to be consumed by the consumption

…either way, you end up paying for it.

Anything we do, anything, requires us to pay for it. Sitting, lying down, running, biking, sleeping all requires a certain level of fuel to feed these activities whether it be a small amount or large amount. The fuel used is to create the product required to carry out these activities. Think of it as putting fuel into your car, but unlike your car which can only use electricity or unleaded gas or diesel fuel, our bodies can use multiple fuels.

If you have read any of my posts, this might already sound somewhat familiar. I have touched lightly on how our body functions with respect to creating energy, but I thought now would be a good time to expand on this extremely interesting and compelling topic. Well, some of you might find it interesting and compelling. I’m not going to delve down deep into the weeds because I don’t think anyone coming here to my blog to read my posts are looking for that amount of detail. If you are, there are other sites out there that get into the exact chemistry on this topic.

My objective is to provide you with enough information that you will have an understanding of how important the right foods are, and also how important it is to focus on the intensity of our exercise. I believe if you have an understanding of this, you will look at what you are eating differently, and you may alter how you exercise.

First Off

Let me spend some time defining a few terms that will help you as you make your way through this post. This might help make it simpler to understand some of the content coming up.

Energy: The ability to do physical work. In this post, I’ll be looking at the conversion of chemical energy (food) into mechanical energy (muscle contraction).

Homeostasis: A state of balance where the demand for energy is easily met by the supply of available energy. An example would be sitting on the couch watching TV.

ATP: Adenosine triphosphate. ATP is energy currency of the body. Anything you do requires you to pay for it in ATP.

Anaerobic: Operating without the use of oxygen. Any activity you do where the process to create energy (ATP) does not require oxygen.

Aerobic: Basically the opposite of anaerobic. Any activity you do where the process to create energy (ATP) requires oxygen.

Getting into it

Now that we know a few terms I want to use the following example to introduce the concept of creating ATP, and how that happens at different levels of intensity.

You show up for your workout ready to go. Your body is currently in a steady state, or a homeostasis state as the body is able to provide the energy needed for the demand. Your instructor starts you off with a warm up of jumping jacks. This takes you out of the steady state as the demand for energy has increased and the body does not have adequate supplies. To meet this demand, the body responds by going to the anaerobic process of creating ATP. The body uses the anaerobic process because the cardio vascular system cannot respond fast enough to supply the oxygen needed to go to the more efficient aerobic system.

Because this is a warm up and being done at a low intensity level, the body should reach steady state relatively quickly and switch to the aerobic fuel system. With the low level of intensity, the body will use fat as the fuel source to create the needed ATP. The process of converting fat to ATP is very efficient in that one unit of fat can create approx. 140 units of ATP.

We’re about over 5 minutes into our warm up and about to start the workout. Your instructor decides that today’s work out is going to be an interval workout using 1 minute of high intense exercise followed by 2 minutes of mid intense level period of recovery.

The instructor signals you to start the 1 minute work period of the first interval. For the interval you are going to do 1 minute of high knees. You start the high knees and you increase your intensity to about 90% of your max. Your body responds by switching from the fat burning process used during the warm up to the glycolytic anaerobic fuel process. The glycolytic process will allow the body to work in the high intensity zone for about 2 – 3 minutes, using glucose and glycogen as the fuel to create ATP. In this phase, 2 – 3 units of ATP are created for each unit of glucose. Since our work period of our interval is only 1 minute, your body will stay in this zone as it is less than the 3 minutes your body is able to stay in this zone.

Our one minute is done, and we are now into our 2 minute active rest period. For the active rest period, the intensity level will be about 60% of our max. Because of the lower intensity, our body is now able to provide the oxygen needed and has changed to the oxidative system which is one of the 2 aerobic systems. At this level of intensity the fuel used is again glucose and glycogen. The other system of the 2 that are aerobic is the one that uses fat as the fuel, which we were using during the warm up. The oxidative system is a little more efficient than the glycolytic system as our body is able to get about 38 ATP units from one unit of glucose.

Our rest period is over and we are back into the 1 minute work period. When we switch to the work period we got a little excited as our instructor was very vocal about wanting more and working harder so our intensity switched to 100%. This switch to our maximum effort has now forced our body to switch to the creatine phosphate phase to create ATP. This system is very limited, and can fuel our body for only about 10 seconds since only 1 unit of ATP is created for every unit of creatine phosphate. But our work period is 1 minute long, and I have only 10 seconds of fuel? Does that mean that at the 11th second of the work period I’m just going to stop and my body is going to shut down? No, of course not. What happens is your body can no longer work at this intensity because it will run out of creatine phosphate, so it will force you to lower the intensity the the level needed where it can switch to the glycolytic system which I mentioned earlier will be able to provide fuel for 2 -3 minutes.

We’re done our work interval and are now into the active rest interval, using the oxidative system. Our body switches back and forth to these different systems as we go through the rest of our interval training. Our interval training is finished off with 15 minutes of low intensity work. Again, because the intensity level is low, our body is using fat as the fuel.

With the above example I have covered the 4 systems our body uses for create energy; creatine phosphate, glycolytic, oxidative, and the fatty acid oxidation system.

The 2 G’s

So what is glucose and glycogen? Glucose is basically sugar in our blood that is used to fuel our cells. Glycogen is glucose stored in our muscles and liver.  When needed, glycogen is converted to glucose. For simplistic purposes you can look at glucose and glycogen as stored carbohydrates. When you eat carbs you are providing the food needed for your body to create glucose and glycogen. Ideally the carbs you should be consuming are complex carbohydrates such as fruits, vegetables, grains, pasta and other sources.

Your body can hold only so much glucose and glycogen. I don’t know if you have ever heard of the phrase “carb loading” commonly used by endurance athletes such as marathon runners. Carb loading is the process of eating as much carbs as you can a day or two before your big event. The idea is that your body will not run out of glucose/glycogen during the race since you topped up your fuel tank through the carb loading.

The 2 G’s are very important for us when exercising as you may have already determined from the above example. For most of the interval training, our body was in either the glycolytic or oxidative system for creating energy. Because of this you want to ensure your body has an adequate store of the 2 G’s before your workout, and also following your workout it is important to replenish what you depleted by having complex carbs for your post workout refueling.


Generally speaking our bodies have enough fat to provide fuel for a long time. The fatty acid oxidation system is very efficient using only 1 unit of fat to create approx. 140 units of ATP.  Yet, I hear all the time about people working in the ‘fat burning’ zone. There is this myth that to lose fat you need to exercise in the fat burning zone. Bullsh*t. Sorry, but It bothers me that this myth is still perpetuated. People are exercising thinking that they are exercising correctly, but unfortunately will not see the results they are looking for. As an example, you exercise for 30 minutes in the ‘fat burning zone’, the low intensity zone, and your body requires 600 units of ATP to do this work. That’s just over 4 units of fat! Not that much is it. And, if you have read my other posts you will already know that one of the benefits of working in a higher intense zone, especially with interval training is that you will have an increased rate of metabolism during your off training time. Read the article if you haven’t already.

Post exercise

Following exercise our body needs to replenish the oxygen used during our workout and it also needs to take care of other tasks such as removing lactic acid, a byproduct of intense work. This process is called EPOC, or Excess Post-exercise Oxygen Consumption. The greater the deficit, the longer it takes to replenish the oxygen back to a steady state. The more intense the workout the more time it takes to reach this steady state. During the EPOC process the metabolism is increased to facilitate this. Are you putting it together? The more intense your workout is, the longer it takes you to reach steady state, the longer your resting metabolism is elevated. This means that when you are recovering following your workout, your body is mostly using fat as the fuel to facilitate the EPOC process. That is one of the benefits of training hard. They don’t tell you this when they talk about staying in the ‘fat burning’ zone. Of course not.

What’s missing?

I have touched on the fuels used, the 2 G’s, and fat. We know that carbohydrates are used to create the 2 G’s and well, fat is used to create fat. What isn’t here is protein. Why? Because protein is not a fuel source! Protein is not a fuel source for the creation of ATP.

Funny isn’t it. In my opinion, again this is my opinion only, this is why I will never have a high ratio of protein in the foods I consume daily. To me it doesn’t make sense knowing what our body uses for fuel. In fact, if your body is depleted in glycogen and glucose, and has to revert to using protein (the body will use protein it if absolutely has to) for fuel it will convert it to glucose via gluconeogenesis or other sources. But this process is really inefficient. But hold on, here’s the worse part about this. The protein the body uses is coming from your muscles! Your body is cannibalizing itself to create energy! All the hard work you put in creating that muscle is somewhat lost as it is now being consumed.

Very much like the water running down the side of hill zigzagging around rocks and other obstructions, your body is taking the path of least resistance. It’s doing what is easiest to create ATP.

That’s it

Did I carry out my objective? Do you have a better understanding of what our body is doing with respect to food, fuel, and energy? Maybe. It’s much more complex than this but I don’t think you need the complexity to better understand how it can impact our exercising, and our eating. The body is an amazing thing, and the more we can understand with regards to what is going on, the better our workouts will be, the heathier we will become. Training smart is much better than the alternative. Use this knowledge and incorporate it into your workouts and into your meal planning. Your workouts will mean much more to you knowing why you are doing these things. It’s very empowering.

Yours in health,









Author: darryl bennett

A certified Canfitpro personal trainer specialist, and a Yondan (4th Degree) black belt in Shorin Ryu Shorin Kan karate, training at Ferraro Karate under Sensei Stephen Ferraro. Also holding a certificate in Plant Based Nutrition from ECornell University. Fitness and health have been a big part of my life, and always will be.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.