People with fat deposits hidden in their muscles have a higher risk of dying or being hospitalized from a heart attack or heart failure, regardless of their body mass index. This is the result of a study published in the European Heart Journal. This “intermuscular” fat is very popular when frying. However, little is known about this type of body fat in humans and its effects on health. This is the first study to comprehensively investigate the effects of fatty muscle tissue on heart disease. The new finding provides further evidence that existing measures such as body mass index or waist circumference are not sufficient to accurately assess the risk of heart disease for all people.
How Obesity Affects Heart Health
The new study was 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 in Boston, USA. She said: “Obesity is now one of the greatest global threats to cardiovascular health, yet body mass index – our primary measure for defining obesity and thresholds for intervention – remains a controversial and flawed marker of cardiovascular prognosis. This is particularly true for women, where a high body mass index may reflect more ‘benign’ types of fat.”
Intermuscular fat can be found in most muscles of the body, but the amount of fat can vary greatly between different people. In their research, the scientists analyzed muscle and different types of fat to understand how body composition may affect the small blood vessels or “microcirculation” of the heart as well as the future risk of heart failure, heart attack and death. The new study included 669 people who were evaluated at Brigham and Women’s Hospital for chest pain and/or shortness of breath, and who were found to have no signs of obstructive coronary artery disease (in which the arteries that supply the heart become dangerously clogged). These patients were on average 63 years old. The majority (70%) were women and almost half (46%) of the women were non-white.
All patients underwent a cardiac positron emission tomography/computed tomography (PET/CT) scan to assess heart function. The researchers also used CT scans to analyze each patient’s body composition by measuring the amount and location of fat and muscle in a section of the trunk. To quantify the amount of fat stored in the muscles, the researchers calculated the ratio of intermuscular fat to total muscle mass plus fat, a measurement they called fat-muscle fraction. The subjects were followed up for about six years and the researchers documented whether they died or were hospitalized for a heart attack or heart failure.
Intermuscular Fat Increases the Risk of Heart Disease
The scientists found that people with higher levels of fat stored in their muscles were more likely to have damage to the tiny blood vessels that supply the heart (coronary microvascular dysfunction or CMD) and were more likely to die or be hospitalized for heart disease. For every 1% increase in muscle fat, the risk of coronary microvascular disease increased by 2%, and the risk of future major heart disease increased by 7%, independent of other known risk factors and body mass index. People with high levels of intermuscular fat and signs of coronary microvascular disease had a particularly high risk of death, myocardial infarction and heart failure.
In contrast, people with a higher proportion of lean muscle had a lower risk. Fat stored under the skin (subcutaneous fat) did not increase the risk. Compared to subcutaneous fat, fat stored in the muscles can contribute to inflammation and altered glucose metabolism, leading to insulin resistance and metabolic syndrome, according to Professor Taqueti. This chronic damage can in turn damage blood vessels, including those that supply the heart, and the heart muscle itself. Since the researchers know that intermuscular fat increases the risk of heart disease, they have another way to identify people at high risk, regardless of their body mass index. These findings may be particularly important in understanding the impact of fat- and muscle-modifying incretin-based therapies, including the new class of glucagon-like peptide-1 receptor agonists, on heart health.
Identify Patients Who May Benefit Prognostically from Targeted Cardiometabolic Interventions
Professor Taqueti and her team are investigating the effects of treatment strategies such as exercise, diet, weight loss medications or surgery on body composition and metabolic heart disease. In an accompanying editorial, Dr. Ranil de Silva of Imperial College London and his colleagues write, “Obesity is a public health priority. Epidemiologic studies clearly show that obesity is associated with increased cardiovascular risk, although this association is complex. In this issue of the journal, Souza and colleagues hypothesize that the quantity and quality of skeletal muscle is associated with CMD and modifies its impact on the development of future adverse cardiovascular events independent of body mass index (BMI).
These interesting results are hypothesis-generating, and should be interpreted in the context of several limitations. This is a retrospective observational study. While a number of potential mechanisms are suggested to explain the association between increased intramuscular adipose tissue (IMAT) and impaired coronary flow reserve, these were not directly investigated. In particular, no details were provided on circulating inflammatory markers, insulin resistance, endothelial function, nutrition, skeletal muscle physiology or exercise performance. The data presented by Souza et al highlight patients with CMD in particular as a patient group at increased clinical risk. Their work should stimulate further research to determine the added value of obesity markers for conventional and emerging cardiac risk stratification (identifying conditions that negatively impact disease) to identify those patients who may benefit prognostically from targeted cardiometabolic interventions.