You Can Have Plenty Of Muscle & Still Miss This Longevity Clue

Building and maintaining muscle is one of the best things you can do for healthy aging. But there's more to muscle health than what you can see in the mirror or measure with a tape measure.

A recent study of more than 55,000 people looked inside the thigh muscles themselves to see what else might be going on.

Muscle naturally contains some fat, but researchers are increasingly interested in what happens when more fat builds up within the tissue. This is known as thigh muscle fat infiltration, or TMFI.

For this study, researchers used MRI scans from 55,120 UK Biobank participants to measure the amount of fat in their thigh muscles.

They then looked at whether those levels were associated with death from any cause and the development of diseases affecting major body systems, including the cardiovascular, respiratory, digestive, endocrine, and musculoskeletal systems.

The more fat participants had infiltrating their thigh muscles, the higher their risk of death and of developing diseases across every major system the researchers examined.

That pattern remained even after researchers accounted for BMI. This is notable because two people with the same BMI can have very different amounts of muscle and body fat.

The link was stronger among people younger than 60 and among men. Those with the highest levels of TMFI also had a shorter estimated life expectancy after age 40 than those with the lowest levels.

Still, the study found an association, not proof that fat in muscle directly causes disease or shortens lifespan.

The results add to a broader shift in how researchers think about muscle. Size matters, but it may not tell us everything about how healthy that muscle actually is.

Several lifestyle factors were associated with TMFI.

Higher alcohol consumption, smoking, and poorer diet quality were linked to higher levels of fat in muscle, while physical activity showed a different relationship.

Sleep also followed a U-shaped pattern, with the lowest levels of TMFI seen at about 7.5 hours per night.

There were genetic differences, too.

A genetic risk score for TMFI was associated with mortality and disease risk in a separate group of participants. This suggests that some people may be more prone to accumulating fat within muscle. Inflammation may also be involved.

Muscle health may be about more than how much muscle you carry. This large study linked greater fat accumulation within muscle with higher mortality and disease risk, suggesting that muscle composition could be another useful marker of healthy aging.