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Does Muscle Turn Into Fat When You Stop Working Out?

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Almost everyone has experienced a situation where they have to step away from their workout routine for one reason or another. Perhaps it’s due to an injury or illness or a life circumstance like a new job, relocation, or new baby, making it impossible to get to the gym with your usual consistency or intensity.

When people stop working out, one of the main concerns is that the muscle mass they’ve worked so hard to build will turn into fat.

But, does muscle turn into fat if you stop working out? On the other end of the spectrum, when you start a workout program, can fat turn into muscle?

This guide will focus on whether muscle turns to fat if you stop lifting weights or working out and whether body fat can turn into muscle when you start lifting weights.

We will cover: 

  • Does Muscle Turn Into Fat If You Stop Working Out?
  • Can Fat Turn Into Muscle If You Start Exercising?
  • Does Muscle Turn Into Fat When You Stop Working Out?

Let’s jump in!

A person measuring their body fat percentage.

Does Muscle Turn Into Fat If You Stop Working Out?

In short, muscle does not turn into fat when you stop working out. 

Muscle and fat are two distinct types of tissue with different constituent cell types. 

Even when you first start exercising, your current body fat tissue does not transform into muscle tissue, and likewise, if you stop lifting weights or working out, the muscle mass that you have built through your training does not transform into fat tissue. 

Once the cell has formed into the type of tissue that it will be—an adipocyte or fat cell for fat tissue or a muscle fiber for muscle tissue—that cell’s fate is determined. 

In much the same way that a neuron (nerve cell) in your brain cannot suddenly become a liver cell, a muscle cell cannot suddenly become a fat cell, and a fat cell cannot suddenly become a muscle cell.

A person flexing their muscular arm.

Can Fat Turn Into Muscle If You Start Exercising?

Even with consistent exercise training and heavy resistance training workouts, your muscles do not grow by creating new muscle cells. Rather, muscle growth, or hypertrophy, through exercise training is primarily accomplished through the enlargement of existing muscle fibers, not the generation of brand-new ones.

Hypertrophy increases the cross-sectional area of the muscle and is mainly due to an increase in the size and number of myofibrils in a muscle. 

Myofibrils are contractile proteins found in muscle fibers.

Let’s examine how muscle growth occurs from exercise.

When you perform intense or challenging exercises that overload the current capacity of your muscles, you create small microscopic tears in the muscle fibers. This is a normal and expected response to strength training.

A person lifting a bar at the gym.

In order to repair the damage, the body initiates a process termed myofibrillar protein synthesis or muscle protein synthesis. During this process, nutrients such as amino acids, which are the building blocks of proteins, are shuttled to the damaged muscle fibers to rebuild and repair the tissue.

As the amino acids are assembled into new muscle proteins, these repaired proteins are inserted in areas of microscopic tears.

Much like spackling a crack in the wall or caulking a gap in the tiles around a bathtub, the new proteins reinforce the areas of damage, thickening the muscle fibers in these regions. This ultimately makes the muscle fiber thicker and stronger. 

Since hundreds to thousands of muscle fibers constitute a single large muscle in your body, the more fibers that are reinforced and repaired, the greater the overall growth or enlargement of your muscles will be.

Ultimately, however, this hypertrophy (muscle growth) is primarily due to a thickening of the existing muscle fibers, not the creation of brand-new muscle fibers.

People at the gym doing bicep curls.

Moreover, with consistent exercise training, muscles also develop more mitochondria, which are the organelles in the cell that help generate energy for the cell, and there can be a growth of new vasculature in the muscle, such as new capillaries, to better perfuse the muscle with nutrients as you exercise.

These physiological adaptations due to training also work to increase the size of your muscle. Therefore, although you do develop larger muscles through proper and consistent training programs, muscle growth is not a product of fat cells becoming muscle cells.

With that said, physical activity burns calories, and if you increase your lean body mass (muscle mass) through an effective strength training program, it’s possible to see a concurrent reduction in body fat as you build muscle in your training program.

This is known as body recomposition.

Body fat is essentially stored energy. Each pound of body fat stores the equivalent of 3,500 calories of energy.

Thus, to lose body fat, you have to dip into your energy reserves, meaning that your body starts burning stored body fat because you are expending more calories than you are eating.

A person with a tape measure around their mid section.

If your workout program starts burning an appreciable number of calories and you either simultaneously make reductions in how many calories you eat or don’t adjust your diet but end up in a caloric deficit (burning more calories than you consume), your body will mobilize some of the triglycerides stored in your fat cells to burn them for the energy you need.

As a result, you will start to lose body fat.

Additionally, since muscle is a metabolically active tissue—meaning it requires more calories per day to support than fat—if you’re building muscle mass through exercise, your metabolic rate can increase.

This, in turn, will help you burn more calories throughout the day, ultimately helping you lose more body fat.

Therefore, although working out won’t directly change fat into muscle, it’s possible to lose body fat while you build muscle through training. 

To further support these body recomposition changes, focus on eating a nutritious, calorie-controlled diet.

The Centers for Disease Control and Prevention (CDC) recommends creating a caloric deficit of no more than 3,500-7,000 calories per week—which would yield 1-2 pounds of fat loss—for those striving to follow an effective and sustainable weight loss plan. 

This caloric deficit can be created through exercise, diet, or ideally both. You can estimate your BMR or even your entire daily caloric expenditure with online calculators.

An arm with a bit of sagging fat tissue.

Does Muscle Turn Into Fat When You Stop Working Out?

Just as how when you start exercising, your fat cells don’t turn into muscle cells, when you stop exercising, your muscle cells don’t become fat cells.

You will see a decrease in the size of your muscles over time, a process termed atrophy, which is essentially the loss of muscle mass due to disuse.

However, muscle atrophy is not occurring due to your muscles changing to fat.

Rather, many of the adaptations you enjoyed because of consistent training are no longer needed, so they are gradually reversed.

Essentially, if you aren’t working out anymore, your muscles don’t need the increased blood flow that they required during training to fuel them with oxygen and nutrients.

This can reduce the size and contract some of the capillaries that had grown from training, and you may lose mitochondria as well. 

A trainer helping someone lift weights.

The resultant size of the muscles will decrease, and the cross-sectional area of your muscles will decrease, but your muscle fibers will all still be there.

It is definitely possible to gain body fat after you stop working out.

The body gains weight when you are taking in more calories than you are burning. A caloric surplus of 3500 calories will result in one pound of fat gain. Excess calories coming in are converted to triglycerides and stored in fat cells (adipocytes), and your body fat percentage will increase.

However, as can be seen, this body composition change is due to storing more fat in your existing fat cells, not a change from your muscle cells becoming fat.

Ultimately, the more consistent you can be with your existing workout routine, the better able you will be to preserve your muscle mass and strength. 

However, even if you need to stop training for a while, your muscles won’t turn to fat. With that said, if you reduce your physical activity level and don’t want to put on more body fat, it’s important to make compensatory cuts in your diet so that you don’t gain body fat.

Remember to gradually build back into working out to reduce the risk of injuries and give your body time to recover and adapt between workouts.

If you are looking for a more precise timetable for gains, you can refer to our article, Want to Regain Fitness? How Long Does It Take To Lose and Regain Fitness? for more information.

A person doing a bicep curl at the gym.
Photo of author
Amber Sayer is a Fitness, Nutrition, and Wellness Writer and Editor, as well as a NASM-Certified Nutrition Coach and UESCA-certified running, endurance nutrition, and triathlon coach. She holds two Masters Degrees—one in Exercise Science and one in Prosthetics and Orthotics. As a Certified Personal Trainer and running coach for 12 years, Amber enjoys staying active and helping others do so as well. In her free time, she likes running, cycling, cooking, and tackling any type of puzzle.

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