People with hidden pockets of fat in their muscles – similar to so-called marbling in beef – are at a higher risk of dying from heart attacks or heart failure regardless of their body weight, according to a study. While “fatty marbling”, or flecks of fat running through meat, is commercially valued in livestock, in humans it has been associated with insulin resistance and type 2 diabetes, researchers said.
It is not yet known how to reduce the risk of heart problems in those with fatty muscles, although academics suggest their findings could be important for ongoing studies looking at the impact of weight loss drugs on heart health. For the study, US researchers looked at 669 people being evaluated at the Brigham and Women’s Hospital in Boston for chest pain and/or shortness of breath, but were found to have no evidence of obstructive coronary artery disease, a condition where the arteries that supply the heart become clogged.
The group were given scans to assess their heart function, while researchers also used CT scans to analyse body composition and measure the amounts and location of fat and muscle in sections of their torso. To determine the amount of fat stored in muscles, the team calculated the ratio of intermuscular fat to total muscle plus fat used a measurement they called the fatty muscle fraction.
Professor Viviany Taqueti, director of the cardiac stress laboratory at Brigham and Women’s Hospital, and faculty at Harvard Medical School, said: “Intermuscular fat can be found in most muscles in the body, but the amount of fat can vary widely between different people. In our research, we analyse muscle and different types of fat to understand how body composition can influence the small blood vessels or ‘microcirculation’ of the heart, as well as future risk of heart failure, heart attack and death.”
Those with higher amounts of fat stored in their muscles were more likely to have coronary microvascular dysfunction (CMD), a condition that affects the small blood vessels in the heart. They were also more likely to go to hospital or die from heart disease.
According to the study, for every 1% increase in fatty muscle fraction, there was a 2% increased risk of CMD and a 7% increase in the risk of serious heart disease in the future. Prof Taqueti suggested that unlike subcutaneous fat – or fat stored under the skin – fat stored in the muscles may be contributing to inflammation and insulin resistance.
“In turn, these chronic insults can cause damage to blood vessels, including those that supply the heart, and the heart muscle itself,” she added. Knowing that intermuscular fat raises the risk of heart disease gives us another way to identify people who are at high risk, regardless of their body mass index.”
Prof Taqueti said it is not yet known how to lower the risk for people with fatty muscles. “For example, we don’t know how treatments such as new weight-loss therapies affect fat in the muscles relative to fat elsewhere in the body, lean tissue, and ultimately the heart,” she added.
She said the findings, published in the European Heart Journal, could be “particularly important” for ongoing studies looking at the “heart health effects of fat and muscle-modifying incretin-based therapies”, including weight loss jabs. Researchers suggest it may be possible that the loss of lean muscle mass as a result of these drugs, known as glucagon-like peptide-1 receptor agonists, may also involve a loss of intermuscular fat.
Prof Taqueti added: “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.
“This is especially true in women, where high body mass index may reflect more ‘benign’ types of fat.”