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How Does Muscle Burn Fat?
Everything You Should Know

Isaac Robertson
Published by Isaac Robertson
Fact checked by Donald Christman, BHSc FACT CHECKED
Last updated: July 19, 2021

This is a common question I get as a certified personal trainer, and it most often comes from amateur bodybuilders who have put in a lot of effort to build muscle mass.

Then comes a cutting phase where they want to burn fat to gain more muscle definition.

And since physical activity burns more calories, it must surely be the muscles that target the flabby stuff, right?

It’s not quite that easy and clear cut, so we’ve put together this quick guide to show you how strength training and losing fat are tied together.

Does Muscle Burn Fat Directly?

man using a dumbbell for his biceps

The simple answer to this is no; muscles don’t burn fat directly as fuel. Instead, muscles send a request to your body for more energy.

Maybe it would be more efficient if muscle cells could simply accept a fat load and use it like a car uses gas from the tank.

But evolution has decided there’s a better way, so let’s not argue with nature here.

Anyway, to understand how muscles use fuel, it’s important to look at two things.

1. Glycogen

glycogen food sources such as apples and spinach

Your muscles store glycogen, a specific type of glucose that can be processed faster than blood glucose.

The body constantly replenishes glycogen stores as the body burns through it.

Still, if you're doing cardio or building muscle, those reserves deplete a lot faster, ultimately leading to tiredness and fatigue.

With more muscle movement, some of that glycogen is oxidized into adenosine triphosphate.

2. Adenosine Triphosphate (ATP)

ATP is the fancy term used for the type of energy that muscle cells use to contract during physical activity [1].

Whether that’s picking up your phone or bench pressing 150 pounds, it all comes down to ATP.

But there is a link to fat, and we’ll explain it shortly.

How Does Your Body Target Stored Fat To Lose Weight?

You’ve probably heard it many times before, but to lose pounds, you have to burn more calories than you’re taking in.

Or put another way, the more energy you consume and the less you exercise, the more you’ll be gaining weight. It’s how the body works.

The science seems simple, but there’s a lot going on behind the scenes to make that happen.

1. Metabolic Boost

woman smiling in a crop top with her waist cinched in a measuring tape

First of all, during cardio or strength training at the gym, your muscles will need more energy in the form of ATP mentioned above.

As they use up glycogen reserves stored in muscle tissue, your muscles will send a request to replenish those reserves.

And because the glycogen reserves don’t last all that long, your metabolism is pretty much immediately called into action.

It’s almost like stopping at a gas station every few miles to make sure you don't run out of fuel.

Only the body doesn’t have to stop moving; it has a more efficient solution.

Your body has a few sources of energy available.

2. More Calories From Other Energy Sources

shirtless buff man flexing his biceps

The first healthy energy source is blood sugar, and it is the fastest way to transform energy into ATP.

But your blood sugar will only last so long, and in the case of high-intensity workouts, it can run out in as little as 30 minutes.

It can even happen sooner if you tend to do fasted exercises [2].

“In general, there’s no need to eat before exercise. If you feel better when you do, then, by all means, keep it up. But if choking down a pre-workout banana or bowl of oatmeal is a chore you do only because it’s supposed to help you avoid muscle loss/fat gain/growing antlers, then it’s time to relax.” - Nick English, Writer at greatist.com.

Next, your metabolism will try to get your digestive system to speed up the processing of macronutrients (fat, protein, and carbohydrates) in your stomach to boost your blood sugar levels.

That means digesting more carbs, fat, and protein to get some extra calories your body needs.

If you plan your exercise well, your metabolism will notice that it can’t get enough resources from your digestive system.

And if or when that doesn’t happen fast enough, your metabolism will target stored body fat.

The more calories your body needs, the more likely your body will target fat as a source of energy.

Related: Does Walking Burn Fat?

FAQs

Does Muscle Mass Turn to Fat When You Lose It?

No, muscle does not turn into fat when you lose it. No process in the human body transforms amino acids from muscles into fatty tissue.

A lack of working out at the gym and a calorie surplus can lead to a loss of muscle mass and an increase in fat.

How Do You Lose Fat but Keep Muscle?

To lose body fat but keep muscle, you have to combine cardio and weight training to maximize calorie burn.

Burning fat depends on how many calories you burn compared to how many calories you take in. Building muscle depends on lifting weights and pushing your muscles to the limit.

Do You Know How To Approach Lean Body Mass Goals?

It’s a common misunderstanding that muscles somehow directly burn fat.

However, the relationship between muscle tissue and fat burning is a lot more indirect.

Losing fatty tissue depends on how many calories you take in vs. how many your body burns. The more active your muscles are, the more calories you’ll burn.

At the same time, you’ll need to boost your calorie intake for the weight exercises. It’s a careful balance during a cutting phase.

If you have some exercise tips to share or any other questions about weight gain, leave a comment below or reach out to us on social media.


References:

  1. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6019055
  2. https://greatist.com/fitness/why-you-should-exercise-on-an-empty-stomach#tl-dr

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