All of these terms refer to what might be described as “Nutritional Failure”, where you haven’t fuelled appropriately for the endurance event you have entered. It can happen to anyone, most recently to this years Tour De France winner Chris Froome on the day he lost the yellow jersey at Peyragudes.
For anyone who has never “bonked”, it can come suddenly after a long ride (or run or tri) and leaves your legs feeling heavy and lifeless. It guarantees a slow painful ride home. It’s more common when you are new to endurance sports and start doing your first long rides, or start doing your first long races >90 minutes. Once you’ve “hit the wall” even riding on the flat can feel like you’re tackling a steep mountain or riding through treacle.
So what is the “bonk” or “hitting the wall”? It is a condition which occurs in endurance sports such as cycling where the athlete experiences sudden fatigue and power loss caused by running out of fuel. The fuel required for higher intensity efforts is carbohydrate, or in its most simple form sugar (aka glycogen) when it is stored in your muscles and liver. Milder versions of bonking are often known as “hunger knock” or “hunger flat” and can be remedied more easily with a brief rest and eating / drinking some quickly digestible carbohydrates. The more serious “bonk” leaves the rider more or less totally depleted of glycogen resulting in lack of energy and power, and will ensure they don’t see the front of the race again that day. There simply is not enough fuel left for the hard efforts required, and the body then turns to fat supplies for fuel at an aerobic intensity. Even if fat is utilised for fuel, sprint efforts at max or near-max capacity are unattainable without the presence of carbohydrates as fuel. In severe cases, athletes can barely see straight or hold a straight line pedalling.
To understand the condition properly, you need to know how your body fuels exercise at different intensity levels. Endurance athletes working over several hours produce energy in one of two ways:
Training can help you raise the power level and the duration that you are able burn fuel most efficiently (aerobically). Fat is the primary fuel source for aerobic intensities and almost everyone has enough fat stored in their bodies to fuel exercise at this lower intensity level, meaning that running out of fat is not why you might need to stop low intensity exercise.
When the intensity increases, your body starts to use carbohydrate more for fuel, with the body gradually burning a higher percentage of carbs compared to fat, the higher the intensity becomes. Unfortunately carbohydrate stores are limited, with untrained individuals on a typical diet storing about 380gm or 1500kcal of glycogen – roughly 90-120 minutes’ worth. However this is spread throughout all of your muscles and may not be completely available to the muscles used in a specific activity like cycling. When the intensity increases your body may burn 600-800kcal or more per hour, resulting in your glycogen stores being depleted in less than 2hrs of continuous cycling. When this happens you’re out of your high intensity fuel and “hit the wall”.
There are two basic parts to preventing this: training (I’d recommend a qualified coach to assist here) and nutrition, both of which can elevate the amount of glycogen stored in your muscles up to 880gm (3600kcal). This will increase the amount of time you can sustain higher intensity exercise.
We’ll focus on nutritional strategies today, and Sports Dietitian Ali Disher from Apple to Zucchini Sports Nutrition will share some of her expertise in how to complete your endurance ride without hitting the wall.
As you see nutrition is a key part of endurance cycling, and nutritional failure can cause you to not finish your cyclosportif or long race. Getting expert advice with a sports dietitian can be the difference between a great day out and a miserable ride home after hitting the wall.