By Dr. Asnat Raziel, bariatric surgeon specializing in obesity treatment, metabolic health and healthy longevity; Director of the Multidisciplinary Center for Obesity Treatment, Assia Medical, Assuta Ramat HaChayal, Tel Aviv
Fat in muscle: why muscle size is only half the story
Fat in muscle is a health marker most of us have never heard of. Strong muscles are one of our most important assets for healthy aging. They help us move, keep our balance and protect our bones. They also regulate blood sugar and help us stay independent as we get older.
However, a new study in the journal Aging Cell, from September 2026, sharpens an important point. The amount of muscle is only part of the picture. What’s inside the muscle matters too, especially how much fat seeps into it. That number may hint at our health and even our life expectancy.
The fat hiding inside the muscle
The term myosteatosis describes fat building up within muscle tissue and between muscle fibers. It’s hard to see from the outside. A muscle can look big, and a body composition test can show impressive muscle mass.
But the fat that builds up inside can hurt muscle quality. It weakens strength and mobility. Moreover, it impairs the way muscle uses sugar and fatty acids. This leads to an important distinction. Muscle mass tells you how much muscle you have. Muscle quality, on the other hand, tells you how healthy and functional it is.
An earlier UK Biobank study showed a similar picture. Low muscle volume combined with a lot of fat in the muscle predicted death from any cause. The link held even after adjusting for grip strength, body mass index and other risk factors.
The study that measured fat in thigh muscles
The researchers analyzed data from 55,120 participants in the UK Biobank. This large research database has collected medical, genetic and imaging data from hundreds of thousands of people. Using magnetic resonance imaging (MRI), the team measured fat infiltration in four areas of the thigh muscles.
Next, they examined the link between fat infiltration and death and disease across different body systems. The analysis adjusted for age, sex and social factors. In addition, the researchers accounted for body mass index, lifestyle habits and metabolic data.
What the researchers found
The results were consistent. As fat in the thigh muscles went up, so did the risk of death. The risk of disease also rose in every body system they examined: the heart and blood vessels, the lungs, the digestive system, the endocrine system and the musculoskeletal system. The link was graded, meaning it grew as fat levels rose.
That has a striking implication. Two people with the same weight and the same BMI (body mass index) can be very different. Their muscle quality may differ, and so may their health risk. In the same way, people at a normal weight aren’t always healthy.
Fat in muscle and life expectancy
The researchers estimated life expectancy from age 40 onward. The quarter of participants with the most fat inside their muscles had a shorter life expectancy than the quarter with the least. The links were especially strong in men and in people under 60. The researchers also found a link to when diseases appeared. People who got sick early had high levels, while those who stayed healthy had the lowest levels.
How to read these findings with caution
The study shows a statistical link between myosteatosis and disease and death. However, it is an observational study. So it doesn’t prove that the fat directly causes disease or shortens life. Fat inside the muscle may also reflect metabolic problems, inflammation or poor fitness.
In addition, the researchers note that the participants were mostly middle-aged or older and of European descent. Most were also healthier and better off than the general population. So we should be careful about generalizing the conclusions.
Genetics plays a role, and lifestyle makes a difference
The researchers identified 79 lead genetic variants linked to fat infiltration in muscle, in 47 regions of the genome. They tested a genetic risk score in a separate group of 362,286 participants. There, too, a high score was linked to death and to a range of diseases.
Alongside genetics, the researchers found clear links with habits we can change. Active people who ate a high-quality diet had less fat inside their muscles. By contrast, smokers, heavy drinkers and people with obesity had higher levels. Sleep showed an interesting pattern too: the relationship was U-shaped. In other words, people who slept too little or a great deal had higher levels. The lowest levels appeared in people who slept about 7.52 hours a day.
Fat in muscle as a link between habits and health
Another finding strengthens the picture. Fat inside the muscle mediated part of the link between lifestyle habits and disease and death. The share it explained ranged from 6.7% to 71.7%, depending on the habit and the disease. Put simply, part of lifestyle’s effect on health runs through muscle quality. What’s more, in people at high genetic risk, unhealthy habits had a particularly strong effect.
The link between muscle and metabolic inflammation
The researchers combined genetic information with data on proteins and metabolites in the blood. They found a strong link between high levels of GlycA (glycoprotein acetyls), a known marker of chronic inflammation, and fat infiltration in muscle. At the same time, high leptin levels also went hand in hand with high fat levels. By contrast, a high proportion of polyunsaturated fatty acids in the blood went with less fat.
Muscle is a very active metabolic organ. It takes up a large share of the sugar after a meal and burns fatty acids. It also releases substances that affect other organs. When its activity drops, a vicious cycle can develop: fat, insulin resistance, mitochondrial damage and chronic inflammation.
Which tests broaden the picture
MRI and computed tomography (CT) scans can detect the condition. However, there is currently no need for them as a routine screening test. Body composition tests give important information about muscle mass, but they only partly assess tissue quality. That’s why a practical assessment includes simple tests:
- Grip strength, a chair-stand test and walking speed.
- Climbing stairs, balance and cardiorespiratory fitness.
Muscle strength now carries central weight in assessing sarcopenia, the loss of muscle mass and strength with age. According to the revised European consensus, low muscle strength is the key sign, while muscle quantity and quality help confirm the diagnosis. So the goal is to combine three dimensions: quantity, strength and everyday function.
How to reduce fat in muscle and build quality muscle
Progressive resistance training, two to three times a week, helps build and maintain strength and muscle mass. Exercises for the large muscle groups are especially important: legs, back, chest and shoulders. You can read about the right combination in my article on aerobic vs. strength training.
Aerobic activity and movement throughout the day support heart and lung fitness. They also improve insulin sensitivity. A meta-analysis of 12 randomized trials found that exercise reduces myosteatosis in adults. On the other hand, a review of 19 randomized trials found no significant difference when people trained without an energy deficit. In other words, the combination of exercise and a tailored diet seems to matter.
A diet rich in whole foods gives muscle its building blocks. Think vegetables, legumes, fish, olive oil and high-quality protein. Finally, enough sleep, quitting smoking and cutting back on alcohol complete the picture.
Fat in muscle and weight loss
This topic is especially important in obesity treatment, including medications from the incretin family. Major weight loss usually includes some loss of lean mass as well. So it’s worth tracking strength, function, protein intake and resistance training, not just weight. The goal is to lose fat while keeping as much muscle as possible. You can read more in my article on preserving lean body mass.
Bottom line: fat in muscle and longevity medicine
This new study invites us to broaden the question. Alongside “How much muscle do I have?” we should also ask “How strong, active and healthy is my muscle?” Longevity medicine focuses on exactly these questions. Ultimately, healthy longevity is built on muscles that carry us forward, in sufficient amount and good quality. That’s why it’s worth assessing muscle quality, not just muscle size.
The information in this article is general and does not replace personal medical advice.
References
Integrative Analysis Uncover the Effects and Multi-Omics Features of Thigh Muscle Fat Infiltration
Adverse muscle composition predicts all-cause mortality in the UK Biobank imaging study
Sarcopenia: revised European consensus on definition and diagnosis
Effect of exercise on myosteatosis in adults: a systematic review and meta-analysis
