How Does Your Body's Energy System Work

​You probably heard many times while discussing nutrition topics with your trainers or reading up on the subject, that food is fuel for the human body, like petrol or gas for a car engine. But how much do you know about how your body generates energy from your food? 

​That's what we'll be looking into today. I could discuss the topic on an academic level to such technical details that this article would be very long and potentially dull if I attempted that. However, I think it's essential that you understand how specific activities affect your body's energy system and what's behind the recovery advice you often receive from The Barracks Gym team. I'll aim to put it into simple terms, but beware; I will need to go into biochemistry just a little bit though.

What is ATP?

ATP is a molecule called adenosine triphosphate, and it's the energy currency for cellular processes. In other words, ATP is the same for our body as money for our society. Nothing happens in the human body without "spending" ATP. Which means we also need to "earn" ATP.
The breakdown of ATP generates energy in your body which is required for ANY muscle contraction to take place.

​In short, in the presence of water ATP breaks down into ADP (adenosine diphosphate) and energy. We have minimal stores of ATP so for the work to continue, the body needs to replenish ATP; this happens through chemical reactions by adding back the third phosphate group to the ADP molecule.

How Does Your Body Produce ATP?

Ultimately it comes down to the food you eat and the type and the intensity of exercise or activity you perform.

Very simply put, the macronutrients we eat will be digested and broken down into their simplest form: carbs into glucose, protein into amino acids and fat into fatty acids, all of which are then transported through the bloodstream to either be metabolised or stored.

The Three Energy Systems

How your body will then produce energy from the available molecules will vary depending on the type of activity or exercise you perform. In most cases, you'll be using a combination or all of the three energy systems we have; the necessary effort will determine which ones.

1. The Phosphagen System​

The Phosphagen (ATP-PC) system provides immediate energy to the muscles for a short period (1 to 12 seconds at maximum effort) immediately after muscles contract. Muscles have a small amount of ATP and similarly small amount of PC (phosphocreatine) stored. The latter would be used to delay muscle fatigue when your body runs out of ATP until another energy system takes over.
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While the ATP-PC system can only support activities for a short burst, because it's available instantaneously, exercises like sprints, heavy 1-3RM lifts or high power movements like throwing a ball or jumping high will require and use this system.

2. Anaerobic Glycolysis

As your body starts to shift towards sustained activities, it also needs to activate the system that's best suited for the effort and the type of exercise you are performing.

The Glycolytic System takes over from the ATP-PC system. The carbohydrates you eat end up either as glucose in your blood or glycogen stored in your muscles and liver as a result of the breakdown process. Your body then breaks them down further to produce ATP through a process called glycolysis; it happens without the presence of oxygen which is why it's called anaerobic. However, oxygen will play a part later when the body is dealing with the by-product: pyruvic acid (“slow” glycolysis).
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"Fast" glycolysis
In activities where you are still trying to maintain maximum effort after your body's ATP stores are empty, you'll enter this phase which can supply energy for up to another 30 seconds.  Maintaining intensity is not possible, and as lactic acid accumulates, moving will be increasingly harder, and you'll feel your muscles getting fatigued.

If you try to keep up and still produce maximum effort at this stage, you may be able to push it for another 50 seconds, but muscle fatigue eventually will take over, and your performance will drop significantly as your body moves onto using oxygen. (Check out our blog on VO2 Max.)

Think of how you feel when attempting a full intensity sprint; when you start feeling you are slowing down and if you keep pushing you'll be forced to jog and then walk.
 
"Slow" glycolysis
If you are interested in the biochemical processes that help you keep going a little further and delay fatigue here, look up what other metabolic pathways there are for pyruvic acid in the presence of oxygen, what the Kerbs cycle is and how it helps to avoid extreme fatigue when expressing significantly less effort.

This slow glycolysis is not exactly anaerobic anymore, it’s more of a transition period before Aerobic Glycolysis comes into play.

Exercises like a 400m moderate run, your beep test, or a start of an endurance activity before a steady state is achieved would all use this system that can supply energy for up to 3 minutes before the oxidative energy pathway has to step in.

3. Aerobic Glycolysis

​You would think once oxygen is in the picture, creating energy would become more straightforward. It's quite the opposite and mainly because, in the presence of oxygen, there are more types of fuel available for the body to use as well as multiple ways.

There are three metabolic pathways to create ATP in an oxygen-rich setting:
 

• Krebs cycle
• Electron Transport Chain
• Beta Oxidation

 
Feel free to check out the above links if you'd like to understand these processes in-depth; I won’t bore you with them here.

In aerobic glycolysis, your body can use both carbohydrates and fats as fuel. This process is much slower than the previous two, so it would only happen during low-intensity longer duration activities: getting through your day by doing your normal activities uses your aerobic system, so does long walks, long duration low intensity runs or gardening.

Breaking down fatty acids takes longer than carbs though. The body will always choose the optimum pathway; first, it will use all readily available carbohydrates before starting to break down your fat stores, which is why bodybuilders are spending hours on end doing steady state cardio when they are cutting for competition.
 
What About Protein?
Breaking down amino acids is such a hard job for the body that unless the carbs are depleted and there is only a minimal fat store, it won't likely happen in a healthy adult. This also means that it’s very hard to lose weight from muscle. You may lose water or glucose that is stored in your muscles, that makes you look smaller, but the protein (amino acids) won’t be lost.

Wrapping Up

​The three energy systems in our body work in close connection to aid optimum performance depending on the intensity and duration of the activity and the type of fuel available.

When you look at it that way, you will understand that finding the right balance in both nutrition and exercise programming is essential to help your body work at its best for the desired results.

Need help figuring out what your body needs for improved performance? Feel free to grab your 2 Week Trial.

Sources:
https://courses.lumenlearning.com/boundless-biology/chapter/atp-adenosine-triphosphate/
https://chem.libretexts.org/Bookshelves/Biological_Chemistry/Supplemental_Modules_(Biological_Chemistry)/Metabolism/Catabolism/Kreb's_Cycle
https://www.khanacademy.org/science/biology/cellular-respiration-and-fermentation/pyruvate-oxidation-and-the-citric-acid-cycle/a/the-citric-acid-cycle
https://bio.libretexts.org/Bookshelves/Biochemistry/Book%3A_Biochemistry_Free_and_Easy_(Ahern_and_Rajagopal)/06%3A_Metabolism_I/6.11%3A_Fatty_Acid_Oxidation
https://www.acefitness.org/fitness-certifications/ace-answers/exam-preparation-blog/3256/the-three-primary-energy-pathways-explained
https://breakingmuscle.com/fitness/understanding-energy-systems-atp-pc-glycolytic-and-oxidative-oh-my