A new study published in the European Heart Journal on Monday reveals that people with fat stored inside their muscles face an increased risk of heart attacks, heart failure, and death, regardless of their body mass index (BMI). This type of fat, known as intermuscular fat, has long been known for its presence in beef steaks but remains understudied in terms of its health implications in humans.
This research is the first comprehensive investigation into the impact of intermuscular fat on heart disease, providing valuable insights into its potential risks. The findings challenge the reliance on traditional metrics like BMI and waist circumference to assess heart disease risk, suggesting they may not fully account for all individuals’ health risks.
The study, led by Professor Viviany Taqueti, Director of the Cardiac Stress Laboratory at Brigham and Women’s Hospital and a faculty member at Harvard Medical School, highlights the limitations of BMI as a marker of cardiovascular health. “Obesity is one of the leading global threats to cardiovascular health, yet BMI, our primary measure for defining obesity, remains an imperfect and sometimes misleading tool, especially in women,” Taqueti said. She emphasized that BMI may sometimes reflect “benign” fat types that don’t correlate with higher cardiovascular risk.
Intermuscular fat can accumulate in muscles throughout the body, but its quantity varies significantly among individuals. This study explored how muscle composition, particularly fat content, affects the small blood vessels (microcirculation) of the heart, and how this, in turn, relates to the risk of heart failure, heart attacks, and mortality.
The study examined 669 patients undergoing evaluation at Brigham and Women’s Hospital for chest pain and shortness of breath. These patients, with an average age of 63, had no signs of obstructive coronary artery disease. The majority of the participants (70%) were female, and nearly half (46%) were non-white. Researchers utilized advanced cardiac imaging techniques, including positron emission tomography/computed tomography (PET/CT), to assess heart function and analyzed body composition via CT scans to measure fat and muscle distribution.
To quantify intermuscular fat, the team developed a metric called the “fatty muscle fraction,” which is the ratio of intermuscular fat to the total muscle and fat in a section of the torso. Over an average follow-up period of six years, researchers tracked patient outcomes, focusing on heart disease-related hospitalizations and deaths.
The results were striking: higher levels of intermuscular fat correlated with an increased risk of coronary microvascular dysfunction (CMD), a condition where the small blood vessels of the heart are damaged. For every 1% increase in fatty muscle fraction, the risk of CMD rose by 2%, while the risk of future heart disease-related events increased by 7%, irrespective of other risk factors or BMI.
Patients with high intermuscular fat and evidence of CMD were found to be at an especially high risk of death, heart attacks, and heart failure. In contrast, individuals with higher levels of lean muscle mass had a lower risk. Subcutaneous fat, which lies under the skin, did not show a similar correlation with heart disease risk.
The study suggests that intermuscular fat may contribute to inflammation and disrupt glucose metabolism, leading to insulin resistance and metabolic syndrome—factors that can damage blood vessels, including those supplying the heart, and impair heart muscle function. This underscores the importance of considering intermuscular fat in heart disease risk assessments.
These findings offer a new way to identify individuals at high cardiovascular risk, independent of BMI, and could be especially useful for evaluating the effects of new medications designed to alter fat and muscle composition, such as glucagon-like peptide-1 receptor agonists.
However, the study leaves open the question of how to mitigate the risks associated with intermuscular fat. “We don’t yet know how different treatments, including weight-loss therapies, impact fat in muscles versus other parts of the body and how this affects heart health,” said Professor Taqueti.
Alongside the study, an editorial by Dr. Ranil de Silva from Imperial College London discusses the complexity of obesity’s relationship with cardiovascular health. He notes that while obesity is linked to increased cardiovascular risk, the new study’s findings suggest that muscle fat quality and quantity may significantly influence this relationship, independent of BMI. However, Dr. de Silva also points out that this retrospective observational study has several limitations, such as the lack of data on inflammatory biomarkers and other potential factors like diet and exercise, which could further elucidate the mechanisms behind the observed links between fat and heart disease.
The study’s findings suggest that further research is needed to establish how fat distribution in muscles and other parts of the body can help identify individuals at higher cardiovascular risk, potentially paving the way for more personalized and effective treatments.
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