There are so many misconceptions surrounding exercise metabolism. Even certain cardio equipment will have an indicator of a “fat burning zone” in relation to heart rate, which is misleading. I would love to set the record straight on this and help explain what fuel sources are being utilized during exercise. Being more informed as to how your body is utilizing fuel will help you understand what is happening while you sit, while you exercise, and post-exercise.
The two main fuel sources in the body are carbohydrates and fat. There will always be a mix of carb and fat being used as fuel, but most often one or the other will dominate. While we are at rest and while we are doing low intensity activities such as walking, cleaning, jogging, cycling or any other exercise that is less than 65% of our absolute maximum cardio capacity (VO2), we are primarily using fat as fuel. Once we go above 65% of our max., we switch to use predominantly carbohydrate. If you take nothing else away from this post, remember this - There is more contribution from carbohydrate with increasing intensity and less contribution as exercise duration increases. And there is more contribution from fat as duration increases, however, less contribution as exercise intensity increases. So, if you want to burn more fat during exercise, increase duration and keep it at a moderate pace. And if you want to deplete glycogen stores, perform short, intense bursts of exercise, whether its cardio or heavy weight lifting. Both will be beneficial for weight loss and I strongly suggest mixing a combination of the two into your workout regime.
How and where we store fuel
Muscle - We store fat and carb in our muscle so that we have fuel readily available for when we require it. We physically cannot store a lot in the muscle though, since muscle cells are used predominately for contraction and not storage, so we rely on additional sources of energy during exercise. The storage form of carbohydrate is called glycogen.
Liver - We also store a certain amount of glycogen in our liver. This is important for regulating blood sugar. Blood sugar is very tightly regulated as there are certain cell types that rely exclusively on glucose for energy such as red blood cells and neurons (though neurons can use ketones as a backup source). Blood sugar is regulated at a concentration between 4-5 mM, in healthy individuals. If it drops below that, our liver will break down glycogen and release glucose into the bloodstream, thus maintaining blood sugar homeostasis.
Adipose - We store ‘infinite’ amounts of fuel in our adipose tissue as triglycerides (storage form of fat). Our fat cells liberate fatty acids via lipolysis and deposit them into the blood to circulate to where they need to go. Compared to blood sugar, free fatty acids aren’t as tightly regulated and we have between 0.2 - 2 mM in our blood at any given time.
Blood - In terms of fuel, we have glucose and free fatty acids circulating in our blood, providing energy to our cells. At higher intensities of exercise, we start to rely predominantly on endogenous substrates found in the muscle cell such as glycogen and triglycerides, previously discussed. Since endogenous substrates are right in the muscle, they provide immediate energy, but remember, there is a very limited amount.
Here’s an interesting fact. This is just hypothetical, as we almost always use a blend of carb and fat, but if we were to exclusively use fat for energy, we could perform 150 hours of marathon running compared to just 1.5 hours using carb as fuel. That’s why it’s said we have an ‘infinite’ amount of fat as fuel and a finite amount of carbohydrate (or glucose, which I’m going to use interchangeably).
And because certain cell types, discussed earlier, rely exclusively on glucose, it is important that we spare some glucose for these cells and not just blow through our carbohydrate stores, because once we do, we fatigue, performance declines and eventually you pass out or, in extreme cases, are barely able to move (Sometimes you see this when watching a marathon or another big event, such as a triathlon. They have completely used all of their glycogen stores and have none left for neurological functions and no endogenous energy sources left within the muscle). The more often you exercise (or the better trained you are) the better you are able to spare glycogen. And how does that work? Well, our bodies are always trying to adapt to make the next time we repeat an activity easier. So, the more we train, the better able we are to utilize fat as a fuel source, sparing our glycogen and prolonging time to fatigue.
How we utilize fuel
As you have likely heard before, mitochondria are the ‘powerhouses’ of the cell. This is where fat oxidation or ‘fat burning’ occurs. Fat oxidation is a series of chemical reactions that turn fat into energy. When you begin to exercise on a regular basis, or you increase your intensity or duration, you increase mitochondrial biogenesis and increase fat oxidation enzymes (which are catalysts for chemical reactions) in adipose tissue, liver and muscle (Romijn et al, 1993 & Rector et al, 2008). With more mitochondria and more fat oxidation enzymes, your ability to utilize fat increases and if you are better at utilizing fat, then you will spare glycogen, which will prolong time to fatigue, increasing performance.
Exercise increases fat transporters (mainly FAT/CD36), so your muscle cells can take in free fatty acids from the blood faster. Exercise also increases insulin sensitivity, which means your body will translocate glucose transporters (GLUT4) to the cell membrane with a smaller insulin signal. And, exercise increases the amount of GLUT4 available to be translocated, further enabling your cells to uptake fuel more efficiently. This increase in GLUT4 and insulin sensitivity actually persists for several hours after exercise, which is great because it means your muscles are taking up carbohydrates long after you finish exercising. This is to ensure your muscle glycogen stores are topped up and you are ready for your next bout of exercise. if you have just exercised and your muscle and liver glycogen reserves are low, then any carb you eat will get sucked up into the liver and muscle, which is great for weight loss because eating carbs when your glycogen reserves are already topped up will cause you to convert the carb to fat and store it in adipose tissue.