Not Just The Waistline: MRI Scans Show Ultra-Processed Foods Fill Muscles With Fat

That bag of chips, that frozen pizza, that shelf-stable pastry grabbed on the way out the door. Americans know these foods aren’t exactly good for them. But a new study says the damage may go deeper than the waistline. Researchers found that people who ate more ultra-processed foods had higher levels of fat within their thigh muscles, even after accounting for factors like body weight, calorie intake, and lifestyle differences.

That fat wasn’t sitting under the skin or pooling around the belly. It had worked its way inside the muscles themselves, the very muscles needed to walk, climb stairs, and keep knees stable. In a country where ultra-processed foods make up more than half of the average person’s diet, that’s a problem that reaches well beyond concerns about appearance.

Published in the journal Radiology, the research drew on data from 615 older adults enrolled in the Osteoarthritis Initiative, a major long-term study focused on knee health. None had diagnosed osteoarthritis or ongoing knee or hip pain at the start of the study, but all were considered at risk for developing knee problems down the road. Researchers wanted to know whether diet quality, specifically how much of a person’s food came from industrial, heavily processed sources, might be associated with degraded quality in the muscles that protect aging joints.

What Counts as Ultra-Processed Food?

Researchers used a system called the NOVA classification, which sorts foods not by their nutritional labels but by how much industrial processing went into making them. Ultra-processed foods are those made with chemical additives, things like artificial flavorings, colorings, and shelf-life extenders, designed to make products last longer, taste better, and look more appealing. Packaged snack cakes, instant noodles, hot dogs, sweetened cereals, and soft drinks all fall into this category. These aren’t foods that were simply cooked or canned. They were engineered in factories.

On average, ultra-processed foods made up about 41% of the participants’ diets. Men ate slightly more of these foods than women, roughly 45% of their diet compared with about 39% for women.

How Researchers Connected Ultra-Processed Food to Muscle Fat

Led by Zehra Akkaya at Ankara University and the University of California, San Francisco, the team analyzed data collected between February 2004 and October 2015. Participants completed a detailed food survey covering 102 items, reporting how often and how much of each they ate over the prior 12 months. Each item was then classified using the NOVA system, and researchers calculated what percentage of each person’s diet came from ultra-processed sources.

To measure muscle quality, the team used MRI scans of both thighs. Two trained observers scored fat infiltration in 10 individual muscles per thigh using the Goutallier grading system, which rates each muscle on a zero-to-four scale. A score of zero means no visible fat in the muscle; a four means more than half the muscle tissue has been replaced by fat. Rather than relying on a single image, observers evaluated 15 consecutive slices covering a roughly three-inch section of each thigh.

Participants had a mean age of about 60 years, and just over half were women. Nearly 65% were overweight and about 24% were obese. People with diabetes, stroke, cancer, or inflammatory joint disease were excluded, an intentional choice to isolate the relationship between diet and muscle fat without the distorting effects of chronic illness.

After adjusting for age, sex, race, education, income, physical activity, smoking, depression, total daily calories, and body mass index, the results showed a consistent pattern. As ultra-processed food made up a larger share of someone’s diet, fat scores in the thigh muscles climbed across all muscle groups examined, including the muscles on the back and inner thigh. Sex made no difference; the relationship held equally for men and women.

Why Waist Size Tells More Than the Scale

When researchers swapped BMI for waist measurement, a more precise gauge of how fat is distributed in the body, the associations grew stronger and extended to every muscle group, including the front of the thigh. The strongest link appeared in the inner thigh muscles. That pattern suggests fat distribution may play an important role in understanding how diet relates to muscle health, beyond what overall body weight can capture alone.

When total dietary fat was added as an extra variable, the results barely moved, suggesting the link may not be explained by fat content alone. Something about ultra-processed foods beyond their fat content appears connected to muscle deterioration.

What This Means for American Dinner Tables

Skeletal muscle makes up roughly 40% of total body weight and contains up to 75% of all the protein in the human body. When fat infiltrates muscle tissue, the muscle works differently. Fatty muscles are weaker muscles, and weaker thigh muscles mean less support for aging knees.

Because the study captured only a single point in time, it cannot prove that ultra-processed food directly causes muscle fat infiltration. Researchers describe their results as “hypothesis generating” and stress the need for follow-up studies that track people over months or years. Still, since this data was collected, ultra-processed food consumption has only grown. Muscles may be quietly deteriorating in ways a bathroom scale cannot detect, and the foods filling grocery store aisles may be a major part of the reason.

---

Disclaimer: This article is based on the findings of a single observational study and should not be taken as medical advice. The research identifies associations, not proven causes. Consult a qualified healthcare professional before making any changes to your diet or lifestyle.

---