What is Skeletal Fat?
Skeletal fat is the colloquial term for the infiltration of adipose, or fat, tissue into the skeletal muscles. Medically known as myosteatosis, this is not a single, uniform type of fat but is categorized by where it accumulates within the muscle architecture. Unlike subcutaneous fat, which is visible just under the skin, or visceral fat, which surrounds internal organs, skeletal fat is embedded within the muscle tissue itself. This makes it an invisible, and potentially more insidious, type of fat.
There are several classifications of skeletal fat based on location:
- Intermuscular Adipose Tissue (IMAT): This is the fat found between muscle groups, separated by fascia from subcutaneous fat. Its presence can be readily observed using medical imaging techniques like CT scans.
- Intramuscular Fat (IMC): This refers to the fat located between individual muscle fibers.
- Intramyocellular Lipids (IMCL): These are lipids stored directly inside the muscle cells (myocytes). IMCL levels can be high in both sedentary individuals with insulin resistance and highly trained endurance athletes, suggesting that the physiological function of this fat varies based on context.
The Health Risks of Excess Skeletal Fat
Scientific research has increasingly highlighted the significant health risks associated with excessive skeletal fat. The presence of this fat is not just a sign of being overweight but is an independent risk factor for several chronic conditions.
Cardiovascular Disease and Mortality
Studies have directly linked higher amounts of fat stored in muscles to increased cardiovascular risk. Research has found that for every 1% increase in fatty muscle fraction, there is a heightened risk of coronary microvascular dysfunction, and a 7% increased risk of future serious heart disease. Alarmingly, excess muscle fat has been found to increase mortality risk as much as smoking or having type 2 diabetes.
Insulin Resistance and Metabolic Syndrome
Excessive fat infiltration can trigger chronic inflammatory pathways and alter glucose metabolism within muscle tissue. This can lead to insulin resistance, where the body's cells stop responding effectively to insulin. Insulin resistance is a key component of metabolic syndrome, a cluster of conditions that includes high blood pressure, high blood sugar, excess body fat around the waist, and abnormal cholesterol or triglyceride levels.
Impact on Muscle Function
Myosteatosis is gaining recognition for its role in the loss of muscle mass, strength, and function, particularly in aging populations. This is because the fat infiltration can interfere with normal muscle metabolic and contractile functions, ultimately contributing to a decline in physical performance and overall quality of life.
Factors Contributing to Skeletal Fat Accumulation
Several factors influence the accumulation of fat within muscle tissue:
- Age: Muscle mass naturally decreases with age, a process known as sarcopenia. As muscle fibers atrophy, fat can infiltrate the vacant spaces.
- Sedentary Lifestyle: A lack of regular physical activity and a sedentary lifestyle contribute significantly to the buildup of fat in muscle tissue.
- Poor Diet: Diets high in saturated fat, processed foods, and added sugars promote overall fat gain, including myosteatosis.
- Obesity: While not the sole determinant, individuals who are overweight or obese often have higher levels of skeletal fat.
- Genetics: Individual genetic factors can influence how and where a person's body stores fat.
Measuring Skeletal Fat
While you cannot see skeletal fat, it can be accurately measured using advanced imaging techniques, unlike simpler methods like BMI, which can be misleading. These methods include:
- Dual-Energy X-ray Absorptiometry (DEXA): Considered a gold standard, DEXA provides detailed scans that can differentiate between fat mass, lean mass, and bone density, including regional analysis of limbs and trunk.
- Computed Tomography (CT) and Magnetic Resonance Imaging (MRI): These advanced imaging techniques are highly accurate for quantifying and distinguishing between subcutaneous, visceral, and intermuscular fat depots.
- Bioelectrical Impedance Analysis (BIA): While less precise than imaging scans, medical-grade BIA devices can provide estimates of segmental body composition, including fat and lean mass distribution.
Strategies to Reduce Skeletal Fat
Reducing skeletal fat requires a comprehensive approach targeting both diet and exercise.
Adopt a High-Protein Diet
A diet rich in lean protein is crucial for preserving and building muscle mass, especially when in a calorie deficit. Protein also increases satiety and requires more energy to digest than carbohydrates or fats, aiding fat loss.
Prioritize Strength Training
Resistance training, using weights, bands, or bodyweight, is one of the most effective ways to combat muscle atrophy and reduce fat infiltration. It builds lean muscle tissue, which is metabolically active and can replace fatty tissue within the muscle.
Incorporate Aerobic Exercise
Regular cardio sessions, such as brisk walking, cycling, or swimming, help to burn calories and mobilize fat stores from all over the body, including intramuscular stores.
Optimize Your Sleep
Poor sleep quality and duration are linked to increased fat mass and weight gain. Aiming for 7+ hours of quality sleep per night can help regulate hormones that influence appetite and fat storage.
Reduce Stress Levels
High stress can elevate the hormone cortisol, which is known to promote the accumulation of visceral and other types of fat. Engaging in stress-reducing activities like yoga or meditation can be beneficial.
Skeletal Fat vs. Other Body Fat Types: A Comparison
| Feature | Skeletal Fat (Myosteatosis) | Visceral Fat | Subcutaneous Fat |
|---|---|---|---|
| Location | Within and between muscle tissue | Deep in the abdominal cavity, surrounding organs | Just beneath the skin throughout the body |
| Visibility | Hidden from view, requires imaging | Not visible externally, increases waist circumference | Visible and "pinchable" on areas like the abdomen and thighs |
| Health Risk | High; linked to cardiovascular disease, insulin resistance, higher mortality | High; strongly linked to heart disease, type 2 diabetes, inflammation | Generally lower than visceral or skeletal fat; acts as energy storage and insulation |
| Metabolic Role | Contributes to inflammation and impaired glucose metabolism within muscle | Secretes inflammatory hormones that negatively impact metabolic health | Largely serves as a passive energy reservoir; some protective effects observed |
Conclusion
Skeletal fat, or myosteatosis, represents a potentially serious health risk that is often overlooked because it is not visible from the outside. Its accumulation, particularly intermuscular fat, is independently associated with an increased risk of cardiovascular disease, insulin resistance, and overall mortality. Unlike simple body fat measurements like BMI, a proper assessment of myosteatosis requires advanced imaging techniques such as DEXA or MRI. The good news is that this type of fat can be managed and reduced through consistent lifestyle changes. Combining regular strength training and aerobic exercise with a healthy, high-protein diet is the most effective strategy to preserve muscle mass, improve metabolic health, and reduce the silent threat of skeletal fat infiltration. For more information on promoting long-term fat loss, visit this guide on Healthline: 12 Ways to Promote Long-Term Fat Loss.
