How to Avoid Bonking and Hitting the Marathon Wall

Mile 20 in the marathon can provoke dread in even experienced runners. Hitting the wall in the marathon is an unpleasant yet common experience. You are cruising along, until suddenly around mile 18-21, you feel as if you can no longer move your legs. You are bonking, and your goal time slips away more and more with each mile.  

However, hitting the wall in the marathon does not have to be an inevitable experience. You can develop a race day strategy to avoid hitting the wall. The strategy to avoid bonking hinges on two key aspects of the race: your nutritional plan and your pacing plan.

Key Takeaways:

• Carb load with 8-10 grams of carbs per kg of bodyweight per day in the 2-3 days before the race. 
• Have pre-race breakfast with a minimum of 100 grams of carbohydrates 2-4 hours before the race.
• Have a gel (or equivalent) and fluids at 10-15 minutes before the race
• Take a gel (or equivalent) every ~30-40 minutes throughout the entire race, aiming for 50-90 grams of carbs per hour. 
• Pace the first 20 miles with control, then push hard in the final 10K. 

Why You Hit the Wall in the Marathon

While the marathon wall is a complex, multi-factorial result of fatigue. Your central nervous system and muscles experience fatigue. However, the common consensus indicates that a significant contributing factor to hitting the wall is glycogen depletion. When you run a marathon, your body uses a combination of fatty acids, glycogen (stored carbohydrate), and glucose (carbohydrates in the bloodstream) to produce energy. The marathon is at an intensity where you cannot rely on fatty acid alone; you need carbohydrates. However, your body does not store enough glycogen to support the duration of the marathon. If you run out of glycogen and glucose, you slow down and feel fatigued 

At your aerobic threshold (first ventilatory/2 mM blood lactate), your muscle glycogen stores can support 80-90 minutes of running. For most runners, the aerobic threshold is slightly faster than marathon pace (2-2.5 hour race pace). The race still lasts longer than your glycogen stores can support. If you do not consume carbohydrates, you run low on glycogen.

When you run out of adequate glycogen, your body switches to primarily fat oxidation. (Complete glycogen depletion would be very, very bad.) As mentioned above, fat oxidation occurs at <60% VO2max, so you slow down to that level or below – and cannot speed back up because your body simply lacks the fuel to do so. Despite common claims, you will not be able to run the marathon entirely off of fat oxidation if you want to race it. Once you surpass 60% of your VO2max (a relatively easy run pace), you need carbohydrates to support ATP synthesis. 

Here’s the catch with glycogen: you can only use what is stored in the active muscle. (Liver glycogen can be used across the whole body; you need both liver and muscle glycogen to support a marathon.) You need to consume carbohydrates during the marathon to avoid hitting the wall. Consuming carbohydrates in the form of whole foods or sports nutrition products helps avoid glycogen depletion. Additionally, maintaining blood sugar levels mitigates fatigue of both the central nervous system and the muscles, as described in this 2010 analysis in PLoS Computational Biology. 

To avoid hitting the wall, you want to avoid glycogen depletion during the marathon. Optimizing glycogen storage, intaking enough carbs to provide energy via blood glucose, and pacing your race appropriately will help you avoid bonking on race day. (All this assumes you have trained properly. Appropriate marathon training will improve your glycogen storage capabilities, fat oxidation/glycogen sparing, and overall aerobic capacity.)

Do Not Skip the Carb Load

Carb loading works – when done properly. Carb loading ensures that your muscle and liver glycogen stores are optimized prior to the marathon, so that availability of muscle glycogen is not a limiting factor. This article delves into the why and how of carb-loading for a marathon. Basically, you want to eat approximately 8 grams of carbohydrate per kg of bodyweight (2.2 pounds) per day for two to three days prior to the marathon.

Finally, you will want a pre-race break-fast to top off your glycogen stores. Ideally, you want to eat it approximately 2-4 hours prior to the marathon. This time window allows for full digestion so that you reduce the risk of GI upset. Full digestion also means your body has a chance to store the consumed carbohydrates as muscle glycogen (it cannot do that if you eat only one hour before exercise). A high carbohydrate meal before endurance exercise will improve time to exhaustion, according to a 2021 study in Frontiers in Sports and Active Living. If there is ever a time when you want to improve your time to exhaustion, it is during a marathon.

An ideal amount is approximately 100-200 grams of carbohydrates two to four hours before. The exact amount may vary based on your weight and how much food you can tolerate. (You do not want to eat so much that you have stomach issues during the race.) That quantity may sound like a lot. However, a plain bagel with 2 tablespoons of jam, a banana, and your preferred sports drink would meet the requirement. 

While the research has reached a consensus that you do not need to carb-deplete prior to a carb load, this old belief persists in some circles. Modern carb-loading protocols are just as effective. The older method has undesired side effects such as gastrointestinal upset, low energy in training, increased injury risk, and irritability. The taper removes extra glycogen demands on the body, which allows you to store more glycogen.

Consume High Amounts of Carbohydrates during the Marathon

Even if you carb-loaded, you still want to take in carbohydrates during the race. Even if you theoretically could complete the race on your glycogen stores alone, do you really want to take that risk on race day?

Fueling during your marathon is scientifically shown to improve performance, especially if you aim for the recommended amounts of 50-90 grams of carbs per hour. In a 2014 study published in the International Journal of Sports Nutrition and Exercise Metabolism, marathoners who consumed 60 grams of carbohydrate per hour of the race finished an average of 11 minutes (4.7%) faster than those who fueled without a set strategy. 

If you are using a gel or serving of chews, each serving is typically 25 grams of carbs. So, a simple and effective method is to take a gel (or equivalent) every 20-30 minutes throughout the entire race.  (Always check the label, as individual products may vary). If you are taking a sports drink with some carbohydrates in it, that can count as part of your carbohydrate intake

Alternatively, you may choose a high-carbohydrate sports drink such as Maurten 360 or Skratch Superfuel and rely solely on that. Some new gels, such as SIS Beta Fuel or Neversecond deliver 30-40 grams of carbs per gel (instead of 20-25 grams). A combination of these gels and sports drinks can help support high carbohydrate intake during the race.

Have a Nutrition Strategy

To avoid hitting the wall, you need more than just a high amount of carbohydrates – you need to have a strategy for how you plan to take them in the race. Ideally, fueling early and often ensures that you maintain a consistent level of blood glucose without overwhelming your gut.

A nutrition strategy is worth the effort. A 2014 study in the International Journal of Sports Nutrition and Metabolism compared two groups of runners, with equivalent 10K race times, in the marathon. The intervention group took 60 grams of carbohydrate and 25 oz of sports drink (with sodium) per hour. The control group did not follow a fueling strategy and fueled based on what they felt they needed. The intervention group finished an average of 10:55 minutes (4.7%) faster compared to the control group, even though their 10K times indicated equal fitness.

Having a fueling strategy can keep you from bonking. You start fueling early in the race to ensure glycogen never drops too low. Later in the race, a fueling strategy helps you stay on top of nutrition, even when you get cognitively tired.

A fueling strategy does not have to be complex. In fact, it’s generally simple. You may opt to take a gel of 25-30 grams every 30 minutes and drink 1-2 cups of the sports drink on course at each aid station. If you are worried about missing a gel, you can set a reminder on your watch or pick easy to remember points in the race (such as every 3 miles).

You will want to practice your fueling strategy for six to ten weeks before your race. Frequent practice minimizes the risk of GI upset on race day. Fueling with carbohydrates in training also improves the cellular response to carbohydrates, according to a 2013 review in the International Journal of Sports Physiology and Performance. This upregulation means you absorb and use more of the carbohydrates you consume. 

Plan Your Pacing Strategy for Even Splits

Even with smart fueling, you can still hit the wall in the marathon if you are overzealous in your pacing. Remember: the marathon is almost wholly aerobic. It should feel as such at the start of the race. 

According to a 2021 study in PLoS One, a significant risk factor for hitting the wall is having run a personal best within the past year. When you think about the psychology of many runners, this makes sense. After setting a PR, it is easy to get greedy for faster results. The desire for another big PR can lead to risky pacing in the next marathon. Starting out too fast can also be a novice mistake.

Starting out too fast in the marathon sets you up for hitting the wall. Marathon pace should be right around your aerobic threshold, which moves pyruvate through aerobic pathways via the Krebs cycle. However, if you start too fast, your body uses a higher ratio of carbohydrates. As a result, you burn through too much energy too soon. Starting out too fast also accumulates lactate faster than desired, which can lead to muscular fatigue and weak muscle contractions. The faster you start out, the sooner you will hit the wall. 

An effective approach to marathon pacing is to split the race into two segments. Think of the marathon as a 20 mile long run (easy to moderate intensity) with a hard 10K progression at the end. As you should in a long run, you ease in the first 2-3 miles. After that, you settle into a comfortable, sustainable effort. Your effort may begin to feel slightly moderate around miles 13-18. However, it should not feel hard until you begin to push more in the final 10K.

Muscle fiber typology, race course terrain, and fatigue resistance may all impact this strategy. However, the general approach can be applied to most scenarios. (This article delves into how to pace your fastest marathon. For tips on pacing marathons in hot weather or on hilly courses, listen to this episode of the Tread Lightly Podcast.)

Disclaimer: I have an MS in Applied Exercise Science with a concentration in Sports Nutrition. I will sit for my C-ISSN exam soon, but am not a registered dietitian. However, you may find you need to work with a registered dietitian for personalized nutrition advice.