EPOC - Excess Post-exercise Oxygen Consumption

If you are anything like me, when I first was learning about nutrition and our body's metabolism I was trying to take in information from everywhere.  It wasn't just good enough to know that eating green leafy veggies was healthy, I wanted to know why?  I didn't want to just accept that High Intensity Interval Training was a better way to burn fat, I wanted to know why?  I think the knowledge of knowing why is so important in making lasting changes to your lifestyle.  You don't just have to take someone else's word for it, you understand the science behind it.  If you guys are ok with it I would love to share some science with you from time to time, those of you that are like me trying to find your way with your fitness and health and want to go deeper you have us as a resource.  So today wanted to share with you the concept of EPOC or Excess Post-exercise Oxygen Consumption.  

EPOC, what is it and how does it affect weight loss?  

The main concept here revolves around creating an oxygen debt during exercise, that later needs to be repaid.  Let's start by explaining how the debt is created in the first place and to understand that I'll briefly explain our body processes energy.  

Energy  Transfer Systems

Our body has three major systems that provide our cells (muscles) with energy.  Let's use running as our activity of choice, specifically running as fast you can.  

ATP/PCr System
When you start running the first system that kicks in is the ATP/PCr system.  This system uses the energy in the bonds between Creatine and Phosphate in our cells.  This system will provide roughly 10 seconds of energy.  

Glycolytic System
After that, your body switches over to the Glycolytic System.  This system uses glycogen stores in your muscle tissue and breaks that glycogen down into glucose.  That glucose is then used to regenerate ATP (our cells energy source).  This system has some byproducts, pyruvate and NADH.  We eventually hit what is called the anaerobic threshold (AT), because ATP generation can't keep pace with energy expended, and we need to switch to the next system which requires oxygen.  A person will usually be able to exercise intensely for about 80-90 seconds before this threshold is hit.  You'll notice this when you are running when you have to make a more drastic slow down in your pace and you notice a little burning in your legs.  

Oxidative Phosphorylative System
Once you hit that threshold your body then kicks into the third system the Oxidative Phosphorylative System.  This system needs oxygen in order to function, hence oxidative in the title that means with oxygen. This system is the real powerhouse in energy transformation and supplies our cells with a bulk of the energy they need after that initial 90 seconds or so.  Activities that do most of the work in this system would be considered aerobic activities because they are done WITH oxygen.  

Repaying your Debt

Oxygen consumption doesn't immediately match up to energy demands when you begin an activity.  As explained above it can take up to 2 minutes before your body kicks into that third stage of ATP generation.  Prior to hitting your anaerobic threshold you are creating an oxygen deficit quickly.  The rate slows as you get into the oxidative phorphorylative system but you never fully catch up.  Therefore, the more intense your workout/activity, will result in a larger oxygen debt or deficit.

This is where EPOC comes into play.  After you are done working out your body needs additional oxygen to do 3 main tasks:*

1. Metabolize additional nutrients
2. Replenish the energy stores that have been used up
3. Reload the depleted oxygen stores in the muscle and blood

Other things that also play into EPOC include elevated post-exercise body temperature, increased activity of the heart and respiratory muscles, and elevated levels of metabolism-boosting hormones.  Excess oxygen is also needed to aid in protein synthesis and recovery of muscles that were stressed during your workout.  

Therefore, exercise intensity directly relates to longer excess post-exercise oxygen consumption (EPOC).  This is why we recommend the workouts that we do versus straight cardio or traditional weight lifting with longer breaks between sets.  It is because of this that higher intensity workouts that involve some strength training is the best solution for fat loss.  If your goal is something else, than there are other options out there.    

Just one specific example of how EPOC works, in one study an individual that did circuit weight training (bench, power clean, and squats), 10 reps, 4 sets of each and completed all of that in 31 minutes, had an elevated EPOC for the next 38 HOURS!!!!  They actually stopped measuring at 38 hours so it actually could have been longer. This is where the magic happens when it comes to boosting metabolisms and fat loss.  It's not what happens during the workout that really matters its what our bodies do to recover from that workout that really affects body composition.  

I'll end with a quote from Alan Aragon:

"Caring how much fat is burned during training makes as much sense as caring how much muscle is built during training."

*Information from The Essentials of Sport and Exercise Nutrition, Certification Manual 2nd Edition