Endurance athletes are known for being able to go on and on for prolonged amounts of time. But how do we do it? What’s the secret? Its fat! Yes, fat. Fat is a pretty important nutrient in the endurance athlete’s diet. During exercise, the body uses fuel stores (carbohydrate, fat, etc.) to create ATP which is the energy your muscles use to move. On average, your body stores somewhere in the ballpark of 80,000 calories while you only store about 14,00-2,000 calories in the form of glycogen (the byproduct of carbohydrates). This fat store is obviously a huge source of energy when it can be tapped into properly. But how does the body know what kind of fuel to burn? Simply put, it has to do with the intensity of the exercise. At lower intensity, your body is primarily burning fat for fuel because its metabolism is a slower process, even though it yields a much greater amount of ATP (fat has 9 kcal/g where carbs have 4). At higher intensities, you burn mostly carbohydrate for fuel because your body needs more energy more immediately and the carbohydrate metabolism process is much faster than fat. Generally, the jumping point at which you move from fat to carbohydrate is around your anaerobic threshold, or where your body begins to produce energy without the use of oxygen in the muscles. Simply, fat metabolism requires oxygen, and carbohydrate metabolism does not.
But why is it important to know about fat and carb metabolism with exercise? Well, there are a few performance benefits you can get from raising that jumping point from fat to carb. First, your energy capacity and intensity increases, meaning your ability to go longer at a greater intensity. If you raise the fat ceiling, your body can burn fat at higher intensities than before. For example, if you can burn fat up until a 7:30 mile pace, if you raise that to a 7:00 pace, then you can now do your long runs at a 7:00 pace without having to worry about burning out of all of your glycogen stores. By raising that ceiling, you also allow your body to reserve your glycogen stores for very high intensity exercise, say the closing few miles of a long race, giving you an extra gear to kick into. And that plays into raising your maximum ceiling of exercise intensity.
So how does someone increase that fat metabolism and become more efficient during exercise? There are a few ways, all of which work together to increase your performance. One of the more important ways to train your body to burn more fat is to time your meals properly with exercise. Most of us eat a lot of carbs because that’s just the food culture we live in. We like breads, pastas, rices, fruits, etc. But eating a lot of these causes you to have a rather high amount of blood glycogen in your system that, without going too much into the science of why, can have an effect on how well your body processes fat. Doing workouts on a fasting glycogen level (for instance, right when you wake up in the morning and haven’t eaten in several hours) can teach your body to rely mostly on fat for fuel because you have a limited source of glycogen. But these shouldn’t be all, or even most, of your workouts. Since your body does need that glycogen source when training at higher intensities, you should really save these fasting workouts for your easy recovery days. Another way to train your body for fat is to avoid using nutritional supplements during training. Like I mentioned earlier, the best way to make your body better at using fat is to actually make it use fat. During your easy intensity miles, your body has enough stored glycogen to get you through to the end without coming to close to depletion. So why would you need energy supplements that are mostly carb? You don’t! Save your GU for your high, race intensity training days. If you’re abiding my the 80/20 rule of training, this shouldn’t be a huge amount of training so you really should not be relying on gels most of the time. Also, if you take a gel on a morning run after fasting all night, you pretty much waste the fact that you’re training on fasting levels of glycogen anyways. The last main way to train your body for fat metabolism is MILES MILES MILES!!! There’s an old saying that miles make champions, and there’s a reason why. While we all should not skimp on our speed work, long miles are important too for other reasons. Doing high volumes of exercise increases the size and density of your cells’ mitochondria, the structures that work as the power plants in your body to produce usable energy. When you have more and bigger mitochondria, you can produce more energy and burn more fuel for that energy. And its not just the total volume either. 2 days of 90 minutes of exercise are not always equal. A single 90 minute run has been shown to induce a higher response in mitochondrial grown than 2 45 minute runs of the same intensity with a break between. But there is obviously a point of diminishing return here. Your body can’t handle 3 hour workouts every day of the week and still be able to recover for the next day’s training. So being smart about your workout planning is as important here as anywhere else.
So let’s recap. Fat is a huge source of energy in the body, as long as you can tap into it correctly. Your body burns mostly fat at lower intensities while it burns carbs at higher intensity. By raising that jumping point from fat to carb, you can increase your exercise capacity and intensity very noticeably while saving your glycogen stores for really, really high intensity efforts. There are a few ways to train this other than just eating a really high fat diet. Time your workouts and meals properly to do your easy days on fasting glycogen levels. Then pour on the mileage to make more mitochondria!
I’m sure you readers may have a lot of good questions since this can be a very deep and complicated topic, so if you have any questions at all, always feel free to contact me by email at firstname.lastname@example.org and I’ll be sure to get back to you with the best answer I can give! And if I get enough questions, a Q&A can always be another good blog post!