Ultra-processed foods are already known to harm the heart and waistline, but new research shows they also damage muscle quality deep inside the body. A study from the University of California, San Francisco (UCSF) reveals that a diet high in processed items leads to fat accumulating within thigh muscles. This hidden fat makes muscles weaker and increases the risk of knee osteoarthritis, even in people who maintain a healthy weight and exercise regularly.
Higher ultra-processed food intake is linked to fatty muscles
Researchers used MRI scans to look inside the thigh muscles of more than 600 older adults. They found that people who ate more ultra-processed foods (UPF) had significantly higher levels of myosteatosis, which is the medical term for fat infiltrating muscle tissue.
The UCSF study findings
The study, presented at the annual meeting of the Radiological Society of North America (RSNA), analyzed data from 666 participants with an average age of 60. Although they did not have osteoarthritis at the start, many were overweight.
The team measured the participants’ diet and compared it to MRI images of their thighs. The results showed a clear pattern: as UPF consumption went up, so did the amount of fat marbled inside the muscle. Crucially, this link remained strong even after the researchers adjusted for factors like body mass index, total calorie intake, and physical activity levels. This suggests that the quality of the food matters more than just the energy it provides.
Fat in muscles weakens them
Healthy muscle tissue is dense and lean. When fat cells build up inside the muscle fibers—similar to the marbling seen in a steak—the muscle becomes less efficient. This condition, myosteatosis, is a known marker of poor muscle quality.
Fatty muscles generate less force than lean muscles of the same size. Over time, this internal fat replacement can lead to weakness, frailty, and a condition called sarcopenia, where muscle mass and function decline faster than normal aging would cause.
What counts as ultra-processed food?
To understand the risk, it helps to know exactly what researchers mean by “ultra-processed.”
The NOVA classification
Nutrition scientists use the NOVA system to categorize food by how much it has been altered. Group 4, or ultra-processed foods, includes industrial formulations made mostly from substances extracted from foods (like oils, fats, sugars, starch, and proteins) or synthesized in labs (like flavor enhancers, colors, and additives).
Common examples include:
- Soft drinks and energy drinks
- Packaged snacks and chips
- Mass-produced breads and buns
- Reconstituted meat products like nuggets or sausages
- Instant noodles and soups
- Frozen ready-to-eat meals
Why they are harmful
These foods are often low in protein, fiber, and essential micronutrients that muscles need to repair and grow. Instead, they are rich in refined carbohydrates, unhealthy fats, and additives. Dr. Zehra Akkaya, the lead researcher, noted that these foods affect the brain’s reward system, making them hard to stop eating, but their poor nutritional profile fails to support healthy muscle tissue maintenance.
Muscle fat increases the risk of knee osteoarthritis
The study found that the fatty degeneration of thigh muscles is not just a cosmetic issue; it is a functional one that threatens joint health.
The mechanical and inflammatory link
The quadriceps muscles in the thigh are essential for protecting the knee joint. They absorb shock and stabilize the leg with every step. When these muscles are infiltrated by fat, they become weaker and less able to protect the knee cartilage from stress.
Dr. Akkaya explained that this decline in muscle quality is potentially associated with the onset and progression of knee osteoarthritis. Furthermore, visceral fat and intramuscular fat are metabolically active, meaning they can release inflammatory chemicals that may further damage joint tissues and accelerate degeneration.
What you can do about it
The findings offer a clear path for prevention. Since muscle quality is linked to diet, changing what you eat can help preserve your strength and joints.
Swap UPFs for whole foods
Focus on whole, minimally processed foods that provide the building blocks for lean muscle.
- Proteins: Eggs, fish, poultry, beans, and plant proteins are linked with better health and provide amino acids for muscle repair.
- Complex carbs: Vegetables, fruits, and whole grains offer sustained energy without the additives found in processed snacks.
- Healthy fats: Nuts, seeds, and olive oil support cell health without promoting inflammation.
Strength training
While the study showed diet plays a huge role, exercise remains critical. Resistance training is the most effective way to build lean muscle mass and reduce intramuscular fat. Combined with a lower-UPF diet, it offers the best defense against frailty.
You should always triple-check these facts with other reliable sources and consult a healthcare professional before making major changes to your diet or exercise routine.
Sources & related information
RSNA – Eating High-Processed Foods Impacts Muscle Quality – 2024
A study presented at the Radiological Society of North America annual meeting found that higher consumption of ultra-processed foods is independently associated with increased intramuscular fat in the thigh. The research highlights a direct link between poor diet quality and muscle degeneration.
AuntMinnie – Highly processed foods affect muscle quality – 2024
This report details Dr. Zehra Akkaya’s findings that dietary quality directly impacts muscle composition, potentially serving as a modifiable risk factor for knee osteoarthritis.
Frontiers in Nutrition – Higher ultra processed foods intake is associated with low muscle mass – 2024
A separate cross-sectional study using NHANES data found a linear association between high UPF consumption and low muscle mass in adults, reinforcing the negative impact of processed diets on musculoskeletal health.
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