Scientists uncover new heart risk hiding between your muscle fibers

Harvard study reveals potentially fatal flaw in how we measure obesity risk

BOSTON — Here’s something to think about the next time you’re choosing a steak: that prized marbling of fat running through the meat might be delicious, but when similar fat patterns appear in human muscles, it could signal serious health concerns.

New research published in the European Heart Journal demonstrates that people with higher amounts of fat hidden between their muscle fibers face greater risks of heart problems and death – and this risk exists regardless of their overall weight or body mass index (BMI). This discovery challenges our traditional understanding of how body fat affects heart health.

“Obesity is now one of the biggest global threats to cardiovascular health, yet body mass index – our main metric for defining obesity and thresholds for intervention – remains a controversial and flawed marker of cardiovascular prognosis,” explains Professor Viviany Taqueti, who led the research as Director of the Cardiac Stress Laboratory at Brigham and Women’s Hospital and Faculty at Harvard Medical School, in a statement. “This is especially true in women, where high body mass index may reflect more ‘benign’ types of fat.”

Think of your muscles like a marbled steak – some people have more fat woven between the muscle fibers than others. Scientists call this intermuscular adipose tissue (IMAT), and the study found it might be particularly dangerous when combined with poor blood flow in the tiny vessels of the heart. Over six years of following 669 patients, researchers discovered that for every 1% increase in the amount of fat within muscle tissue, there was a 7% higher risk of serious heart problems – including heart attacks, heart failure, and death.

The study looked at a diverse group of patients who came to the hospital with chest pain or shortness of breath but showed no signs of blocked arteries – a surprisingly common condition that particularly affects women. Most participants (70%) were female, nearly half (46%) were non-White, and the average age was 63 years. About 46% of the participants were considered obese by traditional measures.

Using advanced imaging technology during routine heart scans, the research team created detailed maps of where fat was stored in the body. They looked at three main types: fat under the skin (the kind you can pinch), fat between muscles, and fat around internal organs. This comprehensive approach revealed something surprising – not all body fat affects health the same way.

“Compared to subcutaneous fat, fat stored in muscles may be contributing to inflammation and altered glucose metabolism leading to insulin resistance and metabolic syndrome,” says Professor Taqueti. “In turn, these chronic insults can cause damage to blood vessels, including those that supply the heart, and the heart muscle itself.”

In simpler terms, fat between muscles might trigger harmful changes in how the body processes sugar and responds to inflammation, ultimately damaging the heart’s blood vessels.

The study found that the traditional ways we measure obesity risk – like BMI or waist size – might miss important warning signs. Surprisingly, the fat layer under the skin (subcutaneous fat) showed some protective effects, while the hidden fat between muscles emerged as a stronger predictor of heart problems.

The research team paid special attention to how well the small blood vessels in the heart were working, measuring something called coronary flow reserve (CFR) – essentially, how effectively the heart’s blood vessels can increase blood flow when needed. Patients who had both high levels of intermuscular fat and poor vessel function were at the highest risk, with their annual risk of serious heart problems reaching 5.1% – significantly higher than those with just one or neither of these conditions.

These findings could help explain why some seemingly healthy-weight people develop heart problems while some overweight individuals remain heart-healthy. The discovery is particularly timely given the growing use of new weight-loss medications. “These findings could be particularly important for understanding the heart health effects of fat and muscle-modifying incretin-based therapies, including the new class of glucagon-like peptide-1 receptor agonists,” notes Taqueti.

The research team is now studying how different approaches – including exercise, diet changes, weight-loss medications, and surgery – might affect this dangerous fat distribution. They don’t yet know which treatments might work best for reducing intermuscular fat or whether reducing it would definitely improve heart health.

For doctors and patients alike, these findings suggest we need to look beyond the bathroom scale when assessing heart disease risk. This could be especially important for women and people without obvious signs of heart disease, who might benefit from more sophisticated screening approaches that consider both muscle quality and blood vessel function.