A diet high in ultra-processed foods may be damaging your thigh muscles, according to research published this week.
In a study of more than 600 adults who were overweight, those who ate the most ultra-processed foods were more likely to have extra fat stored in their thigh muscles, potentially raising the risk of knee pain and permanent joint damage.
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“Muscles with lots of fat stored have poor quality, and this is concerning not only because it indicates poorer muscle strength but also worse overall metabolic health,” or how the body turns food into energy, says lead author Zehra Akkaya, MD, a researcher and consultant for the Clinical and Translational Musculoskeletal Imaging group at University of California in San Francisco.
Weaker Thigh Muscles Leave the Knees Vulnerable
Dr. Akkaya and her research group focus on musculoskeletal health, including osteoarthritis, a common degenerative joint disease that frequently affects the knee.
In previous research, they found diets higher in processed food were tied to greater knee pain. For the current study, they wanted to dig deeper and see how these foods might be affecting skeletal thigh muscles, which support the knee and are critical for mobility. Poor thigh strength can accelerate the development and progression of osteoarthritis.
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For the analysis, they examined magnetic resonance imaging (MRI) scans of thigh muscles from 615 adults who on average were age 60 and had a BMI (body mass index) of 27, which falls into the overweight category. Participants were split roughly 55 to 45 percent women to men; none had been diagnosed with knee arthritis, but were considered at-risk for it. They also submitted food questionnaires about their diets over the previous year.
Researchers determined that ultra-processed foods made up about 40 percent of the participants’ diets on average.
Using a statistical measure called standard deviation, the researchers found that every increase in ultra-processed food as a proportion of someone’s overall diet corresponded to an increase in the presence of fat inside the thigh muscle.
The research team noted that fatty degeneration of the muscle could be clearly seen on MRIs, as streaks of fat replace muscle fibers.
Yet Another Reason to Eat HealthyUltra-processed foods are convenient and tasty, but they’re often nutritionally empty and full of excessive sugars, fats, salt, and chemical additives. A large body of evidence already links diets heavy in ultra-processed foods to higher risks of heart disease, diabetes, and colon cancer.
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For Shane Davis, MD, a sports medicine physician at Tufts Medical Center in Boston, this study highlights yet another way a typical American diet can negatively affect health.
It’s easy to overdo it on ultra-processed foods, he says, because they’re made to taste really good. But then you end up eating less nutritious foods as a result, which may explain why these foods can contribute to more fat in muscle tissues.
“These foods can also have many harmful additives and preservatives,” he says. “Our bodies are evolved to digest and utilize whole foods, not ultra-processed ones.”
GLP-1 Users Have More Cause to Limit Ultra-Processed Foods
People taking GLP-1 drugs for diabetes or obesity may have even more reason to avoid ultra-processed foods because of concerns about muscle loss.
“Protecting muscle during weight loss is always a concern and it becomes an even greater concern the more rapid the loss of weight,” which can occur when people take these medicines, says Donald Layman, PhD, a professor of food science and human nutrition at the University of Illinois at Urbana-Champaign.
The Study Has Some Limitations
As an observational study, this analysis can only show an association between ultra-processed foods and muscle health, but cannot determine with certainty that these foods cause poor muscle quality.
Still, Dr. Davy praised the study for taking into account other factors that could have influenced muscle health, such as total calories consumed and physical activity.
She noted, however, that the research did not consider overall dietary quality.
“People who consume a lot of ultra-processed foods may also have poorer dietary habits, such as a low consumption of fruits, vegetables, and whole grains,” she says. “From this study, we don’t know if it is the poor dietary quality that is linked to negative changes in thigh muscle quality, or if it is the ultra-processed foods.”
Another limitation of the study is that the results applied only to a very specific population (older adults who are overweight at high risk for osteoarthritis), Davy points out.
Despite some weaknesses in the analysis, Akkaya says the overall message is clear: “Be very mindful of your diet — ensure it provides adequate sources of essential building blocks for your tissues and includes limited amounts of ultra-processed foods.”
How to Identify Ultra-Processed FoodsA 2025 report from the Centers for Disease Control and Prevention (CDC) found that Americans get about 55 percent of their daily calories from ultra-processed foods and beverages on average.
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These items include:
Breakfast cerealsMargarines and spreadsPackaged snacksHot dogsSoft drinks and energy drinksCandies and dessertsFrozen pizzasReady-to-eat mealsMass-produced packaged breads and buns
“The simplest way to identify whether or not a food is ultra-processed is by looking at the ingredient list,” says Brenda Davy, PhD, RDN, a professor of human nutrition, foods, and exercise at Virginia Tech in Blacksburg.
“If ingredients are listed that are not used in your home food preparation — such as soy protein isolate, high-fructose corn syrup, artificial colorings or flavorings — then that food is likely to be ultra-processed.”
Nutritionists further caution that some foods claiming to have healthy properties may still be packed with sweeteners and artificial ingredients. These include some flavored yogurts, protein bars, and plant-based milks.
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Fortifying ultra-processed foods with extra protein or fiber, for example, may not do much to make them better for you.
“Sprinkling protein into foods like fairy dust, for example, is not healthy,” says Dr. Layman. “If the food generally doesn’t have protein to begin with, then adding it doesn’t make it much better.”
Protein is an essential building block for muscle, he says. Healthy, whole protein sources include lean meats, fish, eggs, beans, nuts, and seeds.
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