Energy Systems in Your Body

While a calorie is a calorie, the “burning” of calories (aka food) consumed can be very different.

In the human body, there are two separate systems that break down macronutrients to produce energy, and three different energy systems tailored to the type, intensity, and duration of activity.

Energy Currency

To make use of the three different bioenergetic systems, the body uses one predominant molecule to store and transport the energy: adenosine triphosphate (ATP).

Energy is produced when one of the three phosphates from ATP splits off, fueling your movements and warming you up, while also leaving adenosine diphosphate (ADP) and a lone phosphate behind.

ATP ←→  ADP + Phosphate (P) + energy

When you start moving, you begin drawing on your small ATP stores. The three energy systems kick in to help replenish ATP levels and keep you grooving, but your body may prioritize activity of one system depending on the nature of your activity.

Phosphocreatine (PCr) System

The phosphocreatine energy system is prioritized during the first 15 seconds of very high-intensity activity, in the absence of oxygen (anaerobic). This includes activities such as weight lifting, high and long jumps, and the 100-meter dash.

This system exploits the ability of a protein, creatine (Cr), to bind and store phosphate molecules in the form of phosphocreatine (PCr), that can replenish a recently used ATP by providing the phosphate it just split off.

PCr → P + Cr + energy

            ⬇

ADP + P → ATP

This allows for a rapid replenishment of energy currency for a short burst of high-energy output. If you are still exerting lots of energy, your body will be forced to utilize the next system.

Anaerobic Glycolytic System

After the first 10-15 seconds of intensive exercise, and until about 3 minutes, the anaerobic glycolytic system is prioritized, breaking down glucose (carbohydrates) without the presence of oxygen to produce ATP.

The process is relatively inefficient, ‘leaking’ energy as heat, and leaving an undesirable by-product known as lactic acid. This is why you find yourself wiping your brow after sprinting to catch your bus on time.

This system is mainly active during sports events such as the 800-meter sprint, downhill ski racing, and sprints during soccer or hockey games. Still high-intensity activity, but not all out at once.

The final system applies to almost every activity you could think of.

Aerobic Oxidative System

The aerobic oxidative energy system uses the oxygen to help breakdown carbohydrates, then fats, and proteins (as a last resort), to produce ATP. The macronutrients are broken down in a stepwise fashion to efficiently harness the energy and minimize the losses as heat.

This system is active when any activity occurs at an intensity below the threshold of the anaerobic glycolytic system. Daily activities such as walking, jogging, chores around the house, all use this energy system.

Now remember, all systems turn on when activity begins. However, depending on the activity, intensity, and duration, one may be prioritized to better fit your energy needs!

To Sum It Up

The energy currency within the body, ATP, produces energy when one of the three phosphates breaks off. ATP replenishment occurs in three different systems within the body depending on the type, intensity, duration of activity.

The Phosphocreatine system uses a protein called creatine that provides phosphates for almost immediate ATP recovery within the first 15 seconds of very high-intensity activity. The anaerobic glycolytic system breaks down glucose inefficiently to recover ATP quickly, usually working between 10 seconds to 3 minutes after activity starts. The aerobic oxidative system uses oxygen to slowly yet efficiently harness energy from all macronutrients and recover ATP levels.

- Lucas Roldos

Disclaimer: The information provided is meant to spread knowledge and induce interest for educational purposes. It is based on limited research. We try to pull the overall message of the literature, but further research may be necessary.  What is done with the information or suggestions is solely the consumers decision. The information provided is not meant to treat or diagnose any medical condition. References are provided for informational purposes only and do not constitute endorsement of any website or other sources.

Reference:

Bean, Anita. (October 2017). The Complete Guide to Sports Nutrition. London: Bloomsbury Sport.

Wells, GD, et al. (September 2009). Bioenergetic Provision of Energy for Muscular Activity. Paediatric Respiratory Reviews.

Other references provided within the text as hyperlinks