Unbonk!: How to Prevent Bonking During Ultramarathons

How to prevent and overcome ultramarathon “bonks.”

By on January 2, 2024 | Comments

If you’ve ever found yourself “bonking” during a long run or race, then runner Dana Pearcy’s story might sound familiar.

She entered and participated in a local 35-kilometer trail race as a training run. She finished the event but had to gut out a bad patch that lasted for several miles.

“Around mile 14 I started feeling really dizzy and the trail became blurry and surreal looking,” she recalls. “My mind was having trouble processing the trail as I was trying to move across it. I was forced to slow down so I wouldn’t fall. This continued for about 4.5 miles. Around mile 18, I fell, not too bad, just minor scrapes, because I felt super clumsy trying to move through the riverbed I was in. I stopped at the next aid station for a few minutes, ate, and hoped my mind would clear. Shortly after that the feeling passed, but for the rest of the race I felt slow, out of it, and had a hard time focusing.”

Many of us can relate to her story. We’ve all “bonked” or hit the wall. It can creep up on us no matter our running speed, effort, or training background.

Best Heart Rate Monitors for Running - running with arm band heart rate monitors

A bonk can creep up on us no matter our running speed, effort, or training background. Photo: iRunFar/Eszter Horanyi

The Biology of Bonking

In order to appreciate how the bonk occurs, an abbreviated biology lesson is in order. The body uses carbohydrate, fats, and proteins to create muscle movement. Enzymes convert these nutrients into adenosine triphosphate (ATP). ATP doesn’t stick around in the body. It’s created and used in a matter of seconds. Energy, which allows muscles to function, is released through the continual formation and subsequent break down of ATP.

Carbohydrate, stored in the form of glycogen in our muscle, liver, and blood, is the primary, quickest, and most accessible fuel source for medium- to high-intensity activities. The body can turn glycogen into ATP without using oxygen. The unfortunate by-product of anaerobic metabolism is lactic acid, that burning feeling in the muscles when you’re running at high efforts. The trained human body can store enough glycogen for about two hours of hard running.

If we slow our pace, allowing more oxygen to become available to the muscles, we can run for greater distances. Fat, instead of glycogen, is metabolized during these endurance sessions. Fat yields the biggest ATP return and is our most abundant fuel supply, enough to keep us going for days. However, small amounts of glycogen are necessary to complete the fat utilization process. Protein, used for muscle repair, is seldom used as an energy source.

Zach Miller - 2022 UTMB - aid station

Zach Miller topping up glycogen levels at the 2022 UTMB. Photo: Mathis Dumas

So, we’ve got this remarkable system that burns different fuel sources depending on our effort level and exercise duration. The trick is managing our limited carbohydrate stockpile by using our bountiful fat reserves. If we don’t account for glycogen loss in our training and racing programs, we will hit the wall.

“Technically, it’s the depletion of your stored carbohydrate energy or glycogen,” says Robert Kunz, who holds a master’s degree in exercise science and sports nutrition and, now retired, co-founded and built First Endurance, a premier endurance nutrition global company. “Once you run out of glycogen your body must rely on fat and, to a lesser extent, protein. These fuel sources are hard to convert to energy and hence the athlete’s pace is reduced significantly. Some feel queasy and dizzy. It can be hard to think clearly due to the lack of readily available blood sugar.”

How to Stop the Bonk

There are two reasons why we deplete our glycogen stores while racing and training. Assuming we’re healthy and not fighting infection or injury, we struggle in the late miles of an event because we’ve:

  1. exceeded our current fitness level by running at too high an effort, or
  2. neglected to adequately replace our precious carbohydrate sources.

In the rest of this article, we talk about three ways to prevent both these scenarios.

Moderate Your Running Pace to Prevent Bonking

Our tendency on race day is to run too quickly too early. Keeping our effort more in tune with our current fitness will extend our ability to run a steady pace for longer.

“Athletes use primarily fat at slower paces. As the pace increases, so does the reliance of stored glycogen. That is why pacing is so critical,” says Kunz.

Three telltale indicators that you’re tapping your glycogen stores during exercise are forceful, quick breaths, the inability to carry on a conversation, and the accumulation of lactic acid in the muscles. Quite simply, you’ve become anaerobic; your muscles are no longer using oxygen to create energy.

Slow down, return oxygen to the muscles, and allow your fat stores to take up the slack.

Bryon Powell - running in Moab

Bryon Powell keeping the easy runs easy in Moab, Utah. Managing pace appropriately can help to avoid tapping your glycogen stores. Photo: iRunFar/Meghan Hicks

Train Adequately to Prevent Bonking

Bonk proof your body with years of consistent aerobic exercise.

Kunz, who’s finished the Speedgoat 50k, agrees: “The longer you prepare with proper distance training and racing the better your body adapts to the distances and efficient glycogen utilization. This is why athletes should do a 50k before they do a 50 miler and use a similar progression to get to 100 miles. It takes years of training to fully adapt to these distances. An elite ultramarathoner can run a marathon on just a couple hundred calories [of ingested fuel]. Yet if a healthy, but inexperienced 20-something college student jumped in, he would bonk completely by mile 20. The college student does not have the years of adaptation and hence his body burns glycogen at a much earlier stage than that experienced ultrarunner.”

Start with endurance-based workouts or long runs to:

  • Train the body to become efficient at burning fat, its optimal fuel source for long efforts.
  • Improve your ability to amass muscle glycogen, the major form of stored carbohydrates in the body.
  • Increase the size and number of muscle capillaries and mitochondria, the blood vessels and cellular factories that facilitate aerobic energy.
  • Learn to keep going when fatigued.
  • Practice your race day fueling regime with products and ratios that work for you.

Introduce stamina-based workouts into your schedule to further your body’s ability to utilize both oxygen and fuel well. Shift your lactate threshold toward faster speeds and harder efforts.

Prevent Bonking With Adequate Fueling and Hydration

Stockpiling your body’s fuel sources begins well before you toe the start line. Reduce your training volume and maintain a regular meal schedule to allow for muscle recovery and glycogen storage. See this article on proper peaking for more details on how to approach the final weeks before an event.

On race day your goal should be to prevent glycogen stores from being used rather than trying to replace them by consuming fast-absorbing carbohydrates such as gels or your favorite sports drink containing maltodextrin or similar. As easy as it sounds, keeping the bonk at bay is a tough challenge. Here are some common indicators that you’ve hit the wall:

  • leaden legs
  • acquiring a grumpy, down-trodden attitude
  • dizziness
  • tingling lips or cheeks
  • clammy skin
  • lack of motivation
  • lack of power
  • temporary loss of or blurry vision, seeing “stars” or white flashes after blinking

If you do find yourself in this predicament, Kunz recommends, “Immediately consume fast-absorbing sugars and water. Remain at a walk so you allow your body the energy it needs to process this and drive it to the working muscles. After roughly 15 minutes, slowly pick up the pace. At this point you might be hard pressed to bring yourself back up to your normal race pace since you do not have any stored glycogen to pull from. From this point on your body will rely on fat and the oral carbohydrates you’re taking for fueling purposes. You still have plenty of fat energy left and can keep going for long stretches, however it’ll be at a much slower pace.”

Tailwind Endurance Fuel and Recovery Mix - in the wild

Tailwind Endurance Fuel and Recovery Mix are among the iRunFar team’s favorite running and recovery food, which can help with a quick hit of carbohydrates. Photo: iRunFar/Eszter Horanyi

If things begin to feel dire and hopeless during your next ultra, remember this: The brain is the only organ in the body that operates solely on blood sugar. A severe case of glycogen depletion can starve the brain leading to a loss of concentration, confusion, clumsiness, a bad attitude, anger, and depression. Before pulling the plug, sit down, drink, eat, and wait 20 minutes. Sometimes a little food and drink can revive you enough to get you to the finish line.

During the 1998 Western States 100, I bonked so badly that I couldn’t even walk. I spent 45 minutes at the Peachstone Aid Station gathering myself. After some food and fluid (the flute music and Reiki were also nice bonuses), I was able to get off the cot and continue, albeit at a much slower pace and a better eye on my fueling.

While driving, we diligently monitor the gas gauge. Do the same for yourself at your next ultra. Keep close watch over your body’s energy levels, so that gauge never reaches empty.


[Editor’s Note: As one of iRunFar’s best training articles, we’ve worked with author Ian Torrence to update this article before resharing it.]

Call for Comments

    • Bonking horror stories, who’s got one?
    • What have you found works best for you in terms of staving off a bonk in the first place?
    • Have you ever been able to unbonk during a run or race?
    • And where in your pacing, training, and fueling/hydration could you do better to avoid the dreaded energy bonk?
Ian Torrence

Ian Torrence has more than 12 years of experience coaching runners of all levels. Ian has completed more than 220 ultramarathons, with 50+ wins, since his first ultra finish at the 1994 JFK 50 Mile. Ian and his wife, Emily, are online coaches at Sundog Running. Information about his coaching services can be found at SundogRunning.com.