How to Fuel Muscles During Exercise – Understanding the Importance of Carbohydrates

Adopting a regular exercise routine is one of the best things that you can do for your long-term health.

Exercise benefits all tissues in the body, and with each passing year research continues to learn more about the benefits of frequent and consistent movement. Over the course of 10 years of nutrition and fitness research science, I’ve developed a huge appreciation for the benefits of exercise, that seem almost too numerous to count.

Simply stated, exercise benefits all organ systems in the human body, including your heart, blood vessels, muscles, bones, ligaments, tendons, immune system, intestines, liver, pancreas and brain. That’s right, even tissues that have nothing to do with performing exercise receive the benefit of frequent activity.

Understanding how to fuel muscle during exercise is the first step in optimizing your individual athletic recovery program.

In this article, I’ll share some important aspects of muscle physiology that occur during exercise to help you create a personalized plan for optimal athletic recovery.

Exercise is a Beneficial Stress

Exercise is considered a “stress” to many organ systems, but differs from the negative stress of everyday life in that it stimulates the breakdown, repair and growth of muscles, ligaments, tendons and bones in the process of creating a stronger and more resilient body.
During exercise, muscles must perform two main tasks:

  • Oxidize or “burn” available fuel for energy
  • Contract in response to a rush of electrical signals from the brain

By accomplishing both tasks simultaneously, the muscle performs work, generating heat, waste products and the need for increased blood flow. Here’s a behind-the-scenes look at what fuels your muscles are using to power you during your workouts.

Muscle Fuel During Exercise

The muscle is capable of burning multiple fuels during exercise, including glucose (from carbohydrates), fatty acids (from fat) and amino acids (from protein).

In the same way that a car stores fuel in a fuel tank, muscles have evolved the ability to store glucose, fatty acids and amino acids on-board. All three fuels are burned for energy in the mitochondria, organelles within muscle cells that function much like a car engine.

car muscle analogy

Even though a car only has one engine, a single muscle cell often contains hundreds of mitochondria in order to generate large amounts of energy during exercise. In this way, the muscle is specifically designed to generate massive amounts of energy on the fly.

Glucose is Stored as Glycogen

Glucose is stored within each muscle cell as glycogen. Glycogen is specifically designed for quick-burning energy, and fast acting enzymes in the muscle cell allow for easy access to this large reservoir of fuel.

Many people believe that carbohydrates are the enemy and should be avoided at all costs.  On the contrary – dietary carbohydrates contribute to glycogen stores, literally fueling your muscle tissue.

Just make sure that your carbohydrates come from REAL sources, and not from fake and refined products. 

Fatty Acids are Stored as Triglyceride

Fatty acids are stored within muscle cells as triglycerides. Muscle triglycerides are stored in a lipid droplet that can be accessed by a different set of enzymes, providing a secondary fuel source during exercise.

Amino Acids are Stored as Muscle Protein

Finally, amino acids are stored within the muscle tissue as muscle protein. If a muscle cell were a car, the muscle protein is the metal frame of the car that provides structure and rigidity. Think of muscle protein as the infrastructure of the muscle tissue itself. Unlike glucose and fatty acids, there is no storage tank for amino acids in the muscle tissue. The muscle itself is the storage tank.

Here’s another way to look at it:

gastroc muscle


The Choice of Fuel Depends on Exercise Intensity

Even though all three muscle fuels are available for use during exercise, the muscle is making moment-by-moment decisions on which fuel to burn, depending mainly on the intensity and duration of exercise being performed.

At low intensities, fatty acids are the main fuel source and only small amounts of glycogen are broken down.  As the intensity of exercise increases, larger amounts of glycogen are broken down and burned for energy, making glucose the predominant fuel source.  As you can see in the graph below, as the intensity of exercise increases, the dependence on carbohydrate goes up and the dependence on fatty acids goes down.

cross over concept

You may notice that only carbohydrate and fat are fuel sources shown in the above graph.  That’s because amino acids are the lowest priority fuel, given that it is the infrastructure of the muscle tissue itself.  In order to preserve muscle mass, the muscle will burn glucose and fatty acids before resorting to amino acids.

Muscle Microtrauma

Even though amino acids from muscle protein are the last choice for fuel during exercise, microscopic tears result from repeated muscle contractions, called microtrauma.  These microscopic tears are one of the signals that the muscle requires in order to repair during rest.

Think of microtrauma as the repeated wear-and-tear that your car experiences from driving long distances.  In the same way that you replace damaged engine parts with newer and more efficient technology, microtrauma to muscle protein requires repair immediately following exercise.

Lucky for you, when muscle tissue repairs itself, the muscle will overcompensate by creating stronger, more resilient muscle protein so that the risk of repeated damage is reduced.

Overcompensation is exactly why muscles get stronger over time.  Enzymes that partake in the muscle repair process “overreact” to microtrauma in anticipation of future damage.

Amino acids from dietary protein are used as the building blocks for new muscle protein.  This is why protein is often considered an essential component of your post-workout recovery meal.


Muscle is the largest type of tissue in your body, and is extremely malleable because it responds to the type, duration and intensity of exercise that you perform.  Frequently exercised muscle tissue is in a constant state of remodeling, leading to increases in endurance, strength, flexibility and power.

The next time you perform a workout, keep in mind that your muscle is performing a number of tasks at the same time, including the following:

  • Choosing between glucose and fatty acids for energy
  • Protecting muscle protein from being used as a fuel source
  • Contracting repeatedly, hundreds to thousands of times in a single exercise session

In next week’s article, I’ll explain what happens in the post-exercise state so that you can begin to learn what nutrients your muscles are silently demanding following your favorite workouts.

Until then, keep up the good work and thank your muscles for all that they do to keep you active!

Image Credits

Female Track Athlete

Car Analogy

About The Author

Cyrus Khambatta

Diagnosed with type 1 diabetes at the age of 22, I have spent over a decade learning the fundamentals of nutrition at the doctorate level. My goal is to share my knowledge of practical nutrition and fitness with people with prediabetes, type 1 and type 2 diabetes. Diabetes is an OPPORTUNITY to attain excellent health. Reversing the effects of insulin resistance can be a fun and enjoyable process if the right system is in place. That's why I've spent over 10 years developing a rock solid system that can minimize blood glucose variability and insulin resistance.

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