Your body is constantly producing energy to carry out your life. From heartbeats to steps to intense workouts in the gym, you get energy thanks to a little molecule called Adenosine Triphsophate aka ATP.
ATP is present in small amounts in your body but it has to be recycled to keep making energy. You use three main fuels sources to recycle it: creatine, glucose and fats.
In the fitness world, we call these the three energy systems and we will take a closer look at each of them in this article.
Table of content
How ATP is recycled
The phosphagen system
Glycolysis
Oxidative system
How ATP works and how it's recycled
ATP is a molecule that contains three phosphate bonds. To create energy, your body breaks of one of these bonds.
The result is an electrical impulse (the energy), one molecule of Adenosine Diphosphate (ADP) and an of phosphate bond.
Your body is a very smart machine, and once it notices all the ADP and phosphate bonds, it will bind them back up together to make a new ATP molecule.
In order to do this, your body will need to use one of three other molecules:
Creatine (ATP-PC system)
Glucose (Glycolysis system)
Fats and oxygen (aerobic system)
Depending on the intensity of your activity, your body will use different energy systems. The graph below shows you roughly which system is used during which intensities.
Here is a quick cheat sheet.
The creatine system is used for the first 10-15 seconds of intense activity.
The glucose system is mainly used between 15 second and 3 minutes of activity
Any activity lasting longer than 3 minutes will predominantly use fats and oxygen.
The above graph is a rough approximation of what's happening in your body. It's not like you suddenly switches from glucose to fats at the 3 minutes mark. Similarly, you will be using some glucose to replenish ATP during long bouts of exercises, just very little of it.
Now let's take a look at each of them individually.
Creatine: the phosphagen system
Let's pretend you want to catch your bus, and you see it start to leave without you. You start running as fast as you can to catch up to it.
Your brain tells your leg muscles to contract to make you run. You have some ATP stored in your muscles which get's used up in the first milliseconds of the effort. This creates ADP and your body starts using creatine to turn that back into ATP.
Since you are sprinting so fast, you are burning through your ATP stores extremely quick. Thankfully, creatine is stored within the muscles and is immediately available to turn ADP back into ATP. Happy days.
Unfortunately, you don't have that much creatine in your body. You will run out of it within 10-15 seconds. That's why you body will then turn to glucose next.
Glycolysis: When we start burning carbs
The driver hasn't noticed you and keeps driving. You keep on running but now you feel like it's not your maximum speed. You have to slow down. That's because using glucose to recycle ADP back into ATP takes a bit longer.
Glucose is the resulting molecule when we eat any types of carbs. Bread, pasta, fruits, candy, all that get's digested into glucose. We have glucose in our blood at all times, in our liver and muscles in the form of glycogen. Glycogen can be turned into glucose really quickly.
It makes sense when you think about it. Your muscles need help recycling ADP. Whereas creatine was readily available in the muscles, glucose needs to come from the blood stream or you need to convert glycogen into glucose.
Converting glucose into energy is a ten steps process.
Now, once the glucose goes through the chemical reactions that produce ATP, you end up with two molecules of pyruvate, and two molecules of NADH (nicotinamide adenine dinucleotide).
Let's shelve the NADH for now and come back to it in another post.
Let's return to our bus.
You keep running, propelled by glucose. You have way more glucose than creatine, so this this system will power your run up to three minutes.
After three minutes, your body will start to use fats. This is when "fat burning" finally occurs.
The aerobic system: the fat burning furnace
You've been running after your bus for three minutes now, and you start to really slow down.
This is due to the fact that your body now has to get fat molecules and turn them use them to recycle the ADP.
This process takes a while because fat cells, unlike glucose, are not in the blood but stored in various areas of your body. However, it's a very efficient way of producing ATP. When you start using fats as fuel, you slow down considerably BUT you could keep going for much, much longer.
You may remember from early posts that 1 gram of fat produces 9 calories. This means that 1kg of fat would produce 9000 calories. If a person has 10 kg of fat on them, they would have a reserve of 90000 calories. For reference, a 45 minutes HIIT workout will burn anywhere from 300-600 calories. When you hike for a long day (6 hours or more), that's anywhere from 1000-2000 calories.
Now keep in mind this is a theoretical limit. You could theoretically run after that bus until your reach your destination.
Once we start talking about how far the aerobic system can take us, we enter the realm of extreme endurance and fatigue, which is an entirely different discussion than this post.
Conclusion
Your body uses creatine, glucose and fats to recycle ADP into ATP. The duration and intensity of your activity will dictate which one you use. Short and very intense exercises such as sprints or heavy lifting will use creatine. Shortish and more moderate exercises such as group fitness classes, interval training or playing sports will mainly use glucose. Any slow and long exercises such as jogging, hiking, snow shoeing, cycling will use fat.
In a later post we will check out how to train each of these systems to have a solid cardio foundation to make us feel amazing!
Clem
sheeeeesh was not ready for the awesome "hoochie daddy shorts" in the costa rica pic!! 🔥🔥🥵 Great post though! Thanks for the helpful info