By ERIN DEAN
Published: 06:30 EDT, 1 April 2025 | Updated: 06:34 EDT, 1 April 2025
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.
Professor Viviany Taqueti, director of the cardiac stress laboratory, who led the Boston research
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.
‘Fat stored under the skin [the ‘cuddly’ fat’] is less likely to be associated with risk of heart disease,’ says Professor Taqueti.
‘It is quite benign and doesn’t seem to do much. While fat around or inside organs tends to be much more active: it can secrete hormones and chemical messengers into the blood that impacts how well the body can use up energy, which can have a major impact on health.’
Healthy muscles play a key role in how efficiently the body uses energy, by mopping up glucose from the blood. This glucose is turned into fuel for the muscles.
One theory is that if the body becomes less good at moving glucose into cells (which occurs from the age of 45, or if you are obese or have type 2 diabetes), some of this may be laid down as fat in the muscles. This then becomes a vicious circle, as fatty muscles make it harder to use glucose as fuel, as Francis Stephens, a professor of exercise metabolism and physiology at the University of Exeter, explains: ‘Muscles are large and a major place for the body to store glucose, but the fat seems to stop this happening so well,’ he says.
‘The fat within muscles appears to release a shower of chemical messengers [called cytokines] and toxic substances that disrupt the ability of insulin molecules – the hormone which helps glucose move from the blood – to dock with the walls of muscle cells,’ he says. As a result, glucose stays in the blood, pushing up blood sugar levels – raising the risk of type 2.
Higher levels of fat in muscles may also reduce their strength, says Professor Stephens. ‘Having fat may actually impair the ability of the muscle fibres to contract.’
Fat in muscles increases with age, and a number of studies have found people with higher levels are more likely to have falls and find moving and walking harder when older, he adds.
Currently you can’t find out your level of fatty muscle, as it can only be calculated by researchers using CT or other scans.
But the usual healthy lifestyle advice – exercising and maintaining a healthy weight, including losing weight if needed – will reduce the fat in your muscles.
People who are obese or have type 2 diabetes and go on weight loss and exercise programmes shed muscle fat, along with fat elsewhere, says Dr Bret Goodpaster, scientific director at the Advent Health Translational Research Institute, which leads hundreds of clinical trials a year, in Florida.
‘We took two groups of older people between 70 and 89, one started a walking exercise programme, which they were encouraged to build up to five times a week, and the other group didn’t,’ he says. ‘After a year, those who exercised had not increased the fat levels in their thigh muscles while those who didn’t had.’
Currently, the evidence suggests any exercise, including lifting weights or aerobic exercise (swimming, running or walking) has a positive impact on reducing fat in muscles, he says. This is possibly because muscles can take more glucose from the blood when you’re moving, and having lower blood sugar levels may prevent intermuscular fat accumulating or lead to it being used to create energy, says Dr Goodpaster.
One area that doctors are interested to know more about is what impact new weight-loss injections have on this fat. There’s been some concern about the loss of muscle with these jabs, with up to 60 per cent of the weight lost potentially being lean tissue, according to a paper looking at the issue in Diabetes, Obesity and Metabolism journal last June.
This raises concerns about losing both strength and the calorie-burning power of muscle tissue which is more likely to help people maintain weight loss in the long term.
But, in fact, it could be that some of what is being lost from the muscles is actually this harmful fat, rather than just lean muscle, suggests Professor Taqueti. Professor Stephens adds: ‘This would be a good thing. But it is too early to say. Those studies are ongoing.’