Consider the delicate web of fat in a Wagyu steak. The "marbling" that makes carnivore connoisseurs swoon is a visual heuristic for quality flavor.
Now, a new study suggests the very same marbling of fat inside our own muscles points to trouble.
This condition, known as intramuscular adipose tissue, or IMAT, has long been recognized by scientists as a strong indicator of poor health. It's linked to a wide range of diseases: obesity, Type 2 diabetes, neuromuscular disorders (including Duchenne muscular dystrophy) and neurogenerative conditions such as ALS. In some cases, clinicians can even track the progress of a disease by the amount of fat in muscle tissue.
"We wanted to understand the precise function IMAT might play on muscle health," said Daniel Kopinke, Ph.D., an associate professor in the department of pharmacology and therapeutics in the UF College of Medicine. "Now, we have functional evidence that it is an active driver of declining muscle function."
The study shows that intramuscular fat acts as a physical barrier, obstructing the traditional healing process and regeneration that typically follows a muscle injury.
Kopinke's team developed a genetic model called mFATBLOCK that allowed researchers to damage the muscle while preventing the infiltration of IMAT.
When fat cells were present in the muscle, the muscle fibers were unable to properly form and grow. The fat tissue's roadblock led to a disorganized and chaotic healing process that ultimately resulted in smaller, weaker muscle fibers.
"This directly translated to a loss of strength," said Kopinke, whose background is a veritable scientific smorgasbord of developmental biology, mouse genetics, stem cell biology and muscle regeneration.
The muscle with fatty interlopers was not capable of producing the same amount of force as the healthy, unobstructed muscle.
One metaphor Kopinke's students often reference is a forest fire. When everything is burned down and you're seeking to encourage new trees to grow, a boulder is an impediment. Wherever there's a boulder, a tree cannot properly germinate or grow.
Similarly, where space is occupied by fat cells, muscle fibers cannot grow. Notably, without any hampering of the fat cells' growth, the fat tissue ultimately took up 12% of the whole muscle tissue.
This isn't to say that individuals seeking to change their weight are out of luck. Much like weight gain, weight loss relies on an energy imbalance — in this case, expending more energy than is put into your body, often through diet and exercise.
Fortunately, the solution to reducing intramuscular fat is the same method for general weight loss: creating an energy imbalance. By expending more energy than consumed, the body is forced to shrink its fat cells, including those marbled within the muscle. This clears the path for muscle fibers to regenerate and grow.
"If you make the area that fat cells occupy in your muscles smaller, the muscle fibers would have more space to grow into," Kopinke said.
"You can shrink your fat cells," he added. "Based on everything we found, we would speculate that if you make the area that fat cells occupy in your muscles smaller, the muscle fibers would have more space to grow into."
These findings could fundamentally shift the research area's understanding of the role of fat in muscle disease and aging. It poses major implications regarding current therapies for severe muscle injuries and for chronic diseases like muscular dystrophy or age-related muscle loss. Now, experts may incorporate strategies on reducing or removing the physical blockage of fat, rather than solely promoting muscle growth.
"By clearing the path for muscle fibers to heal correctly, we may be able to restore function and improve strength in millions of people affected by these debilitating conditions," Kopinke said.
