April 22, 2026, 1:55 pm | Read time: 6 minutes
If you’re concerned not just about appearance but muscle quality, a new study might interest you: It suggests that muscles can “fatten” from the inside—even if you watch your calorie intake but choose the wrong foods. However, muscle fattening is just the tip of the iceberg.
High Consumption of Ultra-Processed Foods Linked to Increased Fat Deposits in Muscles
As we age, our bodies tend to replace muscle mass with fat tissue. A new study offers specific guidance on how this process can be slowed by avoiding ultra-processed foods (UPF). Researchers from the Osteoarthritis Initiative found that high consumption of these foods is directly linked to increased fat deposits in thigh muscles. “Radiology” reported on the study.1
The study is a secondary analysis of data from the Osteoarthritis Initiative, which seeks biomarkers for knee osteoarthritis.
Fat in Muscles of Over 600 People Determined
Researchers analyzed MRI images of the thighs of 615 individuals. All were at risk for osteoarthritis but had no pain or damage. The team led by radiologist Dr. Zehra Akkaya, associate professor at Ankara University, visually determined the fat content in the participants’ muscles. They examined ten muscles per thigh, totaling 20 muscles per participant.
Participants answered 102 questions about their diet: various foods, drinks, ingredients, and supplements. They also reported how often they consumed a food—on an eight-level scale from “never” to “every day” and in what portion size.
The researchers focused on the consumption of ultra-processed foods, known as UPFs. These include ready meals, snacks, soft drinks, sweets, sausages, and instant products. Popular items like protein powders or drink syrups also fall into this category. One characteristic unites them: UPFs consist of isolated ingredients and additives.
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More UPF, More Fat in Muscle—Even with the Same Calorie Amount
The study’s finding: The higher the proportion of ultra-processed foods (UPFs) in the daily diet, the more pronounced the fat accumulation in thigh muscles. For every increase in the UPF proportion, the fat content in the muscles rose.
This “more UPF, more fat” relationship held regardless of calorie count. This means: Even if two people consumed the same amount of calories, the person with the higher UPF proportion had fattier muscles.
Researchers draw a specific comparison: A participant with a UPF proportion of 87.1 percent showed significantly above-average fattening of the entire thigh muscles. A participant of similar age and weight with “only” 29.5 percent UPF proportion was within the study average.
Each muscle was rated on a scale from 0 to 4 (0 = no fat, 4 = over 50 percent fat). An ideally healthy muscle without visible fat deposits is rated 0. Adding the scores (0 to 4) of all 20 muscles results in a theoretical maximum value of 80.
In direct comparison, people with little ready-to-eat food had a score of 25, while those with very high consumption of ultra-processed foods scored 38.
Belly Fat Apparently Causes Thigh Muscles to “Fatten” More Intensely
An especially intriguing aspect of the study is that belly fat seems to act as an amplifier. Those with a lot of belly fat who also ate many ready-to-eat meals had thigh-supporting muscles that “fattened” particularly intensely.
Researchers attribute this to belly fat shifting the body’s center of gravity forward, impairing stability.
Main cause of unhealthy belly fat: a combination of a sugar-rich diet and lack of exercise. Hormones determine how much visceral fat escalates. However, they ultimately react to lifestyle—and diet is a crucial part of that.
Fattened Muscles Threaten Stability
If thigh muscles fatten due to diet, the aforementioned stability problem intensifies. Fattening causes muscles to lose quality and fail to perform their support function properly. Study leader Dr. Akkaya highlights the link between poor muscle quality and the risk of knee osteoarthritis.
“Our study provides valuable evidence that the consumption of ultra-processed foods is associated with poorer muscle quality […] regardless of gender in individuals at risk for knee osteoarthritis,” concludes the lead author.
Risks Beyond Joints: Colon Cancer
Fattening of thigh muscles is just part of a much larger health problem. While the MRI images of the current study show the gradual destruction of muscle quality, other studies warn of even more drastic consequences: More and more young adults are developing colon cancer—an alarming trend where UPFs are considered a key risk factor. A 2025 U.S. study associated high UPF intake (ten servings or more) with a 45 percent increased risk of adenomas in the colon. If untreated, they can develop into malignant tumors (FITBOOK reported).
In Germany, the current UPF proportion is estimated at 39 percent, while in the U.S. it is over 50 percent.2
At What UPF Proportion Does Muscle Fattening Begin?
Researchers do not specify a threshold UPF proportion at which the effect of muscle fattening begins. Nor do they mention a “safety limit” for an “acceptable” proportion of ultra-processed foods in the diet.
The results were obtained from a participant group that averaged 41.4 percent of their daily diet from ultra-processed sources. At this level, there is a clear link to poorer muscle quality, it says.
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Weaknesses: Study Shows Correlation, Proves Nothing
Data was collected at only one point in time. Thus, the study can only show a correlation, but not prove that diet is the sole cause of muscle fat. There may have been inaccuracies in dietary reporting. The influence of modern weight-loss medications (such as GLP-1 receptor agonists), which can affect muscle composition, was not considered. The study group mainly consisted of older adults at risk for knee osteoarthritis. Therefore, the results cannot be easily applied to younger, healthy individuals or others.
Thus, the study is a strong indicator. But further long-term studies are necessary for definitive scientific proof.
Conclusion—These Foods Should Be Preferred
While the study does not provide a direct diet plan, it offers clear guidance on what to replace in the diet using the NOVA classification system (details can be found here). The most effective way to reduce the UPF proportion is to avoid “ready-to-eat” products and industrially manufactured snacks in favor of fresh, home-cooked foods. This not only invests in one’s weight but apparently also directly in the mechanical stability of one’s joints.