For years, health experts have debated if Body Mass Index (BMI) is the best measure of a person's health. While waist size has been highlighted as a key factor in predicting heart disease, a recent study reveals that neither BMI nor waist measurement is the ultimate predictor.

Instead, a hidden factor, intermuscular fat, the fat stored inside muscles might be a more accurate indicator of heart disease risk. Researchers noted that those having higher amounts of this particular type of fat face a greater risk of death and hospitalization from heart attacks or heart failure, regardless of BMI or waist size.

"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," Professor Viviany Taqueti, who led the study said in a news release.

The study analyzed how different muscle and fat compositions affected the small blood vessels or "microcirculation" of the heart and influenced the risk of developing heart failure, heart attack, and death.

The research involved 669 patients at Brigham and Women's Hospital, with an average age of 63, who were assessed for chest pain or shortness of breath but had no evidence of obstructive coronary artery disease.

The patients underwent cardiac PET/CT scans to evaluate heart function and CT scans to analyze body composition, including fat and muscle distribution in the torso. Researchers introduced a new measurement called the fatty muscle fraction, which quantifies the ratio of intermuscular fat to total muscle and fat.

The participants were followed up for around six years to check for outcomes including hospitalization and deaths from a heart attack or heart failure.

The analysis revealed that higher levels of fatty muscle fraction were linked to a 2% increased risk of coronary microvascular dysfunction (CMD) and a 7% higher risk of future serious heart disease, with every 1% increase in fatty muscle fraction, regardless of other risk factors and BMI.

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

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