It's well known that belly fat accumulating around your abdominal organs is unhealthy, but there's a more insidious form of fat that could be even more hazardous to your health, a new study says.
Fat that infiltrates your muscles appears to dramatically increase your risk of death, according to findings published May 16 in the journal Radiology.
Fatty muscle — a condition called myosteatosis — was associated with a 15.5% increase in absolute risk of death in a group of healthy adults, researchers found.
By comparison, obesity appeared to increase participants' absolute mortality risk by only 7.6%, results show. Fatty liver disease raised risk by 8.5% and muscle wasting by 9.7%.
"The signal [for muscle fat risk] was so much stronger for this otherwise healthy cohort," said senior researcher Dr. Perry Pickhardt, chief of gastrointestinal imaging at the University of Wisconsin School of Medicine and Public Health. "It really stood out as a strong biomarker beyond the things I think we all accept as important measures."
"I think there's going to be profiling of patients where if you line up myosteatosis with a very fatty liver or maybe abundant visceral fat, you might be a lot worse off than if you just had one of those or two of those," Pickhardt added.
Muscle fat has been a subject of increasing interest in the fields of obesity and diabetes, said Dr. Steven Heymsfield, a professor of metabolism and body composition with Louisiana State University's Pennington Biomedical Research Center.
There's a small and healthy amount of fat naturally found inside each muscle cell that can be used to generate energy, said Heymsfield, who was not involved with the study.
The real health concern comes from excess fat that accumulates outside the cells and around the muscle fibers and bundles.
Think of a Steak
"If you think about a steak, the marbling in the steak, that's what we're dealing with here," Heymsfield said. "Over the last decade or two, it's been shown to correlate with adverse health outcomes, as shown in this study."
People on average carry a few kilograms of muscle fat distributed throughout their bodies, Heymsfield said. It's more likely to gather in the legs than in other regions of the body.
Pickhardt and his colleagues conducted their study in a group of nearly 9,000 healthy patients who underwent low-dose CT scans for colon cancer screening, a procedure known as virtual colonoscopy, between 2004 and 2016.
The researchers realized these CT scans could be useful for assessing other potential health problems, given the large amount of data that the scans gather on a person's physique.
"When we look at visceral fat and muscle measurements and aortic calcium or liver fat or bone marrow density, all of these things add together and basically you can end up with this really powerful prognostic sort of virtual physical exam, if you will, and just leverage that off of CTs done for any reason," Pickhardt said.
"We call it 'opportunistic screening' for now, because it's taking data that basically used to be kind of disregarded or tossed away and harnessing that in new ways," he added.
So the research team trained an artificial intelligence tool to extract body composition measures from the abdominal CT scans, specifically assessing each person's belly fat, muscle fat, liver fat and muscle wasting.
The automated software simplified the process. "It would have taken a lifetime to do that with the older methods," Heymsfield said.
The researchers then tracked participants for an average of nine years to see whether any of these measures could be related to major health problems or early death.
Not only was muscle fat associated with the highest death risk, but the association held even after researchers accounted for each person's BMI (body mass index) — the best available measure for obesity.
Lean People at Risk, Too
"BMI was actually a very poor predictor and had a very weak signal," Pickhardt said. "Clearly, there are patients who were not obese who had this bad muscle measure. That's what makes this so important — there are lean folks in terms of BMI out there who actually have a worse profile than you might think."
However, this study can't draw a clear cause-and-effect relationship between muscle fat and risk of death, noted Dr. Angela Tong, a clinical assistant professor of radiology at the New York University School of Medicine.
Muscle fat might be accumulating because of another health problem that's the true risk, said Tong, who co-authored an editorial published with the study.
"I think of it more as a sign that maybe something else is going on, maybe something else in your health that's not letting you be as active," Tong said. "You should look carefully if there are some cardiac issues or diabetes."
Other studies have drawn a link between fatty muscles and poor outcomes. For example, a 2020 evidence review published in Critical Reviews in Oncology/Hematology found that cancer patients diagnosed with myosteatosis had a 75% greater mortality risk than those without fatty muscles.
How Does it Happen?
It's not exactly clear why your muscles might start accumulating fat, Heymsfield said.
"There may be some genetics involved, and it increases as you age, despite your best intentions to lift weights or exercise," Heymsfield said.
Muscle fat also is known to accumulate if your muscles atrophy, Heymsfield said.
"Let's say you have a cast on your leg and the muscle atrophies, sometimes those muscle cells are replaced by fat cells," Heymsfield said. "That's probably the biggest source of what these investigators found."
For example, myosteatosis is a hallmark of certain types of muscular dystrophy, Heymsfield said.
It's also not clear how you can rid yourself of unwanted muscle fat, Heymsfield said.
"The science is evolving, but I think on the most part, if you lose weight and you exercise, I would say those are two really good ways to reduce it," Heymsfield said. "There may be some there that just won't go away no matter what you do, possibly the genetic part or the part that comes from muscle cells dying."
A 2021 evidence review in the Journal of Applied Physiologyfound that "exercise can significantly improve muscle quality in populations at risk of developing obesity," causing fat in muscles to decline.
This study shows that CT scans could be a useful tool for a variety of health screenings, including for fatty muscle, Pickhardt said.
"I could envision a time in the not-too-distant future where this is an intended screening measure," Pickhardt said. "You can do it for about the same amount of radiation as a standard X-ray of the abdomen.
"I'm a little hesitant to call it a virtual physical exam, but that's kind of the concept," Pickhardt added.
Heymsfield said CT scans seem to be the best way to assess muscle fat levels at this time. "You could get an estimate with ultrasound, but not to the same degree of accuracy," he said.
But people shouldn't actively worry about whether their muscles are fatty, given that the science surrounding this is so new, he added.
"I think what's going to happen now is since the AI and other analysis methods are going to become ubiquitous, that radiologists are going to get this data back automatically just when they do an abdominal CT scan," Heymsfield said. "As a result, people are going to start saying, 'Wait a minute, what do I do about this?' The answer is, if you're overweight or you're under-exercising, those are two things you could do easily in response."