As an ultrarunner I have been learning how to burn more of my “fat” stores as energy on long runs. My hope is to incorporate this into my races in the future. Why burn fat? Well, the body has way more fat calories than it does glycogen calories. There are 3,500 calories in just 1 pound of fat. You can run on glycogen stores for 1.5 hours to 2 hours. This is why many people have to take a few gels or drink sports drinks in a marathon. In theory, you could run for many hours on just fat calories. In fact, there has been a man who ran 100 miles on just water and some electrolytes as he burned “fat” as his fuel source (look up ultra runner Mike McKnight).
The tricky part to burning fat is to not have your body switch over to burning up glycogen stores as it’s main fuel source. Once your body switches over you can’t really switch back. This happens generally when you start getting your heart rate to 70% of it’s max and higher. This is why when you are in a race you mostly burn glycogen because our heart rate and efforts are maximal. Therefore, we are not in a lower heart rate zone for long and we cannot just burn fat for long.
In endurance running like in ultra distances, an athlete can implement fat burning as a fuel source into their strategy because they do not need to go out fast and get their heart elevated early on. For my 100 mile run coming up my plan is to start out slow and try to burn fat for the first few hours of my run. I’ll probably switch over to carbs and tap into my glycogen stores eventually but at least it will help me to not have to worry about fuel in my stomach for the first few hours. If I want to finish with a descent time I will have to eventually pick up the pace a little bit and my heart rate will naturally rise over time as my body works harder to maintain the same pace because of fatigue.
When my heart rate passes the 70% of my max area, I’ll be ready to steadily drink my calories or take some in the form of gels. Some people like to eat real food but I’m not really sure how my body will respond to eating food in the later parts of my 100 mile run. Digestive issues can really be a major problem in endurance sports and getting enough calories in and to stay down will be a challenge. This is the price some of us athletes pay as we push to the max limits of what our bodies will allow us to do. The best I can do is to practice and experiment with my nutrition and fueling before the big 100 mile day.
I personally think burning fats first and carbs second makes the most ideal scenario for ultra runners. The pace should be very slow for ultra runners from the beginning anyways and this is the time to take advantage of using up some fat calories and to not tap into the glycogen stores. However, you’ll have to pay close attention to your body because you don’t want to “bonk” or “hit a wall” when you thought you were going to have plenty of time burning fat calories. I have had runs where I suddenly felt weak and exhausted shortly into a run. Lastly, practice, practice, and practice what you want to do on race day. Don’t do anything new and eat anything new on your endurance run race day.