In endurance sports such as cycling and running, “hitting the wall” or “the bonk” describes a condition caused by the depletion of glycogen in the liver and muscles, which manifests itself by sudden fatigue and loss of energy. I’ve always wondered if it was possible to improve my glycogen storage in the hopes that I don’t experience the dreaded wall toward the end of the marathon. I know pacing strategy has a lot to do with pushing the wall back, and I’ve gotten better at this, but improving the glycogen storage and fat utilization through training can not only help you avoid hitting the wall, but actually help you finish strong.
The primary type of training to stimulate better glycogen storage and efficiently burn fat is by running 90 minutes or longer at a reasonable pace – not slow jogs. High training volume also helps. It is also suggested to include two-a-day workouts during a high mileage week. I’ve never done two-a-day workouts, but may begin incorporating them during my next marathon training cycle.
There is also emerging research that denying the body carbohydrates during training runs teaches it to burn fat instead, which spares muscle glycogen stores. A key part of success in the marathon is to conserve as much energy as possible for the last half of the race. So training on empty may train your body to more effectively burn fat, minimizing the depletion of glycogen. Ways to go about this include running early in the morning before breakfast, doing your long runs without gels (just drink water, no carbs), or do a depleted double. However, expect to run slower if you do this.
The body can store about 2,000 to 2,500 calories of glycogen. When you run, the body burns a mixture of carbohydrates and fat. The harder you run, the higher proportion of carbohydrates you use. The slower you run, the higher the proportion of fat you use. When walking, the proportion is about 50/50. A recovery run is 65% carbs, 35% fat. When running a marathon, it ranges between 75 – 90% carbohydrates and 10 – 25% fat. Assuming you burn approximately 3,000 calories during a marathon, it only makes sense that you’ll begin running low in your glycogen stores around the 20 mile mark and the effort begins getting more difficult because the body is forced to burn fat, requiring more oxygen per calorie.
Not sure if I’ve found any concrete solutions regarding this issue, but I find the science behind all this pretty fascinating.