Bonking During Your Marathon? Here’s Why.

Putting the time and energy into marathon training only to bonk before getting to the finish line is a race-day tragedy many of us can, unfortunately, relate to. While several race-day elements are out of our control (like weather) we can control how we do or don’t fuel ourselves before and during the race, which could make the difference between running out of energy or staying strong through the finish.

What is “Bonking”?

Bonking, also known as “hitting the wall,” is an unfortunate race-tanking phenomenon that happens when carbohydrate reserves drop or become depleted, typically occurring after mile 20 in the marathon. Unlike fat stores, the body can only store a limited amount of carbohydrates in the liver (75-100 grams) and muscles (300-400 grams) — and you need upwards of 750 grams to run a marathon — making it essential to (re)fuel during long endurance events.  

Muscle glycogen is the primary source of carbohydrates in the body, and the main fuel used during endurance activity. Once these stores become depleted, performance decreases and runners may experience symptoms such as extreme fatigue, a sudden loss of energy, dizziness, headaches, pain, cramping and a slew of negative emotions. Hitting the wall is as much mental as it is physical. 

What Are The Main Reasons For Hitting the Wall?

Here are some of the key reasons runners may bonk and how to fix them.

1. Not eating enough carbohydrates before the race 

A substantial body of research points to carbohydrates as the key to fueling our muscles and bodies for longer activity, hence, they should make up the majority of your meals and snacks. Practice eating or drinking carbohydrates an hour or two before your run, workout, or race to give your body time to digest them. 

A recent study published in Frontiers in Sport and Active Living notes that “high carbohydrate content in the pre-event meal led to a longer time to exhaustion compared to a meal with a low carbohydrate content, or exercising in a fasted state.” Eating a diet higher in carbohydrates the week before the race can also be beneficial for enhancing glycogen stores and promoting lasting energy during your 26.2-mile race.

2. Your glycogen tank is empty

Taking in carbohydrates during long endurance events prevents muscle glycogen stores from becoming depleted. Once they are depleted, the body has to turn to convert fat and protein to energy, which is a much longer and less efficient process — unless you’ve done significant training to adapt your body to burn fat more readily. As glycogen stores become depleted, blood glucose becomes an increasingly important source of carbohydrate fuel. However, it is used almost instantaneously. Enter hypoglycemia, which occurs when liver glucose output can no longer keep up with muscle glucose needs during prolonged exercise, leading to headaches, fatigue, and nausea. 

How do you avoid this bonk? Aim to take in 30-60 grams of carbohydrates per hour of running. Some runners may even need up to 80-90 grams/hour. In the final miles, there’s some interesting research that a carbohydrate mouth rinse can help keep the brain alert, which can be useful if you’re having trouble getting fuel down. 

3. Your hydration strategy is off

What and when you drink can be as significant as what you eat. Being dehydrated or overhydrated can have serious negative effects during long endurance events.

When dehydrated, everything in the body works less efficiently, meaning you won’t be able to digest and absorb those carbohydrates effectively, which will impact blood sugar and glycogen stores. A water deficit that exceeds 2% of your body weight can impact and compromise your performance as well as cognitive function, particularly in hot weather. Severe dehydration (hypohydration), with water deficits of 6-10%, has more significant effects, which can include decreased cardiac output, sweat production, skin and muscle blood flow, and can be very dangerous. 

Being overhydrated is typically caused by overdrinking fluids in excess of sweat and urinary losses, and/or not taking in enough electrolytes, namely sodium. Overdrinking can lead to hyponatremia (low plasma sodium), which can cause nausea, vomiting, headaches, confusion, respiratory distress, and low consciousness. It can even lead to death if untreated. 

So having a hydration plan specific to your needs is critical. (And be sure to take your gender into account.) Never start a race dehydrated, and make sure you’re taking in sufficient electrolytes with your fluids, especially if your carbohydrate gels or chews don’t have a high concentration of sodium in them.

4. You need to chill on the caffeine

Yes, caffeine can be an ergogenic aid when taken in appropriate doses (3-6 mg/kg of body weight), but too much can have negative consequences on some people, causing nervousness, irritability, GI distress and other runner’s stomach symptoms, dizziness, lightheadedness, and dehydration. 

So be strategic with your caffeine use. Before a marathon or longer race, have a caffeine plan that keeps you from overdoing it. Monitor and limit your pre-race coffee or tea. Space out your caffeinated gels, chews or liquids during the race, and make sure you’re taking in adequate water and electrolytes too.

You’ve heard time and time again but it bears repeating: Practice and preparation are the keys to success for long-distance running. Spend time practicing your fueling, electrolyte, and caffeine plan well before race day to avoid hitting the wall.

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About the Author 

Sarah Schlichter, MPH, RDN, is a registered dietitian based in the Washington, DC area. She helps to fuel runners without strict dieting. Sarah is also a nutrition consultant and writes the blog, Bucket List Tummy, sharing nutrition posts, healthy family-friendly recipes and running tips  Sarah also owns the Nutrition For Running blog, and co-hosts the Nail Your Nutrition Podcast, focused around evidenced-based nutrition tips for athletes.