Fats Vs Carbs: How Our Bodies Use Energy

ENERGY SUBSTRATE UTILIZATION: FAT VS CARBS

Fats Vs Carbs: How Our Bodies Use Energy

We have two primary forms of energy substrate in our body, and a few ways to store that energy. If you’re not up to speed on this, check my last blog post Fats vs Carbs: How Our Bodies Store Energy.

So our body runs on two main types of fuel: Fat and Carbohydrate. The way we use these two fuels, and the mechanisms behind them, are pretty darn different. Of course, there are other ways our body can fuel itself if we need to. You may have heard of ketosis, where we use “ketone bodies” for fuel, and we are also capable of utilizing protein if need be. We aren’t really focusing on these systems though for today. I want you to understand what happens in a well nourished body when it’s asked to perform under normal circumstances because isn’t that the goal anyway?

Fat -

Remember from my last post about this, we are referring to excess body fat, referred to as subcutaneous fat here. We are not talking about the other types of fat around your organs, referred to as visceral fat. Fat Oxidation is the process of “burning” fat for energy.

Carbs -

We also learned in my other Fats vs Carbs post that all carbs break down into glucose, and we store glucose in our body a few ways. We have blood glucose, muscle glycogen, and liver glycogen. Our body relies on these fuel sources for energy, primarily when we are exercising, not as much when we are at rest.
 

We also have 3 Energy Systems which are the processes that our bodies go through to effectively turn materials into energy. If you think back to your biology class days, you may remember good old ATP, adenosine-triphosphate. For ANY work to be done, the cell MUST have ATP. We burn glucose and fat to create ATP. And these are the systems that get that done:

Each of these systems will provide you with energy and produce ATP. They work at different rates though. The PCr system is the first energy system our bodies kick into but it’s also the fastest to die out. We can only sustain energy from Pcr for sprints, or for movements in the “3 or less” rep range. At that point the stored ATP is depleted and we have to dip into the Glycolytic system, which we can sustain for roughly 30 seconds - 2 minutes of work. After that, our bodies realize we are going to need more energy and soon. We usually see a decline in intensity and our body switches into the Oxidative system, where we continue to burn some carbohydrates but ALSO start to oxidize fat for fuel. This is also known as Lipolysis and we know from that other post that we typically have enough stored fat to keep moving and performing for a LONG time.

Now, it is worth noting that the first 2 energy systems listed do not require oxygen for the chemical processes to take place. However, the oxidative system, which is aerobic, does! This is because of the oxidation of fat in the Kreb’s cycle (which is just the name of part of the chemical reaction that occurs to produce ATP). Because the Krebs cycle takes more time to complete than our other energy systems it slows down our power output and overall intensity.

Also, just to clarify, all 3 of these processes are generally going to start at the onset of activity. Our bodies don’t consider these as individual processes and, amazingly, are efficient enough to just use whichever energy system is most appropriate for the demands. We use various percentages of energy from each system depending on what we are doing and shift back and forth until we reach maximal fatigue and simply can’t perform anymore. Our body is using the oxidative system the most purely when we are at rest, for instance when we are sleeping.

Ok, so the science is done but what does that all really mean when it comes to burning fuel for our workouts?

Many people read that and think “Oh my gosh! This means I need to do a lot of low speed, long duration, cardio to burn fat! That’s the answer.” But let’s be real here, if that were the answer, wouldn’t everyone be super fit and lean already? In order to effectively burn fat we have to be good at switching between these processes. Since we preferentially burn glucose, if we have a constant stream of incoming sugar, our bodies won’t ever have to dip into our back up stores and burn fat. We also won’t be very good at if when we need it most and that usually results in some crankiness. We have to train our bodies to be good at all of the energy systems if we want to truly be healthy and fit.


If your first concern is fat loss, we still have to make sure our bodies can switch back and forth well between these systems. To lose fat mass we have to be really good at burning it, and we have to make sure we are setting our bodies up for success. You have to train smart to prime those processes, AND you have to eat strategically so that you know your body has a chance to dip into those fat stores and burn, baby, burn!

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About Ashley:

Ashley is a Pilates instructor and entrepreneur in NYC who specializes in biomechanics and exercise nutrition for women. She uses the science behind exercise physiology to empower her clients to lead healthy, happy lives. Her passion project is writing “The LEAN Life Newsletter” which goes out every Tuesday and focuses on Lifestyle, Exercise, Active recovery, and Nutrition and serves as the backbone for her L.E.A.N. Life online health and fitness programs. She aims to get this information into the hands of as many women as possible so they can feel stronger and more confident in their bodies. For more information head to AshleyBrownPilates.com or @AshleyBrownPilates on instagram, or click here to get on the L.E.A.N. Life List!