Bonking or Hitting the Wall

Carbohydrates (Glycogen) and Endurance Athletes

Why do I “bonk” or “hit a wall” while training or competing in an endurance event?

Here is a simplified explanation of why –

To prevent “bonking” or “hitting a wall” it is important to understand what your body uses for fuel during an endurance event (marathon, triathlon, mountain bike race, century ride, or an adventure race). Your muscles rely on a mixture of fat and carbohydrates for fuel during an endurance training or event. The slower and less intense your training pace, the more your body relies on fat. But as you increase the intensity of your endurance training, the more your body relies on carbohydrate fuel, glycogen. (Glycogen is the storage form of glucose; it is an easy accessible source of energy for exercise)

For endurance athletes it is important to remember that a single long run (1 to 2 hours), ride, or swim can deplete your glycogen levels.  You cannot rely on your fat as a back up because fat cannot be metabolically processed fast enough to keep up with the fuel demands when you are exercising at an intense pace. (Intense pace is key here) Basically, when you deplete your glycogen levels you “bonk” or “hit a wall” and you are forced to stop or slow to a pace where fat can be used as a fuel source.  Most of us have anywhere between 80,000 and 120,000 calories worth of body fat held in reserve at any given time and roughly 2,000 calories worth of carbohydrate fuel (glycogen). Glycogen is stored in your muscles (1,200 to 1,600 calories) and liver (300 to 400 calories) and about 20 to 100 calories is circulating around in your blood stream as glucose which is the preferred fuel for your brain. Therefore, to prevent bonking or hitting a wall, you should start thinking about eating after the first hour of training, the goal is to give your muscles an added source of fuel while maintaining a normal blood sugar level to keep your brain fed to help you think clearly and remain focused during your training or event.  It is important to consume 100-250 calories (25 to 60 grams) of carbohydrates per hour during endurance exercise. By consuming carbohydrates here it will significantly increase your stamina.  Each individual is different; some athletes prefer whole foods (me!) while other athletes prefer Gu’s, sport drinks and bars. It is important to play around with your food intake during training to train your intestinal tract and muscles. Most athletes have a hard time eating and digesting whole foods.

On a personal note, I consume about 120 calories per hour during my long trainings. This past weekend I ran for seven hours and consumed one almond butter & jelly sandwich on sprouted bread, 1 ½ Kind Bars, handful of
cheese, and dried apple rings for about 700 calories. In addition, I drank 100 ounces of water and two electrolyte tablets.

To estimate what you need for a 90 minute to 4 hour event = 1 calorie of carbohydrate per pound of body weight per hour and .25 grams of protein per pound of body weight per hour (a ratio of 4 carbs to 1 protein to help prevent the onset of central nervous system fatigue) *failing to get adequate carbohydrates during intense exercise in this duration 90 minutes to 4 hours can result in muscle wasting.


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Filed under Nutrition, Sports Nutrition

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