Login | November 17, 2025
fitness mythbuster 6.0
PETE GLADDEN
Pete’s World
Published: November 17, 2025
Okay, let’s get out our magnifying glasses and our Sherlock Holmes hats for another edition of the Fitness Mythbuster.
And this edition is a humdinger, a fitness myth that I’ve heard virtually my whole fitness career: If you stop exercising, your muscles will turn into fat.
And I bet I’m not the only one who’s heard this cockamamy statement…right?
Yep, this one’s a doozy, and we’ll begin the conversation with my stating the obvious.
Such a transformation of one type of tissue into another is absolutely impossible.
Muscle and fat are two vastly different entities, both in structure and function, and the thought that one could magically morph into the other borders on ludicrous.
So as always, let’s dispel this myth with a little scientific scrutiny.
The first thing we need to do is to discuss the structure and function of muscle and adipose (fat) tissue.
Muscle tissue
Here we’re talking about a tissue made up of thin, elongated cells - muscle fibers.
Now the nifty thing about a muscle fiber is it’s ability to stretch, a unique property which obviously enables us to move our bodies in a gazillion different ways.
Now digging a little deeper into the composition of these muscle fibers, they contain this structure called a sarcoplasm, and within that are membranes called sarcoplasmic reticulum.
And it’s these sarcoplasmic reticulum that can store and release calcium ions, thereby initiating muscular contractions.
And finally, there’s another structure that wraps around every muscle fiber, the sarcolemma, which is a kind of cloak over the muscle fiber that regulates the passing of substances in and out.
Adipose tissue
So when it comes to fat tissue, we’re talking about fat cells (adipocytes) which are also a type of connective tissue.
And these cells’ claim to fame is that they can synthesize and hold globules of fat in the form of lipids.
Fat cells are found primarily under the skin, but also between muscles, in organs like the intestines, heart, liver etc. and in bone marrow.
So as you can immediately ascertain, fat cells are designed for storage rather than movement, which means that their structure and function is light years away from that of muscle cells.
Now some of the other major differences between these two types of tissues include the following:
Fat tissue is way less dense than muscle tissue, which means that fat takes up less space.
This is because fat tissue contains those lighter lipids, whereas muscle tissue contains heavier proteins and water - and nearly 75% of muscle tissue weight is that water.
What’s more, fat cells are larger and less compact than muscle cells, also leading to a lower density.
Another major feature is the weight of fat tissue compared to muscle tissue.
Fat tissue weighs less per unit volume than muscle tissue.
Thus, fat tissue’s weight and density allows people with more body fat to float in the water easier than people with more muscle.
And for those concerned with weight loss, well, one critical difference between these two types of tissue is that fat tissue is far less metabolically active than muscle tissue (fat tissue burns less calories at rest than muscle tissue).
This means that someone with more muscle tissue and less fat tissue will likely have a higher BMR (basal metabolic rate) than a person who possesses more fat tissue and less muscle tissue.
And remember, BMR is a 24/7 caloric burn.
Okay, so now back to the myth, that muscle can turn to fat if you quit exercising.
My hunch is this: The myth evolved many, many decades ago and centers around strength/power athletes who got muscular via sports training in high school and/or college.
These individuals, who after ending their sport careers went through years of relative inactivity as they pursued job careers and families.
And what happened?
Well, they lost the muscle mass (muscular atrophy) that they’d put on, then their BMR dropped from lack of activity and shrinking muscle mass, and finally they likely continued to ingest the same amount of calories as they did back in their “good ole” days. Subsequently, over the years they gained fat mass.
So how do you avoid this fat replacing muscle dilemma?
Partake in resistance exercises to maintain and build muscle, stay aerobically active, eat adequate protein and don’t ingest junk foods/highly processed foods.
And with that we’ll say adios to yet another crazy fitness myth.