What is The Bonk?
Bonk is a term used by runners and other endurance athletes to describe the moment when their muscles have run out of glycogen. Glycogen is how your body stores glucose in the muscles and liver. Glucose can be quickly and efficiently converted into energy by your muscles during exercise.
Unfortunately, glucose is not a dense form of energy storage and your muscles contain a limited supply. Glycogen is an analog of starch which is a carbohydrate. Anyone who has counted calories knows that a gram of carbohydrates contains 4 calories. A gram of fat contains 9 calories and therefore contains more than twice the energy potential of glucose.
The average person can store 380 grams of glycogen or about 1,500 calories in their body. Through training and carbohydrate loading, a conditioned athlete can store up to 880 grams or 3,600 calories.
Many athletes hit the wall after 1-2 hours of running or cycling, depending on their conditioning and how hard they work. At this point in their workout, or race, their bodies have used up the glycogen stored in their muscles and have switched over to burning fatty acids.
I will use myself as an example.
I weigh about 175 lbs and am in pretty good shape. Not Olympic athlete shape but I can run and finish a marathon. A highly conditioned athlete who has done carbo-loading can store up to 3,600 calories in his body. At my level of fitness and without carbo-loading I can probably store 2,000 calories.
If I run at a 6mph pace, or 10 minute miles, I will burn approximately 800 calories an hour. At about 2.5 hours my body will have burned all of the glycogen it has stored in my muscles and liver. Running at this pace (6mph) for 2.5 hours would get me approximately 15 miles into a marathon.
Most runners can make it to about 20 miles before hitting the wall by replenishing glucose while they run. Carbohydrate gels and drinks like these are typically what most athletes use.
How to avoid The Bonk
The best way to avoid “The Wall” and The Bonk is to consume carbohydrates as you exercise or race. As mentioned above, many athletes can get to 20 miles before bonking by consuming carbohydrates as they run.
For a half or full marathon I carry GU Energy Gels with me. These little packets have 100 calories and 20 grams of carbohydrates, or 80 calories that are quickly turned into sugar and absorbed into my blood stream. I usually feel a surge of energy a few minutes after taking a GU. Some people tell me they do not feel anything.
GUs and other energy replenishment products often also contain potassium and sodium which your muscles needs to work properly. Some products also contain amino acids or caffeine as a boost to your energy level.
I find that it is better to avoid The Wall. Once my muscles have become completely depleted I never seem to be able to regain my energy. My legs feel rubbery and it takes all of my conscious effort just to move my feet, let alone run.
Over the years I have learned to grab Gatorade at the water stops and to carry GUs with me. I do not drink Gatorade at all of the water stops but I always drink it at some of them. For a marathon I often consume 5 GUs. I feel that it is better to have too many carbs than it is to bonk.
Calorie consumption calculator
Here is a link to a web site with a tool to help you figure out how many calories you burn during a workout or race.
You can change the pace and duration of your exercise, you can even change your weight to see how that would affect your energy consumption. If you hit the wall at a race you could also use this tool to estimate how many grams of glycogen your body had.
I hope this information is helpful for you, and thanks for stopping by.