How the human body uses its energy sources

Carlos Anthony Castro
3 min readNov 23, 2022

A client of mine recently did a 4 rep max on his bench press. Totally fatigued, he said he was ready to start again within 1 minute. But I told him to take at least 2 more minutes to recover. He has a particular goal of getting a stronger bench press weight. My client challenged me here. And I know this client in particular is a science geek. So I asked him about the human bodies “cleanest source of energy “, and how long does that energy source take to recover. His answer, at least 3 to 5 minutes, which is correct. My reply, “well why should you only be resting for 1 minute right not after that set “. He laughed, but agreed.

Bioenergetics is a branch of chemistry that explains how the human body uses energy. And its a very complex subject. The purpose of this blog is to simplify it and help people understand what we need to do to reach our personal goals as it pertains to energy systems. So lets get started:

There are 3 basic macronutrients, proteins , carbohydrates, and fats that food consist of. When we consume food, it’s broke down, refined and released into our body for energy, similar to how an automobile uses gas(for our bodies this is called Metabolism). Once released , its stored first as adenosine and phosphate, 2 nerdy molecular terms referred to as ATP, our body’s “cleanest source of energy”. In addition we store carbs in our muscles and liver & muscles (assuming we’re eating carbs, nerdy term glycogen/ carbs). Then theres the remaining amount of what we’ve consume if not burned off, to be stored as body fat( the human body is really good at this one !).

Quick review, so we have energy storage of ATP, carbs/glycogen, and the rest as body fat. Now lets talk more about how the human body uses all of the above for energy.

Imagine this, you’ve got to break out into an all out sprint as fast as you can!Run! The body’s first source of energy ( ATP) would allow the first 15 seconds of energy at the highest intensity. Should you chose to keep running, our cleanest source of energy needs time to recover, so the next source of energy has to take over. The next energy system, known as fast glycolysis is where we break down the next source of fuel, glycogen/ carbs . And it takes a little more time to convert this fuel into energy( ATP). So we’s gotta slow down that sprint. This will generally last us about 60 more seconds at a slower pace.

If we chose to keep running, “ATP our cleanest source of energy” is out of gas, so is the aforementioned “fast glycolysis”. Theres another energy source left, body fat. And as mentioned above we have plenty to store for energy! But body fat breaks down much slower than sugar and our cleanest sources of energy. So the body simply cannot perform at the same level of intensity. So we can keep going, but we’ve gotta slow all the way down .

But the human body is smart. So we aren’t necessarily any one particular energy source at once. It’s generally a composition of all pending the intensity level of what activity we’re doing. I myself am a marathon runner. Marathon running is a light , moderate intensity level of activity, that would primarily rely on body fat as energy. But carb loading is still extremely important . So along that course the human body will use all sources at some point.

As it pertains to the general population that has been mislead to believe that the least/ moderate intensity level is what’s helping reach you goals/ the “fat burning zone”. Let’s take a step back, and understand how the human body uses energy. Bioenergetics is again, a very complex subject, broken down here for the general population.



Carlos Anthony Castro

*Personal Trainer* Marathon Runner * Instagram @iron_endurance_training