A lot happens inside the body long before anything shows up on the outside. A new study suggests one of those quiet changes may sit inside the muscles themselves and it links to the same health markers doctors watch closely for heart disease risk.
Researchers behind the study, analysed MRI scans from around 11,000 adults and looked at the make-up of their muscle tissue. They found that people with more fat stored in and around their muscles were more likely to have high blood pressure, elevated blood sugar, type 2 diabetes and less healthy cholesterol levels. People with less fat in muscle and more lean muscle tended to show stronger heart-health markers.
“Normally, muscles are mostly made of muscle fibres. But over time, fat can slowly build up inside and around the muscle tissue,” explains cardiologist Dr Bharat Sangani. “One important point: someone can look normal weight on the outside, but still have unhealthy fat stored inside their muscles.”
What “hidden” fat in muscle actually means
Body fat does not only sit under the skin. It can also settle deeper, including within muscle tissue. When that happens, the muscle is not purely “muscle” anymore. It becomes a mix of lean tissue and fat, which may influence how the body handles blood sugar and fats in the bloodstream.
Sangani says this type of fat build-up tends to show up more often in people who move less, gain weight over time, eat a less balanced diet or live with diabetes or insulin resistance. Age matters too. Lean muscle naturally declines as the years stack up and that creates more opportunity for fat to move in.
In practical terms, this can mean someone looks fine in clothes, feels mostly okay day to day, yet still carries a higher cardiometabolic risk profile beneath the surface. Muscle quality becomes part of the story, not only total body weight.
What is lean muscle mass, and how can you build more
Lean muscle mass is the muscle tissue on your body that helps you move, stabilise joints and stay strong day to day. It is also “metabolically active tissue,” explains registered dietitian nutritionist and board-certified specialist in sports dietetics Roxana Ehsani. “The more lean muscle mass we have, the more calories we can burn at rest.”
But the bigger win is what lean muscle does behind the scenes. Lean muscle helps pull sugar out of the bloodstream, supports insulin sensitivity and can reduce inflammation, says Dr Sangani. Those shifts connect directly to heart health because high blood sugar, insulin resistance and chronic inflammation can all nudge cardiovascular risk higher over time. Ehsani adds that muscle is also tied to healthy ageing in general: “Research shows that individuals with higher levels of muscle mass tend to live longer and experience fewer age-related health issues.”
The good news is muscle stays trainable. Building more does not require extreme workouts, but it does require two basics done consistently. “The two-ingredient recipe to gaining more lean muscle mass is by first, engaging in resistance training at least twice a week and secondly, consuming enough high-quality sources of protein throughout the day,” Ehsani says. Strength work gives the body a reason to build and maintain muscle. Protein gives it the building blocks to repair and adapt. Ehsani suggests weaving strength exercises into weekly workouts and using high-protein foods throughout the day to support recovery and growth.
The bottom line
“The big lesson is: health is not just about body weight. It’s also about muscle quality,” says Dr Sangani. “Even modest increases in muscle activity can improve long-term heart and metabolic health.”
Newsletter
