Nutrition and the Energy Systems

Nutrition and the Energy Systems:

You Are What You Eat

By Zach Lewandowski, BS, CPT

If you have been in a gym before, I’m sure you have heard the term “energy systems”. Learning about the energy systems in the human body can help us truly understand how we get fuel for our workouts, especially if your goals are performance related. In this article, I will try to simplify the 3 main energy systems in the body and the mechanisms behind them.

Muscle Contraction and ATP

Exercise requires muscle contraction. Muscle contraction throughout the human body can be broken down based on muscle subtype specialization to accomplish its dynamic function. In general, muscle fibers are classified into 2 large categories:

  • Striated muscle fibers  
    • Cardiac muscle tissue (involuntary)
    • Skeletal muscle tissue (voluntary)
  • Smooth muscle fibers
    • Located in the walls of the hollow, visceral organs (involuntary) (1)

In this article, we will be talking about voluntary contraction in the skeletal muscle.

Skeletal muscle contractions are exercises that are dependent on the breakdown of adenosine triphosphate (ATP) and the associated release of free energy. (2) ATP is often called the “energy currency” of our bodies. All of the energy systems in our body work to generate ATP or generate molecules that will further drive ATP production, increasing the amount of energy we can “spend” upon exertion. We cannot store large amounts of ATP within the cells of the body which is why we have to rely on the 3 energy systems in the body: ATP-PCr System, the lactic acid/glycolytic system, and the beta-oxidation system.

While the 3 systems are similar in their outcome goal, to provide energy to the muscles, they differ in the pre-cursers used, products made, maximal rate of ATP regeneration, capacity of ATP regeneration, and their associated contributions to fatigue. It is important to note that the body uses all 3 systems in some fashion, never completely closing one off. The main factor that determines which energy system is used during exercise is the intensity and duration of the workout.

Outline of the 3 Main Energy Systems

  1. ATP-PCr SYSTEM (anaerobic)
    1. ATP – Adenosine Triphosphate
    2. PCr – Phosphocreatine
  2. Lactic Acid System/Glycolytic (anaerobic)
    1. Muscle glycogen (CHO)
  3. Oxygen/Beta-Oxidation System (aerobic) 
    1. Muscle glycogen & blood glucose (CHO)
    2. Muscle TG (triglyceride) & blood FFA (fat)
    3. Protein (aa); minor source

ATP-Phosphocreatine (ATP-PCr) System

The ATP-PCr system is mainly used for short, intense bursts of energy such as a 40-yard dash or a maximal exercise lasting about 1-10 seconds. This system is anaerobic meaning it does not require oxygen. Anaerobic contractions occur when you are performing a high-intensity exercise at 80 percent to 90 percent of your maximum heart rate. When you reach this level of intensity, your oxygen needs will exceed your oxygen supply, and your body will turn to alternative sources of energy, like phosphocreatine, to supply muscle contractions. ATP-PCr involves ATP and creatine phosphate that are stored within the muscle fibers. ATP is used and then resynthesized by using creatine phosphate to generate more ATP. 

Since we do not have large stores of ATP and creatine phosphate in our muscles, this pathway stalls once we run out of creatine phosphate. (3)  This is when supplementation with creatine may give someone the extra edge in competition or lifting. Creatine is naturally found in animal products such as red meat, fish and chicken. It is the building block for creatine phosphate/phosphocreatine. So, when someone is supplementing with creatine it helps to replenish ATP quicker which in turn, helps you get those extra few reps.

The Lactic Acid System

When we run out of creatine phosphate and need to replenish ATP,  our next best source is glucose (aka carbs)). The Lactic Acid System, also known as glycolytic system, is used for bouts of HIGH INTENSITY (maximal rates) of exercise. In this context, high intensity can be defined as something that is all out or close to maximal effort for 30-120 seconds. This could be something like a heavy set of deadlifts for 8 reps or a shift in ice-hockey. This is the dominant system for most sports which is why our nutrition is very important.

This system relies on carbohydrates (glucose or stored glycogen) in order to generate ATP. When our muscle glycogen stores are low, we tend to feel like we “hit a wall” and we cannot perform at our best. This is one reason why our nutrition is so important, especially carbohydrate consumption. Carbohydrates are one of the three main macronutrients and are found in fruits and vegetables, bread, rice, potatoes, etc. When we consume carbohydrates, they are digested, absorbed, and finally, converted into glucose (unless they are ingested as glucose).

The carbohydrates we consume pass through the liver and are mostly either converted to glucose and sent out into the bloodstream or stored as glycogen in the liver. Blood glucose is an important fuel for the nervous system. When our blood glucose drops too low, fatigue increases and our performance has a significant decrease. Your muscles need glycogen to perform at higher intensities over the length of a game or a lifting session, and your nervous system needs blood glucose to tell your muscles to give you all they have.

This is why nutrition is so important because when your opponents are starting to fatigue, you are able to compete at a higher intensity later in the game.

 I am sure everyone has heard of the term “lactic acid”. It has been known to be a negative byproduct of exercise and some contribute it to their fatigue. Recent studies have shown that lactate is in fact beneficial during intense exercise (3, 4]. “Interestingly, exercise, which promotes lactate production, has been recently reported to have a positive effect in many physiological as well as pathological conditions, including brain aging and neurodegenerative diseases.”(5) Production of lactate in muscle during intense exercise is beneficial for removing pyruvate, sustaining a high-rate of glycolysis which, in turn sustains a high rate of ATP generation.

Beta-Oxidation System

The final energy system is the Oxidative System which is aerobic, meaning it requires oxygen to function. The major processes involved are: respiration, circulation (heart pumps blood w/oxygen), peripheral circulation (arteries carry oxygen rich blood to the muscles), and metabolism (muscle takes oxygen that is being used to make ATP which comes mainly from carbohydrates and fat).

The oxidative energy system has two main parts and one lesser part. The two main parts are aerobic glycolysis and aerobic lipolysis. The one lesser part that takes place is aerobic proteolysis which has very limited energy production. Aerobic glycolysis is similar to the last energy system we discussed. It is the oxidation of glycogen or glucose (carbohydrates) during higher intensity but longer duration activities such as a 5k.

Aerobic lipolysis is the oxidation of fats when our bodies need energy for something longer than aerobic glycolysis can handle. Much larger quantities of ATP can be obtained by the oxidation of fatty acids derived from the breakdown of fat in adipose tissue, but the maximal rate of ATP generation is slower yet than that of glycogen oxidation and is more than tenfold slower than that with creatine phosphate like we talked about earlier in the article from ATP-PCr.(6)

Finally, the last part of the oxidative system is aerobic proteolysis, which is the oxidation of glucogenic or ketogenic amino acids. Amino acids are compounds that combine to make proteins. There are a total of twenty amino acids that comprise muscle protein. Nine of those are essential amino acids (EAAs), which means they cannot be produced by the body in physiologically significant amounts, and therefore, are vital components of a balanced diet. (7) Aerobic proteolysis breaks down amino acids to generate ATP although it does not play a large role in the oxidative system and has limited amounts of energy production.


As you can see from this article, our energy systems rely on different sources of fuel. This is why individualized meal plans are important for each person depending on their goals and requirements during activities. No single diet is best for competition because they all have their own demands. This table below summarizes the energy systems, which sources they use for fuel, and the activities in which they will be used.

Remember, no system ever completely shuts off. They are all used in some fashion. That means that we need to be sure to full all of the systems well with a well-balanced and strategic diet. If you would like more information on how to construct a good diet or to learn more about the energy systems, contact us at:

This article does not necessarily represent the views/opinions of MadLab Performance, LLC. This article is not meant to recommend health solutions in place of a doctor or other medical professional and should only be used to help those reading it gain more information prior to making their own decision.

Zach is the Nutrition Specialist for MadLab Performance, providing lifestyle change offerings through nutritional strategies. He is also a Professional Coach, training in person and online. When he’s not coaching, Zach can be found performing sets of 8 reps of deadlifts. Zach may be contacted by email at:


(1) Gash MC, Varacallo M. Physiology, Muscle Contraction. [Updated 2018 Dec 30]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-. Available from:

(2) Baker JS, Mccormick MC, Robergs RA. Interaction among Skeletal Muscle Metabolic Energy Systems during Intense Exercise. Journal of Nutrition and Metabolism. 2010;2010:1-13. doi:10.1155/2010/905612.

(3) Robergs RA, Ghiasvand F, Parker D. Biochemistry of exercise-induced metabolic acidosis. American Journal of Physiology-Regulatory, Integrative and Comparative Physiology. 2004;287(3). doi:10.1152/ajpregu.00114.2004.

(4) Nalbandian M, Takeda M. Lactate as a Signaling Molecule That Regulates Exercise-Induced Adaptations. Biology (Basel). 2016;5(4):38. Published 2016 Oct 8. doi:10.3390/biology5040038

(5) Proia P, Di Liegro CM, Schiera G, Fricano A, Di Liegro I. Lactate as a Metabolite and a Regulator in the Central Nervous System. Int J Mol Sci. 2016;17(9):1450. Published 2016 Sep 1. doi:10.3390/ijms17091450

(6) Berg JM, Tymoczko JL, Stryer L. Biochemistry. 5th edition. New York: W H Freeman; 2002. Section 30.4, Fuel Choice During Exercise Is Determined by Intensity and Duration of Activity. Available from:

(7) Wolfe RR. Branched-chain amino acids and muscle protein synthesis in humans: myth or reality?. J Int Soc Sports Nutr. 2017;14:30. Published 2017 Aug 22. doi:10.1186/s12970-017-0184-9

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