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Sports Nutrition and Supplementation Series: Exercise Physiology (Part 2 of 3)

I analyze the wide, complicated world of sports nutrition and supplementation to build a personal wellness plan that will help me to become the best athlete I can be.

Part 1 of this post series focused on sports supplements. I developed a personal decision-making framework to analyze whether I should take a given sports supplement to improve my athletic performance.

If you haven’t read part 1, I recommend starting there, as it introduces some relevant background information and motivation for the post series as a whole.

This post shifts the focus to exercise physiology. I research the various energy pathways the body uses to power movement, and start to explore the importance nutrition has on these pathways.

The goal in understanding more about physiology is to learn about how the body works so I can understand how to best fuel it. Consequently, this post builds the background knowledge needed for the last post, part 3, which focuses on nutrition.

ATP: Body’s Energy Currency

Muscles contract to power body motion. This takes energy. Additionally, the brain needs energy to maintain ion gradients for nerve activity.

The molecule ATP (adenosine triphosphate) provides chemical energy for the body. In essence, it is the body’s energy currency.

How does ATP provide energy? ATP contains potential energy that is released during hydrolysis (reaction with water). One important implication of this is that you need water in order to produce energy!

When ATP combines with H2O (water), ATP is converted to a different molecule called ADP (adenosine diphosphate), and 7.3 Calories (kcal) of energy are released. This energy that gets released can power cell’s activities like muscle contraction!


  • ATP molecules are the body’s energy currency. They provide chemical energy to power muscle contraction. 

Energy Pathways

The body has several different pathways it can use to generate ATP. The pathways differ not only in their chemical nature, but in their output.

Some pathways produce energy very quickly, but are inefficient. This means they can generate a lot of force very quickly, but cannot produce energy for very long before they are depleted.

On the other end of the scale are pathways that are very efficient but cannot produce energy very quickly. This means they cannot produce a lot of force, but can supply energy for a longer duration.

The following diagram illustrates the nature of this sliding scale, and points out where on the scale the body’s various energy pathways lie.

Human Body Energy Pathways Diagram

An important note is that this is a general scale. and the duration numbers shown are approximate. Also, at any given point during exercise, the body uses multiple different energy pathways to some degree, not just one.


  • The body utilizes several distinct chemical pathways for producing ATP for energy: raw ATP, phosphocreatine, anaerobic glycolysis, aerobic carbohydrate metabolism, and aerobic fat metabolism.
  • Some energy pathways can produce energy very quickly but inefficiently, making them well-suited to produce high amounts of power, but only for short durations.
  • Others produce energy slowly but efficiently, making them well-suited to produce low amounts of sustainable power.

Energy Pathway #1: Raw ATP

Remember that the body’s energy currency is ATP. Usually during exercise the body must generate this ATP. However, it turns out that muscles actually store some raw ATP.

This raw ATP can produce energy very quickly, since the ATP is already generated! However, the muscles only store enough ATP to power about 1-2 seconds of activity.

Thus, this raw ATP is used for extremely explosive movements, like an all-out sprint or a maximum jump, where energy needs to be supplied immediately. No other energy pathways can produce force quick enough to support these types of movements.

When the muscle’s ATP supply is exhausted, ATP can be resynthesized via the “used” ADP molecules. All other energy pathways serve this role: they resynthesize ATP from used-up ADP molecules.


  • Muscles store raw ATP that can produce lots of power for only a second or two
  • All other energy pathways resynthesize ATP from used-up ADP molecules

Energy Pathway #2: Phosphocreatine

This energy pathway was briefly covered in part 1 of this post series when analyzing creatine supplementation. Behind raw ATP, the creatine phosphate energy system produces energy the quickest, but least efficiently.

Chemically, phosphocreatine molecules stored in the muscles donate their “phosphate” groups to ADP molecules, which creates ATP (body’s energy currency). This is a very quick process  relative to other energy pathways.

Thus, the phosphocreatine energy pathway is very well-suited for use cases when the body needs to generate a lot of force very quickly, like sprints and other explosive movements.

However, just like with the “raw ATP” energy pathway, this is not a very efficient way of generating ATP. Typically muscles can only store enough phosphocreatine molecules to power 10 seconds or less of vigorous activity before the creatine is exhausted.

Increasing creatine muscle stores to allow for more energy production via the phosphocreatine energy pathway is the logic behind creatine supplementation.


  • Creatine phosphate is stored in muscles and can be used to produce ATP 
  • It can produce a large amount of power for only a short amount of time (< 10 seconds)

Fuel Metabolism

Once raw ATP and phosphocreatine stores are depleted (or if energy demands don’t necessitate the usage of those inefficient pathways), muscle cells turn to fuel metabolism to produce further energy.

There are 2 types of fuel metabolism. One uses oxygen (aerobic) to produce ATP, and one proceeds in the absence of oxygen (anaerobic). There are tradeoffs between using aerobic vs. anaerobic pathways to produce energy.

Both anaerobic and aerobic energy production involve “burning” fuel to produce ATP energy. Fuel is chemical energy stored in your body, and there are 3 types of it: glucose (carbohydrates), triglycerides (fat), and amino acids (protein). Each of these different fuel types also has tradeoffs that will be discussed.

Fuel Types


Carbohydrates get stored in the body as “glycogen”, which is a bunch of glucose units covalently linked together. Glycogen reserves are stored in the muscles and the liver.

Glycogen is known as the “quick” form of energy for the body. Glycogen gets broken down to its individual constituent glucose units. Then, this glucose can be processed to release energy either aerobically or anaerobically, via different chemical pathways.


Fats are stored as “triglycerides” in the body. A triglyceride consists of 1 glycerol unit connected to 3 fatty acid units. Most fat is stored in adipose tissue, but there are small reserves of fat in other organs, including muscles.

Triglycerides alone account for roughly 84% of the total energy stored in your body! Fat is a very efficient way of storing energy, compared to any other fuel type.

When it is needed for energy, the triglyceride is broken down into the constituent glycerol and fatty acid units. This process is relatively slow, and the breakdown of triglycerides into ATP requires oxygen. This means fat can only be used aerobically to produce energy, unlike carbs, which can be used aerobically or anaerobically.


Protein can be degraded to produce energy, but it is not very efficiently used for this purpose. Protein is better used for rebuilding flesh. For this reason, the body only uses it for energy in dire circumstances, like a precipitous drop in blood sugar, or if no other energy is available.

Protein is primarily stored in skeletal muscle, so when your body is using protein for energy, it is basically breaking down your muscle to do so. This is obviously not ideal.

In order to generate energy from protein, the protein is broken down into amino acids, which can break down to glucose (the same thing carbohydrates break down to).

Summary of How Fuel Metabolism Produces Energy
  • Carbohydrates -> Glycogen -> Glucose -> Small carbon chains -> ATP
  • Fat -> Triglycerides -> Fatty acids -> Small carbon chains -> ATP
  • Protein -> Amino acids -> Glucose -> Small carbon chains -> ATP
  • Carbs (glucose) can be used aerobically and anaerobically, whereas fats can only be used aerobically

Cellular Respiration

To understand any of the next chemical pathways the body uses to produce energy, it will help to analyze how glucose is converted to energy aerobically (with oxygen). All other energy pathways are modifications to this basic pathway.

As a reminder, glucose can come directly from muscles, or from sugars in the bloodstream, or from the breakdown of protein. To produce energy from glucose, muscle cells go through a chemical process called cellular respiration. This process has 3 stages.

Stage 1: Glycolysis

Inputs to this stage are ADP, ATP, and water molecules, as well as glucose. Various chemical reactions take place and produce a net gain of 2 ATP molecules, in addition to 2 molecules of pyruvate.

Stage 2: Krebs Cycle

Pyruvate molecules go through a chemical reduction to produce 1 ATP molecule, as well as carbon dioxide and water.

Stage 3: Electron Transport Chain (ETC)

Both glycolysis and the Krebs cycle produce electrons. These electrons need to be dealt with, so they move through a transport chain, which produces ATP molecules. Eventually, the electrons are accepted by oxygen atoms.

The ETC stage is the only stage of the cellular respiration process that requires direct oxygen. Without oxygen available, electrons build up in the muscle cells, and inevitably jam up the whole cellular respiration process.

Importantly, the ETC stage creates 27 of the total 30 ATP molecules produced by cellular respiration! That’s 90% of the total energy! Remember this fact, it will be important later.

Summary of Fuel Metabolism

  • All energy pathways other than raw ATP and phosphocreatine produce energy from fuel
  • The body has 3 different types of fuel: carbs, fat, and protein
  • Muscle cells can create energy from fuel either aerobically (with oxygen) or anaerobically (without oxygen)
  • Carbs can be used aerobically or anaerobically, but fat can only be used aerobically. Protein is used as a fuel only in dire circumstances, as it breaks down muscle.
  • Cellular respiration is the process used to convert glucose fuel to energy. It has 3 stages: glycolysis, krebs cycle, and the ETC. The ETC is the only stage that requires oxygen, but produces most of the energy of cellular respiration.

Energy Pathway #3: Anaerobic Glycolysis

After ATP and phosphocreatine, the next energy pathway that can produce the most amount of force quickly is called anaerobic glycolysis. Anaerobic glycolysis is the breakdown of glucose (carbohydrates) into energy (ATP) without oxygen (anaerobic).

Anaerobic glycolysis cannot produce energy as quickly as raw ATP or phosphocreatine, but it can produce energy faster than any aerobic energy pathway. However, it isn’t as efficient as aerobic pathways, as it produces less energy overall and is not sustainable.

Thus, the body uses it for short duration, high-intensity activity that lasts anywhere from 10 seconds to 60-90 seconds.

Why is anaerobic glycolysis less efficient than aerobic metabolism? And why is it not sustainable? To answer these questions, let’s go back to the cellular respiration process.


Remember that oxygen was required in the 3rd stage of cellular respiration, the electron transport chain, to absorb electrons produced from the first two stages. Without oxygen, these electrons build up, and cellular respiration has to stop, which means no more energy.

Luckily, as it turns out, there is a modified way muscle cells can produce energy under anaerobic conditions (without oxygen). Instead of utilizing the 2nd stage Krebs cycle to reduce pyruvate molecules, the pyruvate molecules can be reduced through an alternative mechanism: fermentation.

In yeast organisms, fermentation of pyruvate molecules produces ethanol, which can be used to make wine or beer. In humans, fermentation of pyruvate molecules produces lactic acid. It also results in an accumulation of hydrogen ions (H+).

Essentially, fermentation occurs in cellular respiration after stage 1 glycolysis, and replaces stages 2 (Krebs cycle) and 3 (ETC) to produce ATP without oxygen.


It doesn’t produce electrons that need to be absorbed by oxygen via the ETC, thus avoiding the dependency on oxygen. It does, however, produce hydrogen ions and lactic acid.

Of the two byproducts (hydrogen ions and lactic acid), free hydrogen ions are the problematic one. The ions accumulate to cause a drop in pH of the muscle cells, making them more acidic.

The increased acidity ends up inhibiting one of the key enzymes of the 1st stage of cellular respiration, glycolysis. This makes it unsustainable, as the hydrogen ions eventually shut down the anaerobic cellular respiration process.

What do you feel as this happens? Pain. If you’ve ever watched a 400-meter-runner in anguished pain coming down the homestretch (or been that runner), you know what this looks like (or feels like).

Interestingly, you’ll hear people say phrases like “catching lactic”, referring to the lactic acid accumulating in their muscles. However, lactic acid itself is not actually problematic. The accumulation of lactic acid just happens to be highly correlated with the accumulation of free H+ ions, which is the culprit that actually shuts down anaerobic glycolysis.

But, “catching H+ ions” is less cool of a phrase than “catching lactic”. So, lactic acid is widely believed to be the evil bane of existence for many athletes. For more details on what the body does with lactic acid and free hydrogen ions, refer to the Appendix: Lactic Acid and Free Hydrogen Ions.


I asked you to remember a fact about cellular respiration. Pop quiz: what was it?

That the ETC stage produces 90% (27 out of 30) of the total ATP molecules of cellular respiration. If you’re extremely clever, you may have realized an important implication of this fact as it relates to anaerobic glycolysis.

Recall that fermentation eliminates the ETC from the cellular respiration process. This means anaerobic glycolysis skips the stage that produces most of the ATP energy! Thus, it produces far less ATP molecules (energy) than aerobic metabolism, which utilizes the ETC stage.

This is why anaerobic metabolism is way less efficient than aerobic metabolism. It produces 2 ATP molecules instead of 30!


  • Anaerobic glycolysis breaks down carbohydrates to produce energy in the absence of oxygen
  • It is less efficient than aerobic metabolism, but produces energy faster
  • It is used for higher-intensity activities lasting generally from 10 seconds to 60-90 seconds
  • It is an unsustainable energy pathway: free hydrogen ions accumulate and make muscles acidic, causing pain and eventually shutting down the process

Energy Pathway #4: Aerobic Carbohydrate Metabolism

Aerobic carbohydrate metabolism oxidizes glucose (carbs) to create energy (ATP) utilizing oxygen (aerobic).

The chemical process behind this was previously described in the “Cellular Respiration” section, and it produces a net gain of 30 ATP molecules. This makes it incredibly efficient.

For this reason, aerobic carbohydrate metabolism is a primary energy contributor for any activity lasting longer than roughly 2 minutes. All of the other energy pathways previously discussed are not sustainable, and cannot produce energy for this long.

Oxygen Dependency

However, aerobic metabolism requires oxygen. The reason oxygen dependency matters is that oxygen supply is the limiting factor to how quickly cellular respiration can produce energy.

There are bodily limits to how quickly oxygen can be delivered to oxidize pyruvate and accept electrons as part of the ETC. Oxygen is carried to tissues by the protein hemoglobin, but this can only happen so fast.

There are also bodily limits to how much oxygen an athlete can take in at once. This is known as VO2 Max.

Both of these oxygen-based limits are primary factors to the maximum level of effort sustainable via aerobic metabolism. There’s simply a limit to how quickly aerobic metabolism can occur. This limits the amount of force muscles can produce using aerobic energy pathways.

Thus, there are times when the muscular force demands (e.g exercise intensity) are greater than that which can be supplied via aerobic metabolism. In these cases, the body is forced to use the less efficient but faster anaerobic glycolysis energy pathway.

Anaerobic glycolysis does not have to wait for slowpoke oxygen to support cellular respiration, so it doesn’t suffer the oxygen bottleneck that aerobic metabolism does.


  • Aerobic carbohydrate metabolism breaks down carbohydrates to produce energy using oxygen as part of cellular respiration
  • It is very efficient, and is the main energy pathway for any activity duration > 2 minutes
  • It is limited by oxygen supply and transportation, which prevents it from producing energy quickly to achieve large demands for power by muscles

Energy Pathway #5: Aerobic Fat Metabolism

All of the fuel metabolism energy pathways we’ve discussed up until this point involve burning glucose from carbohydrates to produce energy. As it turns out, fat is also an important source of energy for the body.

Fat can only be used aerobically. It is also the slowest energy pathway, so it can’t produce much power. However, it is the most efficient energy pathway, as it produces the most ATP, and can be sustained much much longer than any of the other energy pathways.

For this reason, aerobic fat metabolism can actually be an important energy contributor for any exercise lasting longer than about 30 minutes.

Fat Metabolism

Fat is stored in the body as triglyceride molecules, primarily in adipose tissue. When it is needed for energy, a hormone called glucagon triggers adipocyte fat cells to release their fatty acids. This happens if blood sugar levels drop, meaning glucose supply is running low.

It takes a full 20 minutes for these fatty acids to make it to skeletal muscle and the heart. From there, the acids are carried into cell mitochondria, get broken down, and enter the Krebs cycle of cellular respiration.

However, unlike glucose, fatty acids cannot be metabolized for energy unless the ETC is operating. This means oxygen must be present, meaning fat can only be used for energy aerobically.

Fat takes a full 20 minutes to reach skeletal muscle (seriously, what is it doing? I never know what it’s doing…  mah, the meatloaf!!), and it can only produce energy aerobically. Thus, fat metabolism cannot produce energy quickly. It can only suit very low power demands from muscles.

Glucose, on the other hand, is much more readily available. Glucose is stored in muscles and in the bloodstream, and can be transported way faster than fatty acids. Thus, when the body is producing energy via aerobic metabolism, glucose is used until fat mobilization begins to supplement energy production.


What it lacks in speed, it makes up for in efficiency. In addition to producing the 30 ATP molecules from aerobic metabolism (as opposed to the 2 from anaerobic metabolism), fat is even more efficient than glucose for energy production! Why is this the case?

The glucose chemical structure involves more carbon atoms bonded to oxygen, as compared to fat molecules. This means they are already partially oxidized, and less “reduced”. So in the “oxidation” reaction of cellular respiration, less energy is released as a byproduct.


This makes fat a more efficient means of storing energy: 4 kcal energy is released per gram of carb oxidation vs. 9 kcal energy released per gram of fat oxidation.

The average individual has enough fat stored for up to 2 months without eating, or to run for about a month straight at a very slow pace! To store an equivalent amount of energy in carbs, you’d be a lot heavier!


  • Aerobic fat metabolism breaks down fat to produce energy utilizing oxygen
  • It is the slowest energy pathway, so it cannot produce large amounts of force
  • It is the most efficient energy pathway, so it plays a key role supplying energy for longer duration activities
  • Fat metabolism requires the ETC to be functioning, so it can only occur aerobically
  • Fat is the most efficient way for your body to store energy: it contains the most energy per gram of any fuel type

Energy Pathway #6: Protein Metabolism

Under dire circumstances, like a precipitous drop in blood sugar, the body can break down skeletal muscle to generate energy from protein. This is less than ideal, because protein is better used for other purposes like rebuilding muscle.

It is also a very inefficient way of storing energy, as it contains the least amount of energy per gram of any fuel type. Humans have very little energy stored in the form of protein.

Similar to fat metabolism, protein can only be used aerobically, because it also enters cellular respiration via the Krebs cycle and requires the ETC to be functioning. Protein gets broken down to amino acids to supply energy.


Interestingly, the brain needs energy in the form of glucose, so fats can’t be used. However, amino acids can be converted to glucose, which means both carbs and protein can actually fuel the brain.

This leads to an important implication. Skeletal muscle can actually be sacrificed to produce protein for energy for your brain when glucose is depleted from the body.

This means it is extremely important to not fully run out of glucose. Otherwise, your muscles are getting broken down in order to fuel your brain, which is not very ideal for athletes.


  • The body utilizes protein as a fuel source under dire circumstances. This is not ideal
  • Protein fuel can only be used aerobically
  • The brain can utilize protein fuel, so if your body runs out of glucose, muscles are broken down to feed the brain


ATP is the body’s energy currency. The body has multiple different chemical pathways to produce ATP. These range from inefficient short-term pathways designed to produce energy quickly, to efficient pathways that produce energy slowly but more sustainably.

All of the pathways rely to some extent on inputs/fuel you give your body, in the form of nutrition. Thus, the next post (part 3), which focuses on nutrition, builds off of the knowledge from this post.

By the end of the next post, my goal is to create a personal master sports supplement and nutrition wellness guide. This guide will be grounded in all of the scientific analysis I’ve done, and will contain concrete supplement and nutrition guidelines for how to be the best athlete I can be.


Lactic Acid and Free Hydrogen Ions

Lactic acid and free hydrogen ions are byproducts of anaerobic metabolism. As discussed, blood lactate is not harmful in and of itself. It is, however, highly correlated with free hydrogen ions, which are what inhibit glycolysis and your ability to produce energy.

Nonetheless, blood lactate is more easily measurable, and thus acts as a reasonable proxy of anaerobic strain on the body for studies and analysis.

During Exercise

As exercise intensity increases, blood lactic acid concentration doesn’t increase at first. This is because other tissues can use lactic acid. It can supply energy for the heart, or get reprocessed by the liver.

However, once intensity reaches a certain level, exercising muscles produce lactic acid faster than other tissues can clear it. Once this happens, blood lactate starts increasing suddenly.

This point of intensity is known as the lactate threshold, or anaerobic threshold. It usually occurs at an intensity of about 80% of the athlete’s VO2 max.

Going harder than the lactate threshold results in rapid fatigue due to the acidification of muscles from the free hydrogen ions. Eventually, glycolysis shuts down.

After Exercise

Lactic acid travels in the bloodstream to the liver, where it gets converted back to glucose during recovery after exercise. This process is called the Cori Cycle.

One interesting feature of the Cori Cycle is that oxygen is required to produce the ATP needed to power it. This is one reason why you breathe so heavily after sprinting or intense movement.

Lactic acid can also be oxidized back to pyruvate and used during cellular respiration for aerobic energy generation. This is the rationale behind cooling down after a heavy workout. A slow jog can “burn up” the lactic acid as it is oxidized to pyruvate, clearing it from the muscles.


1: Krebs Cycle Oxygen Requirement:

2: Lactic Acid, Track and Field:

3: Body Energy Pathways Overview:

4: Runners World (The Science Behind Bonking):

5: Exercise Exhaustion:

Published by Analytical Aspergian

I am an Aspergian who loves logically analyzing the world around me. On this blog, I analyze anything that interests me, from economic design to electromagnetism to sports nutrition and recovery.

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