The short answer is that bonking refers to low blood sugar (hypoglycemia), and simply running out of fuel for your body and your brain.
The long answer is a little more complicated. The energy pathways that provide fuel for long-distance exercise rely on adequate and continuous supplies of glucose to keep your muscles contracting. You get this glucose from several sources. You have glucose in your bloodstream, but that gets used up within a few minutes of high-level exercise. You also have some glycogen stored in your muscle cells, and that can carry most people through a 90-minute workout just fine. The liver also releases glucose into the bloodstream to keep up with demand. At rest, this can be plenty of glucose to keep all your bodily functions going strong for hours at a time. But during high-intensity endurance exercise, such as a 5-hour bike race or Ironman triathlon, you can quickly use up all your stored glucose. If you don't continually add some more fuel, that can spell disaster for your body and your mind.
It's not just your muscles that need glucose to function properly. Your brain needs glucose too. So, if you do use up all the glucose in your system and "hit the wall," you won't just have to slow down and stop exercising; you may experience a variety of cognitive symptoms as well.
Signs and Symptoms of "The Bonk"Lack of glucose in your bloodstream has both emotional and cognitive effects in addition to physical effects. When blood glucose levels drop too low, the first thing you'll experience is problems continuing muscle contractions. You will feel slow, heavy and weak. If you keep going, physical exertion becomes increasingly difficult and you may even start to experience muscle trembling, and shaking, sweating, and lack of coordination. You may feel extreme hunger, or no appetite at all.
By now, your body is protecting your brain by shutting down your muscles. Soon, however, your brain and nervous system is affected, and the result is the mental and emotional symptoms of bonking. Dizziness, light-headedness, tunnel-vision and disorientation are all common experiences. Anxiety, nervousness, and even irritability and hostility may develop for some people. Some athletes experience an overwhelming feeling of depression. At its most extreme, hypoglycemia can result in seizures and even coma.
Anyone can bonk if they don’t eat properly during intense endurance exercise. Lance Armstrong bonked during the climb up the Col de Joux Plane in the French Alps during the 2000 Tour de France. At the time, he recalled it as the worst day on the bike he’d ever had. If you watch any Ironman triathlon events, you'll find more than one athlete staggering along the course, appearing dazed, confused, and disoriented. These are all athletes who drastically depleted their glucose stores.
Bonking is more common in cycling than other sports because pedaling causes far less muscle damage than something like running, so you can continue cycling at an extremely high intensity for hours on end. The limiting factor for elite cyclists, with well-trained muscles, tends to be available energy rather than muscle fatigue. However, many runners and triathletes will find themselves on the road to a bonk, particularly during competition.
What To Do If You BonkAs soon as you feel any of these warning signs, especially if you've been exercising hard for a couple of hours, stop exercise, and eat some simple carbohydrates that can be rapidly absorbed. The best source is a sugary drink such as a sports drink (Gatorade or Powerade), fruit juice, or an energy gel washed down with lots of water to get it into your bloodstream quickly. Other options are straight sugar such as hard candies, gumdrops or jellybeans. The energy bars and solid foods full of complex carbohydrates take longer to process, so they are less helpful during a bonk. If you catch the bonk early enough, you might be able to keep riding (slower) and refuel with glucose and carbs. But if you are really hitting the wall, you should stop, eat, and recover before continuing with exercise.
Ask For Help If You Are BonkingOne important factor in recognizing and recovering from a bonk is to be aware that you may not be thinking clearly. For this reason, it's a good idea to ask for help. Have someone keep an eye on you as you recover, and especially if you choose to keep going. Use the buddy system to prevent further decline, and to make sure you are riding safely. For your part, try to pay more attention to what’s happening around you and keep checking in with yourself frequently.
How to Prevent the BonkTo prevent your blood sugar from dropping to dangerous, bonk-inducing levels, it's wise to eat properly for exercise, and to eat at regular intervals. If you exercise intensely for more than two hours, try to eat something small every 15-20 minutes. It doesn't really matter what you eat, as long as it works for you. Some athletes like energy bars for convenience, but foods like fruits, nuts, PB&J, and pretzels all work just fine. Try different foods to find your favorite before competitions. Then stick with your plan.
Recognizing Your Personal Warning Signs of the BonkKeep in mind that developing low blood sugar isn't just related to how far and how fast you've exercised. It also depends on how well your glucose stores were stocked before your ride, how efficient your body is at accessing and delivering glucose, and a variety of other factors including the conditions, terrain and climate. Getting to know your body and paying attention to how you feel is more important than just eating and drinking on a schedule.
In my own case, I recognize a bonk coming on when my legs just feel extra heavy and climbing a hill feels like cycling through molasses. For me, this means I need some fuel. For my ride partner, however, the warning signs are very different. Irritability, frustration and the inability to make a decision usually indicate that my buddy needs some food. Over time, we've each learned our own unique responses to low blood sugar, when it is likely to happen, and how to stop it from getting worse.
Cleveland Clinic: "Low Blood Sugar (Hypoglycemia)." [my.clevelandclinic.org/] Accessed May, 2012.
Jentjens, et al. Exogenous carbohydrate oxidation rates are elevated after combined ingestion of glucose and fructose during exercise in the heat, J Appl Physiol 2006; 100:807-816
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