The Three Energy Systems Part 3

The final instalment in this blog trilogy covers the final aspect of the energy making cycle and the most frequently used system, the aerobic energy system. We use the aerobic system aka the oxidative system primarily during endurance based actives, low intensity exertion and also when we’re at rest. It primarily utilises carbohydrates and fat as nutritional fuel sources and in emergencies uses protein during extreme endurance events or in situations of malnourishment.

During higher demand endurance events like marathons or before the duration of the exertion increases beyond approximately 90 minutes, carbohydrates are used as primary energy substrates. However, for long duration sub maximal/low intensity exertion, the preferred fuel source shifts to fat as an energy substrate. This is why a renowned method of fat loss, referred to as “steady state cardio” is prescribed to some people during their weight loss ventures.

To fully comprehend an understanding of “high demand” and “sub maximal” aerobic efforts, a personal favourite method of measure to explain this, is by using an RPE scale. RPE stands for rate of perceived exertion and is an effective measure to describe how or when we use certain energy systems. 0 – reflects no effort at all and 10 represents maximal exertion. For the purpose of summarising briefly in this blog, I would categorise as follows 1 – 4/5 = Aerobic, 5/6 – 7/8 = Anaerobic and 8 – 10 is the Phosphagen system. Therefore 4/5 RPE is high aerobic exertion (carbohydrate utilisation) and 2/3 is sub-maximal (fat utilisation).

Aerobic energy production is a much more complex chain of events compared to the two other energy system pathways. The aerobic system utilises carbohydrates sources, by initially beginning with glucose (carbohydrates in the blood) or glycogen (stored carbohydrates in muscle tissue/liver) glycolysis (carbohydrate breakdown). Once the body breaks this down, as I’ve explained in my previous blog, it creates a byproduct called pyruvate. If intensity is sustained as a high output, we use the anaerobic system for ATP regeneration until activity ceases or intensity is reduced.

When the intensity is reduced, we subsequently move down the RPE scale to the stated 1-4/5 RPE and subsequently utilise aerobic energy generating pathways. If oxygen is present (which is a requirement for aerobic production) the acid pyruvate, rather than being converted to lactate is transported to the mitochondria in our cell and converted by an enzyme known as pyruvate dehydrogenase, to a molecule known as acetyl-CoA. The pyruvate now acetyl-CoA, subsequently enters the Krebs cycle for oxidation.

The Krebs cycle is an aerobic mechanism to generate cellular energy (ATP) and the reader should refer to a credible source for a more in depth explanation, that this blogs sensible word count wouldn’t accommodate. Summarising the process, pyruvate (a byproduct or carbohydrate glycolysis) has hydrogen atoms removed to create NADH and FADH, which then through oxidative phosphorylation is oxidised in the electron transport chain (ETC). The ETC uses the NADH and FADH molecules to rephosphorylate the spent ADP, back into ATP. 38 molecules of ATP are generated through blood glucose and 39 molecules of ATP are generated is pyruvate was created from glycogen.

Fat is a large supplier of ATP if it is oxidised via the aerobic system. The triglycerides in fat cells are enzymatically broken down to produce free fatty acids, where they also enter our mitochondria and undergo oxidation known as “beta oxidation”. Certain triglyceride molecule can yield over 300 molecules of ATP per fat molecule, thus making it a very powerful supplier of ATP if intensity and nutrition is tailored correctly.

So a long trilogy of blogs has come to an end. I hope the information provided gave you food for thought on how we generate and recycle energy. This topic is a good source to help guide your food and exercise selections. I encourage further reading into this topic and hope it helps you appreciate your body and its functions.

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