August 4, 2026

Myosteatosis: How Fat Infiltration Inside Muscle Drives Sarcopenic Obesity

By Dr. Zakira Naureen
Body Composition Metabolic Health Young Adults

Most conversations about sarcopenic obesity focus on two numbers: how much muscle a person has and how much fat they carry. But there is a third, less visible piece of the picture that researchers increasingly believe matters just as much. It is called myosteatosis, and it describes fat that has infiltrated the muscle tissue itself, sitting between muscle fibers and even inside individual muscle cells, rather than simply surrounding the muscle or collecting around the waist. Unlike subcutaneous fat, this fat cannot be seen or pinched, and it does not show up on a bathroom scale or even on a standard BMI calculation. It can only be identified through imaging or biopsy, which is part of why it has stayed under the radar for so long.

What Myosteatosis Actually Is

Researchers generally separate fat around muscle into two categories. Intermuscular adipose tissue sits between distinct muscle groups, beneath the fascia that wraps each muscle. Intramuscular adipose tissue is finer grained: fat that has infiltrated within a muscle, in the spaces between fibers or stored inside the muscle cells themselves. Together, this excess fat deposition within muscle tissue is what clinicians term myosteatosis. An editorial in the journal European Geriatric Medicine describes it as an early architectural change in muscle, one that can occur before strength and functional problems become measurable, and even before the metabolic abnormalities typically linked with obesity and diabetes become apparent on standard blood work.

Measuring myosteatosis is not simple. Computed tomography is considered the reference method for quantifying intramuscular fat by measuring muscle radiation attenuation, while magnetic resonance imaging is generally used for intermuscular fat and can also estimate fat fraction directly. More common clinical tools such as DXA scans or bioelectrical impedance, the kind found in many gyms and clinics, cannot distinguish this kind of fat at all. That gap matters: a young adult could have what looks like a completely reasonable body composition reading and still be building up meaningful fat infiltration inside their muscle.

The Metabolic Mechanism: From Infiltrated Fat to Insulin Resistance

Fat inside muscle is not inert. It behaves as a metabolically active tissue capable of secreting inflammatory cytokines, and this local inflammation appears to interfere with normal muscle cell signaling, promote further muscle breakdown, and impair the muscle's ability to take up and use glucose. A large cross sectional study published in the Journal of Clinical Endocrinology and Metabolism examined more than 18,000 adults who underwent abdominal computed tomography and found that higher HOMA-IR, a standard marker of insulin resistance, was significantly associated with a greater degree of myosteatosis. The relationship runs in both directions in the research literature: insulin resistance promotes further fat storage in muscle, and fat stored in muscle worsens insulin resistance, creating the kind of self-reinforcing cycle that shows up throughout the biology of sarcopenic obesity.

At the cellular level, several overlapping pathways have been proposed. Excess lipid delivery to muscle leads to accumulation of diacylglycerols and ceramides, lipid byproducts that are thought to interfere with insulin signaling through kinases such as protein kinase C and JNK. Inflammatory cytokines released by both infiltrated fat and enlarged, stressed fat cells elsewhere in the body add to this disruption. No single mechanism has been established as the definitive cause, and researchers studying muscle insulin resistance describe the picture as multifactorial rather than governed by one clean pathway.

Why This Matters Before Middle Age, Not Just After It

Myosteatosis has historically been studied almost entirely in older adults, often in the context of frailty, hospitalization, or age related sarcopenia. That focus made sense given how the field developed, but it has left a real gap in understanding when this process actually begins. A 2023 study in the European Journal of Applied Physiology examined healthy young adults with an average age of about twenty and used MRI along with ultrasound elastography to measure intramuscular fat in the calf muscle. The researchers found a measurable relationship between the amount of intramuscular fat and passive muscle stiffness, meaning detectable, quantifiable fat infiltration and its physical effects on muscle tissue are present well before midlife, not something that appears suddenly in older age.

This lines up with a pattern this site has covered before: a young adult can have a completely normal BMI while still carrying a body composition profile that raises real metabolic risk, sometimes called normal weight obesity or informally, skinny fat (see why your BMI might be lying to you in your 20s). Myosteatosis adds another layer to that same blind spot. A young person can look lean, perform reasonably well physically, and still be developing fat infiltration inside their muscle tissue that standard screening tools are not built to catch.

What the Evidence Says About Reversing It

The most encouraging evidence on myosteatosis concerns whether it can be modified. A systematic review and meta-analysis published in the Journal of Applied Physiology pooled thirteen studies, twelve of which were suitable for meta-analysis, covering 465 participants who were mostly women with overweight. Exercise interventions, averaging roughly 37 minutes per session, produced a significant and moderate reduction in muscle lipid infiltration and a corresponding increase in muscle attenuation on imaging, a marker of improved muscle quality. The authors concluded that exercise can meaningfully improve muscle quality in populations at risk of both obesity and sarcopenia related disability.

It is worth noting what this evidence does and does not establish. Most included studies combined different modes of exercise, and the review did not isolate resistance training from aerobic training as cleanly as would be ideal (readers interested in the resistance training side of this picture can see our earlier piece on building a resistance training habit as a young adult within the fuller guide to sarcopenic obesity risk). Some more recent systematic reviews looking specifically at combining exercise with calorie restriction have found less consistent results for reducing intermuscular fat specifically, suggesting that diet, exercise type, and duration likely all influence how much myosteatosis actually responds to a given intervention.

Practical Implications

For most readers, the practical takeaway is not that a specific test needs to be booked immediately. Widely available myosteatosis-specific imaging is not yet standard outside research settings and specialist clinics. Instead, the more useful takeaway is that the same behaviors that help protect muscle mass in general, regular resistance and aerobic activity, adequate protein intake, and avoiding long uninterrupted periods of sitting, also appear to reduce fat infiltration within muscle specifically, not just fat around the waistline. Because myosteatosis appears to precede more obvious strength and metabolic problems, addressing it earlier in adulthood, rather than only once function starts declining, may carry more benefit than waiting.

Limitations and Open Questions

Several caveats are important here. Much of the mortality and functional impairment data tied to myosteatosis comes from older, sometimes hospitalized cohorts, so how strongly those outcomes translate to healthy young adults is not yet well established. Measurement itself remains a barrier: CT and MRI are accurate but expensive and not part of routine screening, which limits how much population level data exists, particularly in younger age groups. The direction of causation between insulin resistance and myosteatosis is also still debated, with most human data being cross sectional rather than proving which comes first. Finally, the exercise evidence, while promising, is drawn from a relatively small pool of trials with mixed exercise protocols, so precise recommendations about type, intensity, or duration specifically targeted at reducing myosteatosis are not yet settled.

How This Connects to Sarcopenic Obesity

Sarcopenic obesity is defined by the coexistence of low muscle mass or function alongside excess adiposity, and myosteatosis sits at the mechanistic intersection of both halves of that definition. It represents fat literally occupying and altering the tissue that muscle mass and function depend on, while simultaneously behaving as an inflammatory, insulin resistant tissue that worsens the metabolic profile associated with obesity. Understanding myosteatosis does not change the practical prevention advice at the center of this site's guidance, but it does help explain why sarcopenic obesity can develop and compound quietly, years before it becomes visible on a scale or a standard checkup, and why addressing muscle quality, not only muscle quantity or body weight, deserves a place in how the condition is understood and researched, including in our ongoing work with young Omani adults.

References

This post is for general education and isn't medical advice. Speak with a healthcare provider about your individual risk.

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