Does Muscle Turn Into Fat?

November 13, 2015

“I used to be toned, but then I stopped working out and it all turned to fat.”

Sound familiar?

It doesn’t seem like it should make sense – that muscle can turn into fat – but everyone’s seen football players or other athletes after their playing days are over. Sometimes, they get really fat.  So although it doesn’t make sense that muscle turns into fat, since you can’t explain it, you probably have caught yourself saying the same thing for lack of a better answer.

Well, here it is in black and white (and in bold): No. Muscle does not “turn into fat.” Period.

There is no process in the human body by which muscle tissue – which is made up of mainly protein, amino acids, and water – transforms itself into adipose (fat) tissue, which is comprised of primarily of adipocytes.  The human body, no matter how amazing it can be at times, cannot magically turn one tissue into another.

So then, what’s going on? A negative change in body composition.  Specifically, it’s a loss of Skeletal Muscle Mass combined with a gain of Fat Mass occurring at about the same rate, at about the same time.  How does this happen, and how can you avoid this? Let’s take a look.

It’s Not Magic – It’s Body Composition

The illusion of “muscle turning into fat” becomes believable for many people when they don’t see their weight change over time, yet see themselves get fatter.  While muscle turning into fat is a myth, the possibility of your body fat percentage rising over time definitely isn’t, and that’s what’s actually happening.

Here’s what a 7 pound decrease in muscle and a 7 pound increase in fat would look like in someone who weighed 261.9 pounds with a body fat percentage of 13.0%:

Notice how as Lean Body Mass drops, Skeletal Muscle Mass drops with it.  Because the Lean Body Mass decrease matches the Fat Mass increase, this person’s weight doesn’t change.  

However, this person’s body fat percentage increased from 13.0 to 15.7.  The increased body fat percentage combined with the lack of body weight change creates the illusion that muscle is transforming into fat, when in reality it’s just an increase in body fat disguised by no change in scale weight due to muscle loss.

How do things like this happen, and why does it seem to happen to people who are or used to be very fit?  It starts with muscle loss.

Muscle Loss

Although you may not realize it, you “lose muscle” every minute you are alive.  That’s because your muscles, like any other tissue in your body, depends on cell turnover and protein synthesis.  This means that your body is continually breaking down the protein in your muscles and rebuilding them.  You want your body to do this – it’s part of what’s keeping you alive!

Skeletal muscle can be grown and developed through proper nutrition – which includes consuming sufficient protein to provide the necessary amino acids – and through physical activity.  The converse is also true: if you become less physically active and/or your diet can no longer support the development of increased muscle tissue, you will enter a catabolic (tissue-reducing) state known as muscle atrophy.

Muscles that are partially used – using less than 20% of their maximum force – will start to atrophy over the long term.  Complete disuse is even worse:  muscles that are completely unused, such as when someone is bedridden and performs very little movement, can degrade by about 1/8 of their strength per week.

Of course, if you don’t have any major health complications, your muscles are not going to degrade at such a significant rate as someone who is bedridden.  However, if your body was used to operating at a high, athletic level and you suddenly stop, your body won’t see any reason to maintain your muscles at that level and will begin to atrophy.

Your muscles don’t transform – they disappear and they get replaced with something else.  Here’s how that happens.

Same Diet, Different Lifestyle

The second half of this myth: the appearance of fat.  Since we’ve covered that the muscle is disappearing – not transforming – you can see that this fat has to come from somewhere.  So where does it come from?

The same place it always comes from: an energy surplus - caused by eating more than you’re burning.  Although for many people this isn’t exactly news, it can catch people by surprise, especially people who are used to being athletic and fit.

Athletes require massive amounts of energy in order to perform their sport or activity successfully.  Their bodies require large amounts of all major macronutrients to supply the energy their body demands.  In order to get that energy, they need to eat – and eat a lot.  According to an interview given by Susan M. Kliner, a nutritional consultant for the Seattle Seahawks, NFL quarterbacks required somewhere between 4,000 to 6000 calories, spread out over about 6 meals per day in order to be in ideal playing shape.

A major reason why high performance athletes like NFL quarterbacks require so many calories is that they typically have higher-than-average amounts of Lean Body Mass as compared to average people at the same height.  That’s significant because as Lean Body Mass increases, Basal Metabolic Rate (BMR) increases. BMR is the number of calories a body needs at rest, excluding the calories needed for movement and digesting food.

Here’s an example of someone whose body composition falls into the athletic profile:

Notice the high values for Lean Body Mass and Skeletal Muscle Mass.  This contributes towards the BMR value of 2,602.

However, BMR is not the total calories you need a day.  A more appropriate number to determine how many calories someone needs in a day is the Total Daily Energy Expenditure (TDEE), of which BMR is one part.  In order to calculate TDEE, you’ll first need to determine BMR and then multiply it by a factor determined by how active you are.  Here are the conversions below.

Professional football players would fall under “extremely active” as their full-time job involves very high amounts of physical activity.  Taking the BMR in our example and multiplying it by 1.9 would result in a TDEE of 4,943.8, consistent with the statements made by Dr. Kliner.

What’s important to remember is that this number is the amount calories that this individual needs to consume maintain his weight due to his Lean Body Mass, and critically, at his current activity level.

What would happen if this person stopped being so active and took an office job – jobs that are typically sedentary?  TDEE would plummet quickly because the activity level would drop significantly.

Let’s say that this individual decided to continue being active while working at this office job and worked out enough to be classified as “moderately active.”  Assuming that BMR remains consistent (which due to the effects muscle atrophy would be unlikely), this person’s TDEE would be 4,033.1.  That’s a difference of 910.7 calories, or approximately 11 slices of commercially prepared wheat bread.

What this person would need to do is adjust his diet accordingly to take in less calories than he  did when he was extremely active.  The problem is, people become accustomed to eating a certain amount of food, especially when they have lived a certain way for many years.  They develop a mental understanding of how much they can eat, and they often order and/or cook portion sizes that match this mental understanding of how much food they need.

But when you change your activity level, you need to change your caloric intake.  In the example above, if this person continued to eat at the same level they did when they were extremely active, they would be in a caloric surplus of 909.9 calories a day, or 6,369.3 unnecessary calories a week.

What happens when the body remains in a caloric surplus for an extended period of time? Fat gain.

Who’s At Risk and What They Can Do

So tying it all together, it isn’t that your “muscle turns into fat.” From a body composition standpoint, here is what is happening:

  • Your Lean Body Mass is decreasing due to your decrease in Skeletal Muscle Mass
  • Your Skeletal Muscle Mass is decreasing because of disuse. Your BMR decreases accordingly.
  • Because activity level has dropped, your TDEE has also decreased.
  • Energy intake remains consistent, not accounting for TDEE drop. Caloric surplus created.
  • Caloric surplus leads to Fat Mass accretion.

People who are at risk for gaining large amounts of fat are, somewhat ironically, people who are at their fittest right now.  That’s because when you’re at your fittest, you’re likely at an optimal calorie/nutrient intake matched with an exercise level that allows you to achieve your goals.  You’re in balance.

However, if you ever drop your activity level, your diet must reflect this change accordingly or you will risk running a caloric surplus.  That change might be more than you'd expect, too.

Another solution is to find new ways to increase your activity level that works with your current lifestyle.  Although you may no longer be performing at high levels every day, you can find new ways to be active on a schedule that works for you.

Finally, testing your body composition regularly in order to stay vigilant about the condition of your body will be the best way to ensure that you’re staying at the level you want to be.  By testing your body composition, you will be able to track Lean Mass and Fat Mass gain or loss.  With that kind of information, you’ll be able to take action or make the changes you need to ensure that you stay as fit and healthy.