Individuals with fat stored within their muscles have a greater risk of dying or being hospitalized due to heart attacks or heart failure, irrespective of their body mass index (BMI), as revealed by a new study published today in the European Heart Journal.
This type of fat, known as ‘intermuscular fat’, is often sought after for enhancing the flavor of beef steaks during cooking. However, there is limited knowledge about the implications of this fat on human health. This study is the first to thoroughly explore how fatty muscles relate to heart disease.
The results of this research support the notion that traditional assessments, such as BMI and waist size, do not accurately reflect heart disease risk for everyone.
The study was directed by Professor Viviany Taqueti, who oversees the Cardiac Stress Laboratory at Brigham and Women’s Hospital and serves on the faculty at Harvard Medical School in Boston, USA. She stated: “Obesity currently stands as a significant global health issue concerning cardiovascular well-being, yet BMI remains a debated and inadequate measure for establishing obesity and intervention criteria. This is especially applicable to women, where high BMI could represent less harmful fat types.”
“Intermuscular fat is commonly found in various muscles throughout the body, but the quantity can significantly differ from one person to another. Our research analyzes muscle tissue and various fat types to discern how body composition might affect the heart’s microcirculation and its risk for heart failure, heart attacks, and mortality.”
The investigation involved 669 participants undergoing evaluation at Brigham and Women’s Hospital for symptoms such as chest pain or shortness of breath but exhibiting no signs of obstructive coronary artery disease (a condition where the arteries supplying the heart become dangerously clogged). The average age of participants was 63 years, with a majority being female (70%) and nearly half (46%) identifying as non-white.
All participants underwent cardiac positron emission tomography/computed tomography (PET/CT) scanning to evaluate their heart function. Furthermore, CT scans were utilized to analyze each individual’s body composition by measuring fat and muscle distribution within their torso.
To determine the amount of fat within the muscles, researchers calculated the ratio of intermuscular fat compared to total muscle and fat, a measurement termed the fatty muscle fraction.
Participants were monitored for approximately six years, during which any instances of death or hospitalization due to heart attacks or heart failure were documented.
Findings indicated that individuals with larger amounts of muscle fat had a higher likelihood of damage to the small blood vessels that supply the heart (referred to as coronary microvascular dysfunction or CMD) and were more inclined to experience mortality or hospitalization due to heart-related issues. Specifically, for each 1% rise in fatty muscle fraction, there was a 2% increase in CMD risk and a 7% rise in the likelihood of severe heart disease, regardless of other risk factors and BMI.
Those possessing substantial intermuscular fat along with CMD showed the highest risk for death, heart attacks, and heart failure. Conversely, individuals with more lean muscle displayed a reduced risk. Fat located just below the skin (subcutaneous fat) did not correlate with an increased risk.
Professor Taqueti mentioned: “In contrast to subcutaneous fat, fat stored within the muscles may be linked to inflammation and disrupted glucose metabolism resulting in insulin resistance and metabolic syndrome. These ongoing issues can harm the blood vessels, including those supplying the heart and the heart tissue itself.”
“Understanding that intermuscular fat heightens heart disease risk provides another method for identifying individuals at high risk, irrespective of their BMI. These insights could be particularly significant for grasping the heart health impacts of therapies that modify fat and muscle, including the emerging category of glucagon-like peptide-1 receptor agonists.”
“What remains unknown is how to mitigate risks for those with fat in their muscles. For instance, there’s uncertainty about how modern weight-loss treatments influence muscle fat compared to fat in other body areas, lean tissue, and ultimately, the heart.”
Professor Taqueti and her colleagues are currently evaluating the effects of treatment strategies, including exercise, nutrition, and weight-loss medications or surgeries, on body composition and heart-related metabolic diseases.
In an accompanying editorial, Dr. Ranil de Silva from Imperial College London and his colleagues noted: “Obesity is a primary public health concern. Epidemiological studies convincingly demonstrate that obesity correlates with a heightened cardiovascular risk, although this relationship is intricate.”
“In this journal issue, Souza et al. propose that the quality and quantity of skeletal muscle are linked with CMD and modify its impact on future cardiovascular risks independent of BMI.”
“Among this patient group, primarily composed of females with a high prevalence of obesity, the main findings indicated that increasing intermuscular adipose tissue (IMAT) levels were connected to higher instances of CMD, and the coexistence of both elevated IMAT and CMD resulted in the highest rates of future cardiovascular events, independent of BMI.”
“The intriguing results from Souza et al. provoke further inquiry and should be considered within the context of various limitations. This study is retrospective and observational. Although a number of potential mechanisms were proposed to explain the association between elevated IMAT and compromised coronary flow reserve, these were not directly assessed. Specifically, data related to circulating inflammatory biomarkers, insulin resistance, endothelial function, diet, skeletal muscle physiology, or exercise performance were absent.”
“The findings presented by Souza et al. are compelling and crucially underscore the clinical risk presented by patients with CMD. Their research should encourage deeper investigations into the added value of adiposity markers in conjunction with established and emerging heart disease risk assessments, aiming to identify patients who may gain prognostic benefits from targeted cardiometabolic interventions.”
