I have heard so many stories of runners hitting the wall…and have experienced it myself. The way to avoid it is actually pretty simple when you know a little about the physiology of the body. Have you ever bonked or hit the wall? If so, it happened when your body ran out of glycogen too fast.
Glycogen is a sugar that is stored in your liver and muscles and is the fuel that a distance runner’s body prefers to use. Your body chooses to use glycogen first because it is easy to burn and fat is used as the backup fuel. A finite amount of glycogen can be stored at any one time: 400 grams in the muscles and 100 grams in the liver. This is enough to get you through 20 miles. The calculation is as follows: 500 grams of carbohydrates (glycogen) at 4 calories per gram = 2,000 calories. The burn rate for calories on a run is around 100 calories per mile. So 2,000 calories would last 20 miles.
How Glycogen and Fat are Used for Energy
Glycogen is broken down into glucose for your body to use as energy. The harder you run, the more you burn. It is important that you don’t burn through it too quickly and that you replace lost glycogen throughout the run so the key is to consume calories before you feel tired. Why? Because it takes about 30 minutes for the glucose to reach your bloodstream after you consume it. You have also been training your body on the long runs to switch over to burning fat when you get into the later miles. But burning fat for fuel is less efficient and you will slow down, so it is best to keep the glucose levels steady.
What is Happening When You Bonk or Hit the Wall?
When liver glycogen depletes, is causes a drop in blood sugar. You might feel light headed, lack concentration and have heavy legs. Serotonin increases, which makes you tired and dopamine decreases—which is the feel good hormone. To get back on track slow your pace after taking some nutrition and wait to feel better. The way to avoid the wall altogether is to consume calories while running so that you don’t deplete the stores.
Carb Loading Before the Race
In the three days leading up to the marathon, you should increase your percentage intake of carbohydrates to 70%. Your caloric intake should remain the same so as you increase carbs, you will decrease your protein and fat calories. You also need to increase water consumption because water is needed to help store glycogen. For every gram of carbohydrate that you store, you also store three to five grams of water—so you may go up a bit in weight, but it is just water—so do not worry, you will burn it off at the race! The bonus is that you will be well hydrated. Cutting back on miles leading up the race helps your body reduce glycogen consumption—and your stores will be full by race day. In the hours leading up to the marathon, eat high glycemic index foods to increase blood sugar and glycogen stores. Some good options are: bananas, potatoes, white rice, oatmeal, popcorn, white bread and rice cakes. In the two hours leading up the race, do not eat anything.
Replacing Glucose During the Race
If you are using energy gels, energy blocks/chews or beans, start taking these an hour into the race. You will want to take 100-200 calories per hour. Follow this with a full 8 ounces of plain water. A good way to remind yourself to take in calories is that when you are coming up on a water stop, take your nutrition in advance of getting there and then grab a cup or two of plain water. If your digestive system is sensitive to eating on a run consider liquid nutrition like Tailwind or Skratch which includes both calories and electrolytes. If you combine this with other semi-solid and solid nutrition, be sure that you follow it with plain water.
Hopefully by now, you have experimented with this and are feeling good by mile 20 on the training run. Try to stay mindful throughout your long training runs and marathon—you will be so thankful that you did and will enjoy the experience so much more.