You might have been told that if you exercise relatively slowly, you would be using fat as a fuel source, and if you are going relatively faster, you are using carbohydrate as a fuel source. Of course, the goal of endurance athletics (especially at events longer than 2-3 hours, such as marathons, half Ironmans, and Ironmans) is to use fat as a fuel source, to spare limited supplies of carbohydrates.
There might be two questions that come to mind:
1) At what intensity am I optimally burning fat as the predominant fuel source?
2) At what intensity have I reverted to burning glucose/sugar/glycogen as my main fuel source?
This is something that is extremely hard to pinpoint, and as a result, there are significant flaws with the “practical” methods that coaches typically recommend these days:
“Estimations” and their flaws:
- Anything below 70% of Max HR should be fat burning, and above that, we go into a more exclusive burning of glucose or sugar for energy
- 65%-80% of VO2 max, you are using fat
- If you are “conversational” you must be burning fat
- If you are 75% of functional threshold power, you are burning fat
As you can see, these are all estimations. Is it really possible that everyone is in their fat burning zone if they are exercising at 70% of Max HR? Do you even know what your Max HR is? The worst part about these generalizations is that they work for a normal range of the population, say 60%; however, for the other 40% of the population, these are really poor estimations.
How to identify the zones a bit more precisely:
Let’s go through a bit of basic science with respect to how we produce energy and movement. We will assume “ATP” (adenosine triphosphate) is what is required to produce the necessary energy for movement. Of course, it is a more complicated process, but let’s just stick with this assumption.
And, let’s assume that there are three zones for metabolism, i.e. the production of energy:
1) Fatty acid (fat) metabolism via Beta Oxidation – this is where fatty acids are used to produce ATP. This requires a substantial amount of oxygen in the mitochondria to yield the ATP necessary for energy production. However, for a comparable amount of glucose, fatty acid yields far superior ATP amounts. The limitation is the oxygen delivery to the cell and mitochondria for usage. The process is also the slowest.
2) Glycolosis – usage of glucose/sugar to yield ATP. If you demand a level of energy that requires faster generation of ATP, one where the beta oxidation method cannot provide quickly enough, then glucose is required help produce the requisite ATP. However, this process is not as efficient as the fatty acid metabolism as described above. In addition, the metabolism of glucose in the production of ATP produces carbon dioxide. Yes, you will end up exhaling this. There are different degrees of glycolysis; the higher the demand for ATP, the quicker the glucose is depleted, and the more carbon dioxide is produced.
3) High-energy Phosphagen – “all out” – when your body demands energy and you cannot supply it via either oxidating fatty acids or glucose, you will end up producing energy through the phosphagen pathway. This is similar to the energy that is produced during sprinting or weightlifting.
What does respiratory gas exchange have to do with this?
Some scientists use RER (respiratory exchange ratio) to determine whether carbohydrates or fat are responsible for generating the energy that you are using. Other scientists use the Fraction of Air that is Expired Oxygen (simply, “FeO2”) to determine what kind of zone you are exercising in. While there are benefits and drawbacks to both measures, the next time you are conducting a VO2 max test, don’t just pay attention to the final VO2 number at the end; ensure that you get the data print out from the exercise science practitioner so that you can analyze the trends in RER or FeO2. You will notice that when the exercise becomes more difficult, the RER or FeO2 curve will change in slope. The reason is that as a higher intensity is reached, you will need to produce ATP quicker, which in turn requires a “quicker” fuel (such as sugar), and as a result, less oxygen is used in the generation of the energy (and more carbon dioxide is produced).
At these points where the curves change in slope, you will know that something has changed in your body’s metabolic processes: most likely the requirement to deliver ATP at a quicker pace.