk of it.
What the study looked at
The 615 participants came from the Osteoarthritis Initiative, a long-running federally funded cohort. Their average age was 60 and their average body mass index was 27, which puts the typical participant in the overweight range.
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None had signs of knee osteoarthritis when they were scanned.
Diet was tracked through a food frequency questionnaire covering the previous year.
Each one-step increase in ultra-processed food intake was tied to a small but measurable rise in muscle fat, according to the Radiological Society of North America.
The effect stayed significant after the team controlled for calorie intake, fat intake, physical activity, BMI, and sociodemographic factors.
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In other words, two people eating the same number of calories can end up with different muscle quality if one of them fills more of the plate with ultra-processed products.
"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," lead author Zehra Akkaya, M.D., of the UCSF Department of Radiology and Biomedical Imaging, said in the release.
What it means for your plate
The paper is observational, so it cannot prove that ultra-processed foods directly damage muscle.
It shows a consistent association that survives the obvious explanations. It fits a broader pattern in the research: ultra-processed diets have also been linked to a higher risk of severe fatty liver disease, poorer weight loss, and other health harms.
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The practical takeaway is about swaps, not perfection. Whole foods on repeat, such as beans, eggs, plain yogurt, oats, fruit, vegetables, fish, nuts, and unprocessed meat, crowd out ultra-processed products without demanding that you count calories.
Combined with regular movement, that pattern gives the thigh muscles a better chance of staying lean and doing their job for the knee.
Knee osteoarthritis is not inevitable, and scientists say lifestyle changes can shift the risk in your favor. Building meals around whole ingredients, and moving the joint regularly, is where the evidence keeps pointing.
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.