Eating too much ultraprocessed food could take a toll on muscle health, according to new research published Tuesday in the journal Radiology.
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Ultraprocessed foods, which include salty snacks, sugary drinks and fast food, make up the majority of calories in Americans’ diets. The negative effects they have on heart health and diabetes risk is well-established.
“What is not so well-known is that diet also has a significant impact on musculoskeletal health,” said Dr. Thomas Link, chief of the musculoskeletal imaging section at the University of California San Francisco and the senior author of the study.
Muscles store fat in two ways: in “streaks” of fat that sit between healthy muscles, called intermuscular fat, and in droplets stored in muscle fibers, called intramuscular fat.
Everyone, regardless of weight or physical ability, has some of both types, but you typically won’t find thick streaks of intermuscular fat in extreme athletes, said Christopher Fry, co-director of the Center for Muscle Biology at the University of Kentucky.
The difference is due to how the body uses fat in the muscles, said Fry, who wasn’t involved with the new research. In athletes, fat stored in muscle — predominantly in muscle fiber droplets — is an important energy reserve that the body taps into when a person exerts an extraordinary amount of energy. When those energy reserves aren’t being used or a person has a metabolic disease such as Type 2 diabetes, fat begins to build up, particularly in streaks between the muscles.
“Everyone had a little bit of fat between their muscles, but any expansion is going to not be good,” Fry said.
In familiar terms, “we want a sirloin steak, not a ribeye,” he said, referring to one notoriously lean and another commonly marbled, fatty cut of beef.
To study diet’s effects on muscular fat, Link and his team looked at data from 615 people with an average age of 60 enrolled in the National Institute of Arthritis and Musculoskeletal and Skin Diseases’ Osteoarthritis Initiative. Higher levels of intermuscular fat are a risk factor for knee osteoarthritis, Link said. Everyone in the database was at risk of knee osteoarthritis when they enrolled, either because they had obesity, they were overweight or they had knee injuries.
Fat build-up in muscles changes muscular structure and alters how muscles exert force, Fry said. That changes how the knees absorb the force of the thigh muscles moving the legs, which can break down cartilage and lead to arthritis, he said.
The researchers analyzed everyone’s body mass index and what they ate and MRI scans of their thighs, which showed how much fat each person’s muscles stored. In total, about 65% of the people in the study were overweight, and about 24% had obesity. However, the MRI scans revealed that regardless of people’s BMI, how many calories they ate and how much they exercised, people who ate diets that were high in ultraprocessed foods had more fat streaked through their muscles.
More research is still needed to establish cause and effect; that is, whether eating a lot of ultraprocessed foods leads to more intramuscular fat.
“We thought maybe it was just obesity or belly fat, which you get with high caloric intake. But it was more than that,” Link said.
Protein is a key macronutrient the body needs to build and maintain muscle, but omega-3 fatty acids and iron are also critical. Ultraprocessed foods are often high in calories and low in vitamins, minerals and nutrients needed to maintain muscle health, Link said.
Dr. Tamiko Katsumoto, a clinical associate professor of immunology and rheumatology at Stanford University, said that if ultraprocessed foods are proven to cause fat build-up in muscles, the phenomenon most likely isn’t unique to the thighs. Neither are the health implications, she said. “Knee function and stability comes from having strong leg muscles,” she said. “But when other muscles store fat, it can cause other issues, too.”
A study published last year found that each 1% increase in muscular fat raised a person’s risk of cardiovascular disease by 7%. Studies have also linked more intramuscular fat with a higher risk of Type 2 diabetes. Researchers are still trying to understand why, but Link said fatty muscles release inflammation-causing molecules including cytokines, which is one way the phenomenon could cause health problems.
It is possible to reverse fat build-up in muscles, but that requires eating more whole foods and fewer ultraprocessed foods, along with boosting exercise to maintain muscle mass as you lose fat, Link said.
Losing weight throughout the body can help, Fry said, but because fat doesn’t accumulate in muscles overnight, getting rid of it will also take long-term lifestyle changes.
“Habitual dietary choices influence the health and longevity of our muscles,” he said.