Demystifying the BONK


By Sunny Blende, M.S., Sports Nutritionist

Bonking, hitting-the-wall, crash and burn, total muscle fatigue, whatever the name – as an ultrarunner, this is your worst nightmare. When you are beginning to lose it, the cause of your meltdown matters less than the vision of your training, goals and dreams drifting out the window. But there are different kinds of bonking and a variety of causes. Besides the obvious lack of adequate physical training, which we will not discuss here, most causes begin with a nutritional imbalance. Whether you’re sitting dejectedly on the side of the trail, mentally finished but with plenty of fuel still left in your leg muscles (low blood sugar), looking up at a hill, mind focused on pushing to the top, but legs shaky and depleted (low muscle glycogen) or you’ve reached the hallucinogenic Big Screen where rocks and trees are sprouting legs and beginning to pass you (central fatigue), you are not in your happy place anymore.

Many ultrarunners have made the mistake of consuming improper or excessive calories in the belief that more food will provide more energy. Athletes do not have an infinite capacity to store fluids or food before or during exercise and attempting to do so will result in stomach distress. On the other hand, inadequate intake spells trouble too. Ultrarunning is a calorie-deficit sport to begin with and under-fueling is asking your fellow equally-trained, fueled runners to pass you. We all get fatigued at some point and understanding the different kinds of depletion while recognizing early symptoms can help us avoid, or at least postpone, the ever-lurking bonk.

A Word about Dehydration
If you are dehydrated, all guidelines concerning sports nutrition are off. Dehydration is the single biggest enemy of ultrarunners, especially in the longer distances, because race conditions are changing throughout the day. It is pretty easy to stay hydrated in a 50 km, harder as the distances and time-on-the-trail creep up to the 100-mile or more, due to changing weather conditions, temperatures, daylight or nighttime conditions and increasing core temperature. When your body becomes dehydrated, your blood volume decreases. This sets off a cascade of events affecting your cooling systems. Losing even one or two percent of your fluid volume causes a decrease in your cooling efficiency and an increase in your heart rate, known as “cardiac drift” (the slow increase of your heart rate and oxygen consumption over time with the same intensity of effort). Dehydration can lead to central fatigue, as well as a host of other serious conditions. Know your sweat rate (UltraRunning, April 2006) and drink accordingly.

Low Blood Sugar
When carbohydrates are ingested, they enter your stomach, are broken down and travel to the small intestine where they are absorbed into the blood as glucose or other types of sugars. These molecules then travel to the liver for more processing. Glucose is dispersed to the brain, a small amount is stored in the liver, more goes to exercising muscles, and, if there is any left over, it is converted to fat and stored in the fat cells. If we are exercising with some intensity (think flight-or-fight) we have a mechanism that can send glucose directly to the exercising muscles (legs of an ultrarunner) effectively bypassing the liver and assisting in a survivor reaction (saving ourselves from the tiger). The pathway to the muscle cells is mostly a one-way path, in other words, glucose cannot go from muscle cells to the starving brain and low blood sugar is the result. We are mentally finished but there is still plenty of fuel in our leg muscles to get on down the trail.

Solution: Take in some glucose and you should feel better in about 10 to 15 minutes. (100-200 calories, followed by consistent fueling.) Liquids will be absorbed quicker than solids, so try sports drinks or gels first. Bars or foods with protein will take too long to reach your famished brain.

Low Muscle Glycogen
It is possible to continue to have mental focus and still have your legs buckling under you – totally fatigued. This is a symptom of low or depleted muscle glycogen. This doesn’t mean you can’t continue with the race, for awhile anyway, it means you are out of the muscles preferred fuel – glycogen. Your choices become stopping, continuing at a much slower pace, breaking down some of your muscles and making some glycogen from them for your screaming muscles, or eating some carbohydrates, slowing down only long enough for the fuel to be digested and reach your legs. If you have reached the upper end of your aerobic level, you may be producing more lactic acid at a faster rate than you can convert and re-synthesize it for fuel and this also means you must slow down until your processing equals your production.

Solution: This is entirely dependent on pre-exercise fueling followed by a consistent intake of carbohydrates during the race. Everyone will, eventually, run out of fuel unless they slow to a crawl. To continue strong to the finish line, you must take in the maximum calories that you can absorb. This is about 240 – 340 calories per hour for most trained individuals. Start early and don’t get behind because you cannot “make up” shortages. Begin the race with your levels of glycogen topped up and keep up the feedings. Most spurts of energy will create lactic acid that you can process in a short amount of time. As the race wears on in hours and hours, you may need to slow your pace somewhat.

An Aside about Low Liver Glycogen
We only store about 250 – 500 calories of glycogen in our livers, but this small stockpile is important because while we sleep, our trained bodies are metabolic furnaces. Even though our muscles are mostly inactive, our brains are still demanding glucose and our livers are the providers. When you wake up in the morning you will need to restock the liver and boost your blood sugar. When you are running (anytime and through the night as well), your liver makes up any shortfalls in glucose, so low liver glycogen levels become just as serious. All consumed carbohydrates, except glucose must go to the liver to be processed before they can be used. The problem for ultrarunners eating mostly glucose (maltodextrin products), which can go directly to the muscles for energy, is that the liver stores remain low, and therefore the brain is still starved for glucose. A starved brain senses trouble and begins to shut bodily functions down…as in central fatigue.

Solution: Consume some food upon waking up in the morning, water too. About two glasses of water and 150 – 250 carbohydrate calories should restock your liver stores and rehydrate your body. When running through the night, eat some fructose or a bar with more than just glucose. These kinds of carbohydrates will have to go to the liver to be processed therefore your liver will then send some to your brain, forestalling possible central fatigue.

A Word about Protein
Long-duration exercise uses a higher percentage of amino acids (protein), preferring the branched chain amino acids (BCAA) of leucine, isoleucine and valine. Studies have shown that BCAA depletion leads to elevated levels of tryptophan in the brain, which as anyone who has become sleepy after Thanksgiving dinner knows, depresses the central nervous system and causes the sensation of being physically fatigued. This is why it is a good idea for ultrarunners to include some protein after a few hours of running and keep up their level of BCAA. Protein also aids in the absorption of carbohydrates, another benefit for the ultrarunner. However, if you are feeling fatigued, haven’t been ingesting much protein and it is the middle of the night, turkey sandwiches might not be your best choice of food.

Central Fatigue (or, the total collapse of the ultrarunner – body and soul – weakness, heavy breathing, dizziness, loss of mental coherence)
This is when the little elves start coming out from behind the trees and begin to talk to you. When the brain is starved of glucose, neurons in the occipital cortex can misrepresent incoming images. I was pacing a friend in Western States when she suddenly grabbed me and said, “Don’t you see that skunk about ready to spray you?” It was a tree branch in the trail throwing some shadows. Another time while pacing/crewing a runner in Badwater, he insisted on weighing his gloves separately at the Medical Check, saying “Trust me on this one”. He got to stay at the Medical Check for a while.

Our brain is the ultimate processor for our body during an ultramarathon and it is monitoring blood volume, sweat rate, core temperature, blood sugar, muscle glycogen, liver glycogen, levels of branched chain amino acids and mental states. With all this juggling and supervision of information, the brain may sense impending oxygen deficiency and increased heart rate – the equivalent of a meltdown for an ultrarunner. Its response? Immediate reduction in the mass of muscle that can be recruited by the brain, also known as central fatigue. Reduced muscle activity reduces the demand on the heart and thus prevents heart and brain damage. This mechanism exists to restrain the over-vigorous use of exercising skeletal muscles and becomes activated when sensory impulses are received by the brain’s motor cortex that the heart and/or brain is not getting enough oxygen. When this low oxygenation level is reached, the motor cortex stops recruiting more muscle and fatigue is experienced. Since “fatigue” is only experienced by the brain – not by the muscles where it seems to be coming from – there is reduced output from the muscles even though the muscles themselves may have more glycogen to use.

Solution: Besides training your muscles for superior contractility and training your heart to produce a greater blood flow (cardiac output), your best choice of action during a race is to slow down until your brain stops detecting oxygen deficiencies and starts recruiting more muscle. Usually you have no choice. Then you need to keep hydrating and fueling. Heart rate monitors can help you keep your heart rate in check so the process doesn’t start, but this may just mean slowing down earlier.

Bonking can result from a combination of dehydration, low glycogen levels, gastric issues, training errors and other blunders and many of these errors may be related less to extreme metabolic reasons than just forgetting common sense. Heavy training and long races with under-fueling equals frequent bonking. Pay attention to your pre-race fueling and your during-race fueling. Start early and don’t get behind. Concentrate on the following:
1. Have a fluid replacement drink that supplies some calories made up of maltodextrin or glucose polymers and some sucrose or fructose.
2. Have a carbohydrate/protein drink of approximately one part protein to 3 or 4 parts carbohydrate to start consuming after a few hours into the race.
3. Make sure your fluid replacement and gels have some Sodium (Na) in them. About 140 – 250 milligrams. You can supplement with salt tablets as the weather heats up.
4. Drink and eat at regular intervals. This cannot be over-stated. Absorption is a delicate balance – too much or too little both present problems. Your body will perform best with a steady stream of liquids and calories.


About Author

Sunny Blende, M.S. is a Sports Nutritionist who writes and counsels individuals and teams on fueling for enhanced performance and making healthy food choices. Currently she writes the nutrition column for UltraRunning magazine and runs ultras herself. She has presented at the National RRCA Convention, the National Rowing Convention, Nike San Francisco Marathon Expo, and the Runners World San Francisco Marathon and worked as an assistant with the Los Angeles Marathon Association. An avid master competitor herself, she trains and competes in the San Francisco Bay Area and beyond. Sunny received her Bachelor of Science degree from University of Southern California and her Masters in Human Nutrition degree from University of New Haven. (Photo at left, by Luis Escobar).


  1. Sunny as a fellow health care practitioner and runner (Physiotherapist and Exercise Physiologist) I wish to commend you on this excellent blog. Very informative and easy to ‘digest’ (sorry couldn’t help it)! POGO Physio Australia

  2. Sunny as a fellow health care practitioner and runner (Physiotherapist and Exercise Physiologist) I wish to commend you on this excellent blog. Very informative and easy to ‘digest’ (sorry couldn’t help it)! POGO Physio Australia