You probably hear metabolism in the context of eating and how it effects the way your body looks aesthetically.

“I have a fast metabolism” usually translates to “I eat a lot and I’m still skinny”. While “I have a slow metabolism” usually translates to “I can eat moderately and still have excess fat.”

In reality, your metabolism is a set of chemical reactions that happens at the cellular level to both break down AND build up the body. It works in both directions so the slow metabolism/fast metabolism dichotomy and its associations with certain types of body image are misconstrued.

A fast metabolism, if we are talking about the reactions that build up the body, would theoretically build the body fast. Therefore, a skinny person would not be considered to have a fast metabolism, they would have a slow one.

The purpose of this article is to explore the set of reactions that occur in the muscles to break down stored energy, and convert it to kinetic energy for movement.

Armed with this knowledge, you can start to make decisions about how to train smarter, not harder to stop spinning your wheels and reach your fitness goals!

We will be going over some science today that, without much background, may be hard to digest. I realize this and simplified the information to the greatest degree I could and sprinkled it with practical examples and advice. Don’t let words like “enzyme” and “cytochromes” turn you off! Skip it if you need and move on to the practical recommendations at the end of each section.


ATP, or adenosine triphosphate, is the basic unit of energy that our cells use to carry out their specific functions. ATP is an adenosine molecule with 3 phosphate molecules attached to it. Our muscle cells have a small amount stored to do their job, but how do they use it?

Each unit of ATP has potential energy that is released when an enzyme (a catalyst for a chemical reaction) removes one molecule of phosphate from ATP. When energy is released from the splitting of one phosphate from ATP in a muscle cell, it provides the energy for our muscle cell to contract for movement.


We now have adenosine diphosphate (ADP) and a free phosphate molecule floating around in the cell. In order to release more energy, ADP and the phosphate molecule need to be reattached to form ATP. This action requires both an enzyme to catalyze the reaction and energy. But if ATP is the basic unit of energy the cells can use, where does the energy come from to reattach the phosphate?

It comes (mostly) from our body’s stored sources of fat and carbohydrate! Enzymes break the bonds between the carbon and hydrogen molecules in fat and carbohydrate which releases the energy necessary to reattach the free phosphate molecule to ADP forming ATP.

Our cells then break ATP back down using an enzyme to remove a phosphate molecule and release the energy necessary to do work. Next time you’re pondering the meaning of life remember that, at least on a cellular level, it can be broken down to this process of breaking down and regenerating ATP!


Because each cell has a limited store of ATP, and exercise increases the energy demand of our cells, there are 3 major metabolic processes that our cells can use to regenerate ATP to meet our energy demands.

For the purpose of highlighting each metabolic pathway individually, we will explore what happens when you take off on an all-out sprint and continue running indefinitely.

Later, I will highlight how each system is at play during common fitness events and CrossFit benchmark workouts so that you can get an idea about what is going on in your body during different types of exercise.

Imagine this: you wake up early, put on your running shorts, a t-shirt, and lace up your shoes. You have a swig of water and walk outside. After crossing the dewy grass in your lawn, the soles of your shoes meet the pavement. Looking down the street at the horizon you put one foot forward, one foot back and strike your best sprinter’s pose.

An imaginary gun goes off and you take of sprinting as hard as you can, imagining yourself chasing Usain Bolt.

  • After about 3 seconds, as you are accelerating, your muscle cells will use up most of their stored ATP.
  • As you continue sprinting as hard as you can, your phosphocreatine system kicks on to quickly regenerate ATP from the ADP + Phosphate floating around your cell.
  • After about 10 seconds, you start to slow down. You have depleted your muscles stores of creatine phosphate, and your glycolytic pathway uses stored muscle glycogen (carbohydrate) to regenerate ATP.
  • You continue running as hard as you can for about 80 seconds longer and then your legs start to feel sore and heavy. Even if mentally you try to push harder, you aren’t going any faster. Now your oxidative pathway is shouldering the burden of regenerating ATP to sustain exercise. Depending on your pace at this point, exercise could theoretically continue until you have burned all of your body’s stored fat (21 consecutive marathons for a 180lb male at 10% body fat).

The first two systems, the phosphocreatine and glycolytic are considered “anaerobic”. They regenerate energy without the aid of oxygen. The third system, the oxidative, is considered “aerobic” because it uses oxygen to regenerate energy.

Below is a graph showing the percent of energy that each system regenerates over time, assuming maximal effort output such as in the above running example.

3 metabolic pathways and percent of total energy generation over time. Phosphagen, glycolytic, oxidative.

Now lets look at each system individually, briefly explore how they work, and how to “feed” each system.


Phosphocreatine (PCr) is a creatine molecule bonded to a phosphate molecule. We have about 6 times as much PCr in our cells as ATP, but this is only enough to regenerate ATP for an additional 10 seconds.

This metabolic system works with the help of the enzyme creatine kinase. Creatine kinase breaks the bond between the creatine and phosphate molecules in PCr releasing energy and a phosphate molecule to regenerate ATP from ADP.

The enzyme ATPase then breaks the bond between ATP and one of its phosphate molecules releasing the energy necessary for muscular contraction. This will be the theme of each metabolic system, ATP is regenerated, and a phosphate is removed to release energy for muscular contraction.

Phosphocreatine molecule regenerating ATP Phosphagen system metabolism

The phosphocreatine system is responsible for most of the energy supplied during heavy weightlifting. When you are doing a heavy set of three cleans for example, the work probably takes you 6-9 seconds. After your ATP stores are depleted, PCr is regenerating the energy for continued movement.

If you push a loaded prowler or sled, you are going to slow down significantly after about 12 seconds because your PCr stores are depleted.

This is why creatine supplementation is so popular with power and strength athletes. By increasing the creatine pool in your muscles, more creatine phosphate can be synthesized, and you will have more ability to regenerate ATP for short duration, very high intensity exercise like weightlifting or sprinting.

Creatine supplements are well researched and safe if used properly. Research has showed that creatine supplementation is more effective for vegetarians than omnivores, and this makes sense because there is about 1-2 grams of creatine per pound of meat!

Over a dozen studies have found that creatine monohydrate supplementation is effective in increasing muscle creatine content at doses between 5-20g per day. These doses were also effective in significantly improving power output, and in most studies improved anaerobic running capacity (sprinting).

Armed with this knowledge, if you are a CrossFit athlete, weightlifter, sprinter, or any athlete that requires short bursts of high-intensity exercise creatine monohydrate supplementation is a great way to “feed” your phosphocreatine system to improve your performance.

With that said, research on creatine also shows that it causes weight gain due to water retention. If your current primary goal is to lose weight, I would suggest holding off on creatine supplementation unless you are regularly testing your body fat and benchmarking your progress on total body fat percentage vs. the number on your scale.

Lets now take a look at the glycolytic pathway, and how our cells regenerate ATP after our PCr stores have been depleted.


The glycolytic pathway utilizes stored muscle glycogen, blood glucose, and to a lesser extent glycerol from triglycerides (fat) to regenerate ATP from the ADP + Phosphate floating around in our cells after our PCr stores have been depleted.

This system is actually activated at the same time as the phosphocreatine system, but because it regenerates ATP at a much slower rate it doesn’t significantly impact energy transfer in our muscles during the first ten seconds of exercise.

The glycolytic pathway is much more complex than the phosphocreatine system. Whereas PCr requires one enzyme, creatine kinase, to break its bond to release energy the glycolytic pathway utilizes 10 chemical reactions requiring enzymes to regenerate ATP.

The glycolytic pathway regenerates 4 molecules of ATP per molecule of glucose compared to phosphocreatine system’s one molecule of ATP per molecule of PCr, however the glycolytic pathway “costs” 2 molecules of ATP in the process.

The net energy gain per unit of glucose is 2 molecules of ATP. This system is effective at producing more energy, but at a slower rate than the phosphocreatine system which is why you slow down after about 10-12 seconds of sprinting. The glycolytic pathway can continue to supply energy for about 80 seconds after your PCr stores have been exhausted.

Think for a second about how you feel after a max-effort 500 meter row or 400m run. Your legs feel HEAVY. Probably even on the verge of cramping. You have to walk it off and your legs STILL feel sore. You probably started slowing down towards the end. No matter how hard you wanted to push the sprint in the last few seconds, you just could not go any faster!

Most people blame soreness on lactic acid. This is a common misconception. Lactic acid is actually created when pyruvate, a product of glycolytic metabolism “grabs” hydrogen ions- the real source of your soreness- and removes them from the cells so that you can continue exercising. Another molecule, NAD+ grabs hydrogen ions forming NADH. Later we will look at how lactic acid and NADH can actually be used as a source of energy in the oxidative pathway!

Simple diagram of glycolysis. Glycoliytic metabolism. Glucose in, 2 ATP and NADH and Pyruvate out

When hydrogen ion production from a rapidly running glycolytic pathway exceeds the ability of pyruvate and NAD+ to remove them, muscle contraction will slow down and we begin relying on another energy system that I will address in a minute.

Before we move on it is important to know how to “feed” our glycolytic energy pathway. The primary source of glycolytic metabolism is going to be stored muscle glycogen. When we do short (10-90 second) bursts of high intensity exercise, our muscle glycogen stores are depleted via the glycolytic pathway.

After this type of exercise, our muscles are “hungry” for carbohydrates, and your muscle cells become sensitive to the storage hormone insulin. Any carbohydrate in your blood after exercise and in the presence of insulin is going to be sucked up by your muscles and stored as glycogen. This is a HUGE part of recovery.

I mentioned this in the article on macronutrients for CrossFit athletes, but your muscles primary goal after you exercise at high intensity is going to be glycogen replenishment, even before tissue repair. If you do a long duration workout consisting of high intensity intervals- carbohydrate post-exercise is a MUST.

So far we have discussed two metabolic pathways that utilize carbohydrate and other energy sources, but when do we burn fat? Fat is burned during lower-intensity activities, and the metabolic pathway that fuels this type of activity is the oxidative pathway.


The oxidative pathway can utilize BOTH fat and carbohydrate to regenerate ATP in the presence of oxygen. It can even break down proteins to supply energy needs if fat and carbohydrate aren’t readily available. The oxidative pathway can even use lactic acid created during glycolytic metabolism for energy!

Whereas the PCr and glycolytic systems have upper limits at which they can continue, the oxidative system runs indefinitely.

We saw that the glycolytic system relies on 10 enzymatically controlled reactions to regenerate a net of 2 ATP molecules, which made it slower than the PCr system. The oxidative system requires an even more complex process but it has a HUGE ATP yield!

If running on carbohydrate the oxidative pathway can regenerate 36 molecules of ATP. This is impressive, but when fat is run through the oxidative pathway, it generates even more ATP than carbohydrate…over 10 times as much! Through beta oxidation, one triglyceride (fat) molecule can regenerate 457 ATP molecules!

The oxidative system breaks down fats, carbohydrates and proteins into a chemical called acetyl-CoA and works to regenerate ATP through two processes. These processes take place in the mitochondria of our muscle cell. Think of mitochondria as little generators, or power plants, in each cell.

The first step in the oxidative pathway is the Krebs cycle- an 11 step process that requires nine different enzymes.

The Krebs cycle produces 1 unit of ATP and 8 hydrogen ions. This seems like a waste doesn’t it? Hydrogen ions are what made our muscles sore and shut down the glycolytic pathway, and 11 reactions to generate just 1 unit of ATP?

This is where the second step of the oxidative pathway, the electron transport chain, comes into play. The hydrogen ions created in the Krebs Cycle are bound to FAD+ and NAD+ (B-vitamin derived chemicals) to form NADH and FADH which shuttle the hydrogen ions into the electron transport chain.

The electron transport chain process is sort of like hydroelectricity generation. Imagine the protons of each hydrogen ion like water rushing through a dam. As the protons are pumped from the outer chamber to the inner chamber of the mitochondria’s five cytochromes there is a huge energy yield of 32 ATP per molecule of acetyl-CoA.

Fat, sugar, protein, oxygen into mitochondria energy out. Oxidative pathway, oxidative metabolism

Now that we know how the oxidative energy system works, and the huge ATP yield it is capable of regenerating, how do we “feed” it?

Acetyl CoA, the entry point to the Krebs cycle, can be created as a result of fat, carbohydrate or protein breakdown. We have enough of these nutrients available in storage to feed this system indefinitely.

What is important to keep this system functioning properly, is enough B-vitamins, specifically B2 (Riboflavin) and B3 (Niacin) as the electron transport chemicals NAD+ and FAD+ are derived from these vitamins.

I do not suggest supplementation with synthetic vitamins or eating “fortified” foods like rice and cereals that contain these nutrients. Instead, you should seek out whole food sources.

Vitamin B2 is abundant in dark leafy greens (especially spinach), mushrooms, eggs, and dairy. Vitamin B3 is most abundant in meat, but for plant-based eaters it is also abundant in mushrooms, asparagus, and tomatoes.

If you do not eat enough of the previously mentioned foods, chances are you are not getting enough B-vitamins for an efficient oxidative metabolism. You should seek to get more of these foods in your diet to make up for any deficiencies.

If you already eat plenty of B2 and B3 containing foods, eating more does not necessarily mean you will have a more effective oxidative energy pathway. Instead, to improve your oxidative metabolism you can focus on things that will improve mitochondrial health and efficiency.

It would take another post entirely to explain how to do that, but in a nutshell regular exercise and certain antioxidants are most effective.


Lets take this knowledge and examine some workouts you have probably seen or completed before.

First, lets look at the classic CrossFit benchmark workout, “Fran”. Fran is 21, 15, and 9 reps each of squat thrusters with a barbell and pull ups. The top athletes in the world complete this workout in under 2 minutes and do it without stopping.

You could consider this workout a sprint, and based on the full sprint example from earlier, you would expect that ATP supplies energy for the first 3 seconds, PCr until 12 seconds, and glycolysis until 90 seconds, and oxidation for the remainder.

This assumption, however, would be incorrect. Our metabolism does not always work in such a straightforward way and exercise adaptations can help athletes improve the efficiency of each metabolic pathway.

Because the squat thrusters are at a relatively light weight for high-level athletes, and pull ups are a common skill, they may be using their aerobic metabolism in addition to glycolysis for a portion of each set even at a full sprint pace.

For the first few seconds of movement they will be using their ATP and PCr systems until their glycolytic and oxidative systems can support most of the work load. During the transitions between movements the short, low intensity period leaves enough time for NAD+ and pyruvate to clear hydrogen ions from the muscles to allow glycolysis to continue.

As the end of the workout is in sight, the athletes will “turn it on” and push through the pain of hydrogen ions building up and causing acidity in the muscles.

By the end of the 2 minute workout, they will have pushed the limits of their glycolytic system and will probably collapse in a pool of sweat, squirming around unsuccessfully searching for a ‘comfortable’ position in which to rest as they struggle with “Fran lung”.

From this example, you can see that metabolism isn’t straightforward in the context of most high variety, high intensity workouts you will see.

It is, however, pretty straightforward in instances such as running and weightlifting. If you are performing a CrossFit Total- the sum of your best press, deadlift, and back squat performed in one session with 3 attempts at each- or an Olympic Lifting total- the sum of your best snatch and clean and jerk performed in one session with 3 attempts at each- you will be almost exclusively using your ATP and PCr systems.

It is also straightforward in the case of running. As you accelerate at the start of a run, your muscle cells will utilize stored ATP and PCr as the gylcolytic and oxidative systems kick in. If your pace is conversational, meaning you could hold a conversation with a running partner, the energy burden will be shouldered by your oxidative pathway, and fat should be the preferred energy source.

If you are running shorter (400 meters or less) sprint intervals, you will be relying on your PCr and glycolytic systems almost exclusively.

But what happens in between your sets during weightlifting and sprint intervals for running/rowing/etc.? Because your PCr and glycolytic systems are considered “anaerobic” and do not require oxygen to create energy, these systems create what is commonly referred to as an oxygen debt, or properly as Excess Post-Oxygen Consumption (EPOC).

In order to make up the “debt”, your body consumes more oxygen during the rest between and after sprints or high-intensity lifts. Because your body is using oxygen to regenerate energy during the rest, you are using the oxidative system which will put the byproducts of glycolytic metabolism and fats through the Krebs cycle and electron transport chain.

EPOC after exercise, oxygen debt

This is significant because although you only burn fat for fuel in the oxidative pathway, anaerobic metabolism can lead to increased rate of fat burn at rest.

As we just explored, energy metabolism is not always as straightforward as in our sprinting example. different combinations of exercise change which metabolic pathways our muscle cells rely on to regenerate energy.

We also touched on the fact that training-induced adaptations can improve the effectiveness of each pathway. In the next section we will explore how to effectively and efficiently train each system based on our goals.


Now that you have an idea as to how your muscles break down stored energy to create kinetic energy for movement, you can intelligently choose the types of exercise that will be conducive of your goals.

If your goal is to burn fat, it would seem obvious that activities which rely almost solely on the oxidative system would be your best choice, but people spend years spinning their wheels running themselves to the grave and never dropping the weight they want to lose.

As we explored earlier, anaerobic activities actually increase our rate of fat burn at rest. They also have the benefit of supporting the anabolism (build up) of muscle mass. Muscle is some of the most metabolically active tissue in the body as we just discovered, and more muscle mass means higher energy needs. Long duration exercise is actually catabolic (opposite of anabolic- breaks down) to muscle.

Therefore, a program that carefully mixes anaerobic exercise and aerobic exercise is ideal for fat loss. I would suggest interval training for aerobic training sessions. Below is a chart that outlines the ideal intervals for training each of the 3 metabolic pathways we have reviewed in this article.

Interval training, phosphagen, glycolytic, oxidative

If your goal is to put on size, then you will want to avoid or minimize exercise that relies heavily on the oxidative metabolic pathway because of its catabolic effect and focus on anaerobic activities such as resistance training (weightlifting, gymnastics) and sprint intervals. You should “feed” these metabolic pathways as we discussed earlier providing the body with carbohydrates post-workout.

If your goal is improved athletic performance, it is important to understand the unique energy demands of your sport, and engage in exercise that will help you make adaptations within those energy systems.

A soccer player, for example, must have supreme aerobic capacity to last 120 minutes in a match. Therefore, they should focus most of their effort on exercise that utilizes the oxidative pathway. Goalies, however, only spring into action for short bursts and therefore will be better served to train anaerobically so that they can dive for shots.

Percent energy from phosphagen ATP PCr, Glycolysis, oxidative. Metabolism and energy in sport

Armed with this new information, how can you adapt your training to better support your goals?

If you liked this article please share it with any athletes, trainers or fitness professionals you know would benefit from the information and sign up for my newsletter to receive exclusive video content that I do not post on the blog!



  1. Berardi, John. The Essentials of Sport and Exercise Nutrition. 2nd ed. N.p.: Precision Nutrition, 2013. Print.
  2. “Vitamin B2 – Riboflavin.” Vitamin B2 – Riboflavin. N.p., n.d. Web. 17 June 2015.
  3. “Vitamin B3 – Niacin.” Vitamin B3 – Niacin. N.p., n.d. Web. 17 June 2015.