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Fat Burning During Exercise

Updated: Apr 30, 2023

Exercise metabolism is a complicated thing.

Our bodies run on a molecule called adenosine triphosphate (ATP), to power virtually everything in our body. We make ATP by breaking down the macronutrients from the food we eat. These 3 macros are carbohydrate, fat, and protein. For ATP production, carbs and fat are the most important. There are 3 systems at play that power our workouts. ATP-PCr, glycolysis, and oxidative phosphorylation.

ATP-PCr breaks down phosphocreatine for short bursts of high intensity for up to 15 seconds or so (sprints or 1 rep max). If exercise continues past 15 seconds, glycolysis breaks down the carbs in our bodies from glucose or muscle glycogen for the next 90 seconds or so of work (sets of bicep curls). And finally, as exercise continues after that, we call call upon the aerobic system, also known as oxidative phosphorylation, which utilizes both carbs and fats (jogging).

Figure 1. Energy systems used during exercise. (3)

The oxidative phosphorylation system is made up of the Krebs Cycle and the electron transport chain and it is the only way we utilize fat for ATP production. When using carbs, we first use glycolysis, and when oxygen is available in the muscle, the pyruvates from glycolysis enter the Krebs Cycle which powers the electron transport chain. For fat, however, this process is much different, and the exercises we choose can impact how much fat gets broken down and used during exercise.

Our fat is stored within the body as triglycerides, which are composed of a glycerol and 3 fatty acid chains. We store these triglycerides in adipose tissue, and in smaller amounts in the muscle fibers as intramuscular triglycerides (IMTG). Muscles that are more oxidative (type I muscle fibers) have a better ability to burn fat and have more of these IMTGs than the type II fibers which can’t burn the fat as efficiently.

The first step in actually using fat to fuel our exercise is to stimulate lipolysis. Lipolysis is simple the fancy term for the breakdown of the stored triglycerides, so that we can access the fatty acids for ATP production.

The process of burning fat is slightly different depending on it coming from adipose tissue or IMTGs, with some similarities. First, lipolysis takes place via the action of the enzyme, hormone sensitive lipase (HSL). Second, they both are converted into a molecule known as fatty acyl-CoA in the muscles prior to what is called beta oxidation.

HSL is activated in the presence of epinephrine and norepinephrine, the hormones glucagon, growth hormone, and cortisol. While insulin inhibits this enzyme and instead promotes the synthesis of more triglycerides (1).

When epinephrine and norepinephrine are released via exercise, HSL is activated to catalyze the separation of the glycerol and fatty acids. When this happens in adipose tissue, these two molecules are sent into the blood so that they can be transported to muscle. Since IMTG’s are already in the muscles, they bypass this step.

The fatty acids cannot flow freely in the blood like glucose can. They need the help of a transport protein to reach the muscle, so they bind to the protein albumin. Exercise that is lower intensity increases blood flow to the adipose tissue to collect these fatty acids, bind them to albumin and bring them to our muscles. High intensity exercise, however, drives more blood to our muscles, reducing blood available for the adipose tissue. While at the same time, blood lactate concentrations rise, prompting re-synthesis of the glycerol and fatty acids back into triglycerides (1). Thus, to increase the likelihood of burning fat during exercise, keeping our intensity low, can increase availability of fatty acids to be taken up by the muscles.

When the fatty acid albumin complex is at the capillary bed of the muscles, it attaches to an albumin binding protein on the capillary wall and passes through the endothelial cells of the capillary to the muscle membrane. Here two more transport proteins help to bring the fatty acids into the muscle cell, these are fatty acid binding protein (FABPpm), and fatty acid transporter protein (FAT/CD36).

For the muscles uptake of fatty acids to continue, there must be a lower concentration of fatty acids inside the muscle compared to outside of it (1). So as exercise intensity increases, less blood becomes available to adipose tissue, which may reduce the amount of fatty acids in the blood, reducing the concentration outside the muscle, thus limiting muscle fatty acid uptake. Another reason why to burn fat, lowering the intensity is a key factor.

Once the fatty acids are present in the muscle, from adipose tissue or IMTG, they are converted into fatty acyl CoA, via the enzyme fatty acyl synthetase.

The fatty acyl CoA must be transported into the mitochondria (the powerhouse of the cell), by binding to the molecule carnitine, and is assisted by the enzymes, carnitine palmitoyl transferase I and II (CPT I & II). This is the most important step in fat metabolism, and is the reason why many fat burning supplements use carnitine, carnitine precursors, or CPT I and or II as their main ingredient.

Figure 2. fatty acyl-CoA transport inot mitochondria. (2)

Once in the mitochondria, beta oxidation can take place. In this chemical process, enzymes work to cleave two carbon units at a time from the fatty acyl chain to produce acetyl-CoA, which can enter the Krebs Cycle, and be used to support the electron transport chain. This process produces enormous amounts of ATP, anywhere from 100 to 300 molecules of it, depending on the length of the fatty acyl chain. Beta oxidation of fatty acids provides substantial amounts of ATP for continuous exercise that is of long duration but low to moderate intensity.

For us to use fat during exercise we have to understand what regulates beta oxidation. Unfortunately, it is not a simple answer considering the complexity of exercise metabolism. However, the general consensus is that the rate of lipolysis and beta oxidation is regulated by the rate and amount of carbohydrates used during exercise (1).

This is where research comes in. A study used an exercise protocol that started with a treadmill walk of about 3.7 mph, and increased the incline by 1% and the speed by 1 mph, every 2 minutes until exhaustion. As the protocol increased, so did the intensity and the % of VO2max being used. This study was intense enough to bring participants to 85 % of their VO2max. By analyzing the oxygen and carbon dioxide gas exchange using breath by breath analysis, they showed that carbs were providing 96-97 % of the energy for the prescribed intensity (4). Thus, providing evidence how the more intense we workout, the more carbs we use.

At higher intensity exercise, we need ATP at a very fast rate. This is done by utilizing carbohydrates. However, carbohydrate metabolism produces metabolic byproducts that inhibit the activity of certain enzymes required for beta oxidation of fatty acids.

During high intensity exercise, glycolysis produces rapid amounts of acetyl-CoA, and when there is too much acetyl-CoA within the muscle, it can get converted into malonyl-CoA. Malonyl-CoA can inhibit CPT I and limits the amount of fatty acid being taken up by the mitochondria.

In addition, some of this acetyl-CoA can bind to carnitine, reducing the availability of it for the fatty acyl chain transport into the mitochondria. Thus, if we want to keep the steady flow of fatty acids used during exercise, limiting the rapid amount of acetyl-CoA from high intensity exercise may be a good place to start.

When we train at higher intensities, we cannot keep our work output long enough to utilize fatty acids. Our muscles fatigue due to the byproducts from carbohydrate metabolism. For fat burning to be most effective during exercise, we need to be exercising long enough to utilize oxidative phosphorylation, which happens when we exercise at low to moderate intensities for a long duration (1).

This does not mean we have to run on the treadmill, bike, or be on the elliptical for hours. We can use different methods while still applying the principles of fat burning (low to moderate intensity & long duration).

Now there is nothing wrong with going on a piece of equipment for 30 to 60 minutes or a light jog around the neighborhood, it is just that these can get repetitive and we do need to increase our difficulty of exercise and provide some variation as we get better. So, I want to provide you with some strategies that may shake up your fat burning focused workouts.

One strategy is to alternate through different machines. For example, running on the treadmill for 10 to 15 minutes, then moving over to the bike for another 10 to 15 minutes. This is good for those that need to get an aerobic foundation and can help to break up the workout so that it is not as repetitive.

However, for those of you that are more experienced, the incorporation of circuit or block style training can be a great option.

If you are serious about training and you don’t have a way of tracking your heart rate and workouts, I highly recommend doing so. It really helps to see both progress and how hard you are working.

When performing circuits with fat burning as the intent, be sure to select exercises and weights that allow you to keep your heart rate between 50-70% of your estimated heart rate max. To find your heart rate max, I like to use the Tanaka heart rate estimation formula, 207 – (0.7 x age). If you do not have a heart rate monitor, your level of intensity should be at or just above conversation pace. This is an intensity at which you can maintain a conversation while you are exercising. This is rough estimate, but it is a good baseline.

Here are two examples that can hopefully give you ideas. Remember, it is always important to get a solid foundation of aerobic base and fat burning prior to trying a new method of workout. So, for those that are new to fat burning or exercise in general, I highly recommend easing into it. It will be hard to hold the ego back, but it will beneficial throughout your fat loss and exercise journey.

Figure 3. Polar heart rate monitor graph during exercise. Dark pink are is fat burning heart rate.

The above graph is from my Polar H10 Heart Rate Monitor. I followed a workout that followed the same as the second example above. The dark pink shaded area is fat burning zone heart rate. As you can see, my heart rate did not get excessively high, which allowed me to maximize my time using the oxidative phosphorylation to burn fat throughout the workout.

Remember, throughout this workout be sure to keep the heart rate between 50-70% of the estimated heart rate max. If the heart rate is getting to high, adjust the weight or the tempo that you are using. Adjust the exercise, weights, and duration based upon your level and goals.

Fat and carbohydrates are both used throughout exercise and are both very important to overall health and performance. But the degree to which one is used more relies on the intensity and duration. Low to moderate intensity exercise improves fat burning during exercise because it allows a steady flow of blood to adipose tissue, it limits glycolytic inhibition of lipolysis and beta oxidation, the ATP demand can be met, and it allows us to work for longer duration. There are a ton of other factors that regulate how well we burn fat such as diet, sleep, stress, and genetics.

However, I hope this article has helped to provide a better understanding of fat loss and how selecting the right exercises can help maximize its use during exercise.

References 1. Jeukendrup AE, Gleeson M. Sport nutrition. Champaign, IL: Human Kinetics; 2019. 2. Qu Q, Zeng F, Liu X, Wang QJ, Deng F. Fatty acid oxidation and carnitine palmitoyltransferase I: emerging therapeutic targets in cancer. Cell Death & Disease 2016;7(5) 3. 4. Peric, Ratko, et al. “Fat Utilization During High-Intensity Exercise: When Does It End?” Sports Medicine – Open, vol. 2, no. 1, 2016, doi:10.1186/s40798-016-0060-1.

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