Your aerobic energy system is always on; you cannot switch it off.   Your body is using oxygen to produce Adenosine Triphosphate (ATP) to fuel survival first and movement second.  If the demand for ATP increases beyond the capacity of the aerobic system, the anaerobic lactic system switches on to help meet the energy demands.  If the need for ATP are even greater, the creatine phosphate (ATP-CP) system jumps onboard so the body can produce its highest levels of energy.  As you know from the previous ATP-CP post, the ATP-CP system can only last for 10-15seconds before it fatigues and levels the energy production to the anaerobic lactic and aerobic energy systems.  After 30seconds the anaerobic lactic systems starts, and the aerobic system becomes the only system left to power the body.  The aerobic energy system is limited in its energy output, and depending on the energy demands it may not be able to keep up


During states of rest, the aerobic energy system is used to recharge the depleted anaerobic systems.  More often than not, the athlete must stop moving and breath deeply to allow the aerobic system to restore the anaerobic pathways.  Many people get confused when they look at the above diagram and think that the first 10-15seconds of any workout if fuels by the ATP-CP system.  The rate of energy production, duration of energy production, the proportion of energy expenditure and the work-to-rest ratio are critical components that dictate which energy systems are being used.  If the workout calls for heavy lifting or explosive power the ATP-PC system will be involved.  If an athlete is going for an easy jog, the aerobic energy system can often meet the energy demands without needing to call upon the other energy systems.  

Each energy system can be broken down concerning energy requirements:

  1. Rate of energy production - how rapidly ATP is regenerated during the work period. High rate = high power (weightlifting, track and field, powerlifting).  Maximum Rate = Anaerobic (lactic) driven

  2. Duration of energy production - how long increased energy must be produced for (triathlon, cycling, marathon). Long duration = almost entirely aerobic driven energy production

  3. Rate of energy expenditure - athletes with high levels of skill and quality movement patterns will have a lower energy cost of movement than those with poor movement quality and low levels of skill

  4. Work to rest ratio - Variation between length of work and rest periods. High peak power and longer rest - greater anaerobic contribution.  Short rest periods and/or longer work periods = higher level of aerobic contribution

Many people have the wrong impression of cardio training and for years the “F**k cardio” attitude has been growing, especially towards Low-Intensity, Steady-State (LISS) aerobic training.  You may think I’m crazy, and that aerobic training is a thing of the past, but let's not write it off before we know how the energy systems work.  Let's not simply drink the “Kool-aid" of high-intensity marketing hype. 

Aerobic training is a conditioning tool that when used correctly can improve weight loss, increase cardiovascular health, improve athletic performance, and aid in recovery. The aerobic energy system is the MOST IMPORTANT energy system as it produces the sustainable amounts of Adenosine Triphosphate (ATP) (energy) that fuels aerobic training and is critical for anaerobic recovery.  You can think of your aerobic capacity as the base of the fitness pyramid, the height of a pyramid is dictated by the size of its base.

In the last blog post ee mentioned the three aerobic pathways, the glycolytic, the kerb cycle, and the electron transport chain (ECT). We also started to look at some of the ways to improve the aerobic pathways.  Aerobic capacity training is the single greatest contribution a competitive fitness athlete can add to their programme.  The greater an athletes aerobic capacity, the harder the athlete can work before stepping into the anaerobic energy systems.

If you’ve been paying to attend to the details of our energy system pathway blog series (post 1, post 2, post 3, post 4), you would have noticed that glucose (carbohydrates) has been the primary source of energy production in the anaerobic creation of energy.  Except for creatine in the ATP-PC pathway.  Both protein and fat CAN NOT be turned into energy without oxygen.  Non-endurance athletes must recognise that the energy demands placed on the body are most efficiently fuels by carbohydrates.  If your training goals include building strength, explosive power, high-intensity intervals, circuit training, or interval based sports (most team sports), your diet needs to add quality carbohydrates.  Endurance based sports (marathons, triathlons, adventure racing, cycling, etc…) do not require as many carbohydrates because the low level of energy demands can be fuelled by fatty acid oxidation if correctly trained.  

By understanding how the body produces the energy we can start to make smarter decisions around our nutrition.  For example, non-endurance based athletes who are training for sports performance would be foolish to consider a KetoDiet; it just makes no sense when you look at the biology of ATP production for particular physical demands.  Long-term low carb diets mixed with “high intensity” training also have significant adverse impacts on thyroid, stress and sex hormones. We will talk more about this in future posts.

Knowledge is power, understanding energy production and energy expenditure can enhance your health and your performance.  You have the ability to choose your training modality and your nutrition to support your goals. This week we are going to investigate the biology of the Krebs cycle and build an awareness of how it produces energy.  Next week we will talk more about the Electron Transport Chain (ETC).

The Biology of the Kerb-Cycle:

If biology is not your thing you might want skip over this section. 

Glycolysis Pathway:

If we start with a glucose (carbohydrate) molecule the Glycolysis energy system pathway will take glucose to produce:

  • 2 ATP (energy)

  • 2 pyruvate (pyric acids) molecules

  • 2 Nicotinamide Adenine Dinucleotide NAD+ (crucial coenzyme in making ATP)

The glycolysis pathway can produce these compounds with (aerobic glycolysis) or without (anaerobic glycolysis) oxygen, but glucose is the key.

Pyruvate Oxidation:

The pyruvate molecules can then be oxidised (aerobic) to create:

  • Acetyl CoA - used by the Krebs cycle

  • 1 NADH - transformed from the NAD+ molecules

  • 2 Carbon dioxide (CO2) - a byproduct of oxidation 

The Acetyl CoA becomes one of the main enzymes used to power the Krebs cycle. The Acetyl CoA mergers with Oxaloacetic Acid to form Citric Acid.  

The Krebs Cycle:

The kerbs cycle takes place within the mitochondria.  The mitochondria are located within the human cells (we walk talk more about mitochondria in a future post). The Citric Acid is the first step in the Krebs cycle that is oxidised (aerobic) to produce:

  • 1 ATP (energy)

  • 3 NADH

  • 1 FADH2 - an input to the Electron Transport Chain (ETC)

  • 2 CO2 - a byproduct of oxidation

  • One Oxaloacetic Acid - an input for the Kerb Cycle

As we mentioned at the start, there were two pyruvate produced from the glycolysis pathway, which means we multiply the resulting molecules from both pyruvate oxidation and the Krebs cycle by 2 to get:

  • 2 ATP from Krebs

  • 2 NADH from Pyruvate Oxidation + 6 NADH from Kerbs = 8 NADH

  • 2 FADH

  • 4 CO2 from Pyruvate Oxidation 4 CO2 from Kerbs = 8 CO2

  • 2 Oxaloacetic Acid

Adding these values to the glycolysis pathway results in the following important molecules:

  • 2 ATP from glycolysis + 2 ATP from Krebs = 4 ATP

  • 2 NADH from glycolysis + 8 NADH from pyruvate and Krebs = 10 NADH  - an input to the Electron Transport Chain (ETC)

  • 2 FADH2 - an input to the Electron Transport Chain (ETC)

The NADH and the FADH2 will be used to fuel the ETC to produce 32 ATP, in a supper efficient healthy cell.

Fat and Protein in The Krebs cycle 

The above results started out with glucose as the input to the glycolysis process.  The kerbs cycle can also be used to catabolise proteins or fats.  Both protein and fats can be turned into Acetyl CoA, which can then pass through the Krebs cycle.  However, the resulting outputs of protein and fat catabolism are limited by what occurs in the Krebs cycle, the steps of the glycolytic pathway and the pyruvate oxidation are not taken:

  • 1 ATP (energy)

  • 3 NADH  - an input to the Electron Transport Chain (ETC)

  • 1 FADH2 - an input to the Electron Transport Chain (ETC)

  • 2 CO2 molecules

  • One Oxaloacetic Acid - an input for the Kerb Cycle

Protein and fats are not as ATP rich as carbohydrates in terms of fuel efficiency in the Krebs cycle, and they don’t supply as many FADH2 and NADH molecules for the ETC.


What about gluconeogenesis?

For every given macronutrient (carbs, fat and protein) the aerobic energy system can produce the largest amount of ATP.  The aerobic energy system has metabolic flexibility, which means it can use different fuels to power the body, making it adaptable to our nutrition.

Glycogen is stored in the muscles for quick use, it’s the primary fuel our body draws on to perform movements. It's not possible for fats or protein to be converted directly into glycogen because they are not made up of glucose, but it is possible for protein (amino acids) to be indirectly broken down into glucose, which can be used to create glycogen.  This process is called gluconeogenesis, and there are multiple pathways the body can use to achieve this conversion. Gluco-neo-genesis (glucose-new-creation) generally occurs only when the body cannot produce sufficient glucose from carbohydrates, such as during starvation or on a low-carbohydrate diet. This is less efficient than producing glucose through the metabolising of carbs, but it is possible under the right conditions. 

The gluconeogenesis pathway is driven by the Krebs cycle as the amino acids can be used as inputs to the Krebs cycle and produce Oxaloacetic Acid (the critical component of the Krebs cycle).  Some specific fatty acids can be input into the kerb cycle, but not all.  Fatty acids, in general, are not a primary fuel source for gluconeogenesis.

NOTE: Its important to understand muscles lack the enzyme glucose-6-phosphatase. Glucose stored in muscle as glycogen is unable to re-enter the bloodstream and is for the muscle and the muscle alone to use.  In other words, muscle glycogen is a stranded asset of glucose in the body to be used only by the muscle.  However, glycogen stores in the live can be converted back into glucose and released into the blood steam to fuel the muscles and the brain.  Why is this important?  Macronutrient timing can be used to restore glycogen in the muscle cells, improve insulin sensitivity, speed up recovery, and limit fat storage.

Proteins are vital molecules to the human cells, including muscle.  Do we want to break down our amino acids to produce energy?  In states of long periods of starvation, we want to try and save our muscles and not break down our amino acids.  Low levels of starvation cause our bodies to break down our lean muscle mass.  How many people are under eating in hope to burn body fat, but are actually creating muscular breakdown and therefore lowering metabolism.  

  • Our bodies need glucose to maintain a certain level of blood sugar (homeostasis).  Many so-called “experts” argue that our bodies don’t need carbohydrates.  If you stop and think about it for a moment, our bodies need glucose.  Our body needs glucose so much; it has a process to turn protein (and some fats) into glucose.  We could argue that glucose is essential to life and the most efficient way to sustain life would be to eat carbohydrates. The quality of the carb is important, eating crappy processed foods (pasta, bread, flour, artificial sweeteners, etc..) is not the same as eating fruit and vegetables.

  • A long-term low-carb, low-calorie diet mixed with “high intensity” anaerobic training (think most Crossfit programmes, F45, most circuit based classes) is the worst possible combination for health, athletic performance, fat loss, and hormone status.  And yet, how many people are following this approach? Even worse, how many coaches are suggesting it?

Its frustrating to watch so many stressed out people training hard, eating less, feeling like crap, and not getting the results they want.  They move from training programme to training programmer, from diet to diet, and they never make any real progress.  They fail to draw the link between energy production and energy expenditure because they are told to "move more and eat less."  Aerobic training is not the devil, quality carbohydrates are not the devil, calories are not the devil, marketing and sales of products, gym memberships that only promote "high-intensity,” and uneducated coaches are the devil.  I hope this posts made you stop and think a little more about your training and your nutrition.