Hidden fat in children’s muscles: what it reveals about their cardiometabolic health

The hidden side of childhood fat

Today, more than 159 million children worldwide are living with obesity, making it one of the greatest health challenges of the 21st century. Childhood obesity is linked to serious conditions such as cardiovascular diseases, diabetes, and even some types of cancer, which is why prevention and treatment are crucial for the health and quality of life of future generations.

When we talk about obesity, it is often defined as abnormal or excessive fat accumulation, usually measured by the body mass index BMI. However, BMI does not tell us the whole story. Not all fat in the body behaves in the same way, and where fat is stored might make a big difference for our health. Fat can be accumulated in several organs and tissues – for example, the heart, liver, pancreas, or muscles.  

In recent years, researchers have been particularly interested in the fat stored within muscles, called intermuscular adipose tissue (IMAT). While invisible from the outside, it plays a significant role in health. In adults, high levels of IMAT have been linked to insulin resistance, type 2 diabetes, cardiovascular disease, and even a higher risk of mortality. IMAT refers to fat depots interspersed between muscle fibers or muscle groups. For instance, studies show that just a modest increase in IMAT is associated with a 40% higher risk of mortality over 10 years.

Why does this matter? Because muscles are one of the body’s main engines for controlling blood sugar. They are the largest insulin-sensitive tissue in the body and are crucial for regulating energy balance and metabolism. When muscle quality decreases-whether due to aging, obesity, or lifestyle factors - it can contribute to disability, frailty, and serious health complications.

Looking inside children’s muscles

But… does the link between IMAT and health problems already exist at childhood? To answer this question, more scientific evidence is needed.

In one of our earlier studies, we observed that IMAT was linked to metabolic dysfunction-associated steatotic liver disease (MASLD) in children with overweight or obesity, highlighting the importance of measuring the fat hidden in the muscles.

Our recent research, with 170 children - most with overweight or obesity - from two research projects conducted in Spain (EFIGRO and MICROKID), used advanced MRI scans to look inside their abdominal muscles. When we compared these images with measures of the children’s cardiometabolic health, such as their blood lipids, blood pressure, and insulin resistance, we found something striking. Children who carried more fat within their muscles, -or had less muscle relative to their body size - already showed signs of poorer cardiometabolic health. What’s more, this pattern was independent of their total body fat. In other words, two children with the same body fat percentage could have different health profiles, depending on what was happening inside their muscles. Furthermore, the combination of poor skeletal muscle quality and low quantity, placed children at the greatest risk, contributing to a poor cardiometabolic health profile.

Taken together, these findings highlight that the story of obesity in children is more complex than it appears from the outside. It’s not only about how much fat a child has, but also where that fat is stored.

Stronger muscles, healthier futures

If hidden fat as IMAT can already harm children’s health, the next question is: can we do something about it? Encouragingly, the answer is yes.

In one of our recent studies, we found that adding a 90-minute supervised program of aerobic and strength exercise 3 times a week to a family-based lifestyle and psychoeducational program made a real difference. Children who took part not only reduced the fat accumulated within their muscles by about 6%, but also lowered other dangerous fat depots, such as visceral fat and pancreatic fat, by as much as 18–20%. These results show that targeted exercise can improve muscle quality and reduce unhealthy fat storage, even in children already living with overweight or obesity.

The message is clear: building strong, healthy muscles from an early age matters. Regular physical activity, balanced nutrition, good sleep, and supportive family environments help children not only grow fitter but also protect their metabolic health in the long run. By encouraging healthy habits in childhood, we give them the foundation for a healthier adulthood. For families, educators, and healthcare professionals, the takeaway is simple but powerful: prevention starts young. Promoting exercise and healthy lifestyle behaviors early in life is not just about helping children manage their weight - it’s about safeguarding their future health, reducing the risk of serious diseases.

 Reference: 

Abdominal Skeletal Muscle Quality and Quantity are Independently and Jointly Associated with Cardiometabolic Health in Children. Miranda-Ferrua E, Cadenas-Sanchez C, Medrano M, Ruiz JR,  Idoate F, Osés M,  Concepción M, Cabeza R, Labayen I. International Journal of Obesity, 2025.