Endurance Athlete's Guide to Fueling

Back in 1978 when the first Ironman triathlon was held, the winning time was 11:46:58. Today, athletes regularly break this time and the current record is 7:12:12—more than four and a half hours faster. Obviously, advances in training protocols and gear technology have a lot to do with this progress. However, we can’t discount the impact that fueling science has had on performance.

Race fueling—what you consume during the race—can make or break athletes and is so important that it’s often considered one of the disciplines of a triathlon. Race fueling is equally important for other endurance athletes like long-distance runners, cyclists, and swimmers.

If you are tired of hitting the wall or feel like your performance is stuck regardless of how hard you train, this guide is for you. We’ll take you through the science of endurance fueling so you can create the right plan for your body.

Quick Recap:

• Preventing Glycogen Depletion: During endurance exercise, consume fast carbohydrates to avoid hitting the wall (bonking). Slow carbohydrates like fiber, protein, or fat should be avoided during fueling, as they can slow absorption and cause stomach discomfort.
• Carbohydrate Guidelines:
• For activities lasting 1 to 2.5 hours: Aim for 30 to 60 grams of carbohydrates per hour.
• For activities lasting more than 2.5 hours: Increase intake to 90 grams of carbohydrates per hour.
• Best Fuel: Choose energy gels, chews, or drinks that contain glucose and fructose in a 1:0.8 ratio. Other carbohydrates can be used for fuel, but may cause problems like stomach discomfort.

What is Bonking?

Bonking is what happens when an athlete’s body runs out of energy. Even though the term gets thrown around loosely, it’s more than just extreme fatigue from intense training. It’s an energy crash that causes severe symptoms like dizziness, confusion, shakiness, and even an altered mental state.

If bonking happens during a race, you can consider it game over.

The Science of Bonking

Scientifically speaking, bonking occurs when the body’s blood sugar levels become critically low due to depleted liver glycogen stores.

When your blood sugar levels run low, the body turns to glycogen in the liver. Glycogen is a stored form of the carbohydrate glucose. The liver breaks down its stored glycogen into glucose and sends it to the bloodstream, where it can then be used as fuel.

However, if you exercise for an extended period without replenishing your carbohydrate stores, the glycogen in your liver becomes depleted. Since the liver can no longer release glucose into the bloodstream, the body suffers from hypoglycemia (low blood sugar).

Pro Fact: The brain relies heavily on blood glucose to function, which is why the mental symptoms, such as confusion and lack of focus, are often the first signs of bonking.

What about “Muscle Bonking”?

The body also stores glycogen in the muscles. Like liver glycogen, muscle glycogen gets broken down into glucose. However, muscle glycogen doesn’t get released into the bloodstream. It is used directly by the muscles where it is stored. Muscle glycogen can also become depleted during prolonged exercise, making it difficult for the muscles to sustain contractions, causing the muscles to feel heavy and unresponsive.

Muscle Bonking Is Not True Bonking

Even though the term “muscle bonking” often gets thrown away, true bonking is from critically low blood sugar levels related to depleted liver glycogen.

Depleted muscle glycogen causes fatigue, not true bonking: your muscles will feel tired but your brain still functions normally.

How Much Glycogen Do We Have Stored?

A typical adult can store 500-800g of glycogen in their muscles and liver. For shorter races, these glycogen stores are usually enough to keep the body fueled. However, in endurance races lasting over 90 minutes, glycogen stores can run out, leaving the athlete with an energy shortfall.

Pro Fact: The purpose of carb loading –when athletes eat high amounts of carbs leading up to a race—is to top off glycogen stores.

Why Doesn't the Body Use Fat for Energy?

When blood sugar levels are low, the body typically turns to fat stores for energy. However, fat metabolism is a slow process and requires oxygen–something that is in short supply during high-intensity exercise beyond VO2 max. Thus, fat metabolism is limited in high-intensity exercise.

Likewise, the body can turn muscle tissue into glucose for fuel. However, this is definitely not something we want to happen because it can lead to muscle depletion.

So, how do we prevent bonking?

The solution is to fuel with carbohydrates.

Carbs: Your Fuel Source

Like gasoline in a car, carbohydrates are the fuel that allows your body to go. By consuming the right carbohydrates during intense activity, you can keep blood glucose levels high, allowing you to spare liver glycogen stores and preventing bonking. Research shows there is a correlation between increased carbohydrate intake and improved endurance performance.

So why don’t we see cyclists chomping down on bagels for the duration of the race? Because the body is limited in how many carbs it can absorb at once.

Carb Absorption: Why Combine Glucose and Fructose

Simply put, absorption is the process of a nutrient moving from the intestinal lumen into the bloodstream. The body uses transporters to help absorb carbohydrates. Think of transporters like a doorway in a concert hall. The doorway allows people (carbs) to enter. However, there’s a limit to how many people can enter at once.

It was long believed that transporters could only absorb up to 60 grams of carbohydrates per hour. The main transporters our bodies use to absorb glucose are the SGLT1 and GLUT2 transporters. However, we also have another doorway: the GLUT5 transporter. This transporter is used for absorbing fructose.

By combining glucose and fructose, it is possible to absorb more than 60 grams of carbohydrates per hour.

There is a lot of research that shows combining fructose and glucose during high-intensity exercise improves performance compared to just glucose alone. This makes sense because being able to absorb more carbohydrates per hour increases the amount of energy available to the athlete.

How Much Glucose and Fructose to Consume?

Most sports fueling guides say it is possible to absorb 60g of glucose and an additional 30g of fructose per hour, for a total of 90g of carbohydrates per hour. While these numbers make everything easy to understand, there is no conclusive evidence that these are the maximum amounts the body can absorb.

The truth is that it is impossible to measure carbohydrate absorption. Researchers measure oxidation, and then equate it with absorption–even though the two are not necessarily the same. Further, we know that everyone’s body is different, and even the same body can process nutrients differently based on various conditions.

The 90 Grams Per Hour Guideline

Current research shows that the body can process somewhere between 30g and 80g of glucose per hour, and between 20g and 60g of fructose per hour.

While it may be possible for some elite athletes to absorb more than these amounts per hour, for most people, a good guideline is to consume up to 90 grams of carbohydrates per hour as a mixture of glucose and fructose.

Pro Tip: If you aren’t used to fueling at these levels, you’ll need to gradually increase your carb intake so it can handle 90g/hour without GI distress. This is referred to as “gut training.”

Should You Consume 90g of Carbohydrates Per Hour?

For intense activity lasting more than 2.5 hours, aim to consume 90 grams of carbohydrates per hour. This will provide the body with fuel for energy and keep glycogen stores from being depleted. More than 90g/h is unlikely to improve performance, as no further glycogen sparing will occur.

However, consuming 90g/h isn’t always necessary. For activities lasting less than an hour, the body should have enough stored glycogen. Consuming fuel during the activity won’t help with performance or glycogen synthesis.

For activities lasting 1-2.5 hours, some fuel can help performance. However, athletes generally only need 30-60g/h. While consuming 90g/h isn’t necessarily bad, the additional carbohydrates can make it difficult to maintain an energy balance since you’d need to reduce those extra calories from your diet later on.

Fueling Guidelines

• Activity lasting less than 1 hour: No or little fueling required; glycogen stores should be adequate.
• Activity lasting 1-2.5 hours: Consume 30 to 60 grams of carbohydrates per hour. Most simple carbohydrates will be well-absorbed at these amounts, but a mixture of glucose and fructose is still preferable.
• Activity lasting more than 2.5 hours: More aggressive fueling is required to prevent glycogen depletion. Consume 90 grams of carbohydrates per hour. To allow for absorption, the carbohydrates must be a mix of glucose and fructose, ideally in a 1:0.8 ratio.

Want a more detailed fueling breakdown? See our Fueling Calculator.

Ideal Ratio of Glucose to Fructose

While the research on how many grams of carbohydrates per hour we can absorb is limited, it is clear on one aspect: the ratio of glucose to fructose matters.

Research by Prof. David Rowlands and his team tested the various glucose to fructose ratios and found that a 1:0.8 ratio is ideal.

The 1:0.8 ratio allows:

• Maximum carbohydrate oxidation
• Reduced change of gut distress
• Improved GI comfort

Note that a lot of sports supplements use a 2:1 ratio based on the idea that the body can absorb 60g of glucose and 30g of fructose. But, as established before, these amounts aren’t fully accurate. The research shows that a 1:0.8 ratio of glucose to fructose provides the best results for endurance activities, which is why Nduranz uses this ratio in their energy products.

Types of Endurance Fuels

In the world of endurance sports, the carbohydrates you consume during intense activity are called fuel. There are many options for fuel. We’ll go over them here. However, for endurance activities lasting over 2.5 hours, the best fuel is an energy drink, gel, or chew that contains glucose and fructose in the ideal 1:0.8 ratio. The fuel should also have simple dosing so you consume the right amount of carbohydrates.

Energy Drinks

Energy drinks formulated for endurance athletes contain a combination of fluids, electrolytes and carbohydrates. This allows athletes to meet their hydration and energy needs simultaneously.

Gels

Energy gels contain concentrated carbohydrates, typically in convenient single-serving pouches. Because they don’t require any chewing, gels are easy to consume even when breathing heavily. Nduranz energy gels are formulated with electrolytes, allowing athletes to replace essential salts while they fuel.

Chews

Like gels, chews also contain concentrated carbohydrates in convenient single-serving sizes. Some athletes prefer chews because they tend to have a better taste and texture than gels (though this isn’t always the case!). However, they require chewing, which can throw off breathing during intense exercise.

Sports Bars

There are many types of “sports bars.” Some athletes like these because the act of chewing can help offset the feeling of hunger. However, most sports bars contain fat and fiber, so are not good for endurance sports. The fat and fiber can slow down absorption and also cause digestive issues.

Another problem is that it is difficult to chew properly when breathing deeply. Because bars are solid foods, they can bounce around in the gut when running, causing discomfort. For these reasons, many endurance athletes avoid sports bars.

Gummy Bears

Gummy bears and other gummy candies are typically made from sucrose. Sucrose contains both glucose and fructose, so can be a good source of fuel for endurance sports. However, gummy candies can be hard to chew during intense activity and tend to stick to the teeth, which can lead to tooth decay.

Coca Cola

Coca Cola contains simple sugars that the body can quickly absorb and turn into energy. It also contains caffeine, which can provide an energy burst. However, a lot of serious athletes stay away from Coca Cola as fuel because the carbonated water can cause stomach issues during intense activity. A lot of athletes also avoid it because of its artificial ingredients and acidity that can cause tooth decay.

Pro Fact: Bananas became a popular fuel for endurance athletes in the early 20th century because they are a good source of simple carbohydrates and also contain some electrolytes. However, it’s hard to know exactly how many carbohydrates you are getting and the fiber can cause digestive issues, so today’s elite athletes use precise fuel formulas.

Looking for Fuel?

Nduranz energy gels, drinks and bars are specifically formulated for endurance athletes based on the latest science. All of our energy products use the 1:0.8 ratio of glucose to fructose so you get maximum energy without digestion issues. It’s fuel that makes sense, so you get results.

Check out our products here. Or, want to learn more about the science of endurance sports nutrition? Read our guides.