Slim and thin people are at a higher risk of a deadly heart attack, a new study conducted by Harvard Medical School revealed. According to researchers, it can happen due to hidden pockets of fat in their muscles.

Scientists compared this type of intramuscular fat to marbling in beef, which makes it juicy and flavourful. Even though Wagyu beef, these luxury cuts, are the most sought-afin humans, it can have serious health consequences.

Researchers also said women with this type of fat were at higher risk of dying from heart attack or heart failure regardless of their body mass index. In fact, every one per cent increase in the amount of fat stored in muscles increases the risk of serious heart conditions by 7 per cent.

The study said those with higher amounts of lean muscle had a lower risk. However, researchers said any kind of fat stored under the skin did not increase the risk of any deadly heart condition. Fat stored under the skin didn't increase the risk of a heart attack.

According to the research, more than 650 people—both men and women—were evaluated at the Brigham and Women's Hospital in Boston for chest pain and breathlessness 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 was given scans to assess their heart function, with the researchers using CT scans to analyse body composition and measure the amounts and location of fat and muscle in sections of their torso.

"Intermuscular fat can be found in most muscles in the body, but the amount of fat can vary widely between different people,” said Professor Viviany Taqueti, director of the cardiac stress laboratory at Brigham and Women's Hospital and a faculty member at the Harvard Medical School.

"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,” he added.

Those with higher amounts of fat stored in their muscles were more likely to have coronary microvascular dysfunction, a condition that affects the small blood vessels in the heart.

Researchers say for every 1 per cent increase in fatty muscle fraction, there was a 2 per cent increased risk of CMD and a 7 per cent increase in the risk of serious heart disease in the future. Experts also said that unlike subcutaneous fat—or fat stored under the skin—fat stored in the muscles may be contributing to inflammation and insulin resistance.

The findings of the study, published in the European Heart Journal, said it could be "particularly important" for ongoing studies looking at the impact of weight loss drugs on heart health. Researchers suggest it may be possible that the loss of lean muscle mass as a result of these drugs may also trigger the loss of fat hidden in muscles.

Scientists said the metric for defining obesity and thresholds for intervention remains a controversial and flawed marker of cardiovascular prognosis, especially in women, where a high body mass index may reflect more 'benign' types of fat.

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