Carbohydrates for Endurance Athletes: A Comprehensive Guide

Carbohydrates, especially simple sugars, often get a bad rap because their high intake is associated with health issues like obesity and diabetes. While it’s true that many unfit, sedentary people could benefit from reducing their carbohydrate intake, research consistently shows that carbohydrates are crucial for endurance athletes.

Athletes who limit their carbohydrate consumption during training often experience diminished performance, slower recovery, and decreased focus. As any athlete who tried a keto diet can tell you, training is a miserable experience marked by extreme fatigue.

Further, there is an increasing consensus that carbohydrate restriction—and not just calorie restriction as previously thought—is the cause of Relative Energy Deficiency in Sports (RED-S), which can significantly impact an athlete's health and performance.

By contrast, athletes who adequately fuel with carbohydrates consistently perform better.

Despite how important carbs are for endurance sports, there are still a lot of misconceptions about their function and importance. Therefore, we believe it’s important to clarify the fundamentals of carbohydrates for endurance athletes so they can better understand loading, fueling, and recovery.

What Are Carbohydrates?

Carbohydrates, along with protein and fat, are one of the three macronutrients and provide the body with approximately 4 kilocalories per gram. There are many types of carbohydrates but, in the world of endurance sports, carbohydrates are broken down into two types: fast and slow.

Quick Fact: In science and nutrition, "kilocalorie" and "Calorie" are often used interchangeably, though technically, kilocalorie is more accurate. 
  • A kilocalorie (kcal) is the amount of energy needed to raise the temperature of 1 kilogram of water by 1 degree Celsius. 
  • A Calorie (with a capital “C”) refers to a kilocalorie. 
  • A calorie (with a lowercase “c”) is the amount of energy needed to raise 1 gram of water by 1 degree Celsius, so it is 1000x smaller than a Calorie.

Fast Carbohydrates

Fast carbohydrates are the ones that the body breaks down rapidly. They are quickly absorbed by the gut and converted into energy that your body can use. This is why simple carbohydrates are used as fuel for athletes during endurance races.

    Slow Carbohydrates

    Slow carbohydrates take longer for the body to digest, resulting in a slower release of energy instead of the rapid rise in blood sugar that comes with fast carbohydrates.

    Many slow carbohydrates contain fiber, which slows down the absorption of carbohydrates into the bloodstream, preventing rapid spikes in blood sugar. This is why an apple (which contains fiber) is considered a slow carbohydrate but pulp-free apple juice is typically a fast carbohydrate. Fat can also slow down digestion. So, while white rice is a fast carbohydrate, adding peanut butter to it can turn it into a slow meal.

      slow vs. fast carbohydrates examples for endurance athletes

      Simple vs. Complex Carbohydrates

      Another way of categorizing carbohydrates is as “simple” or “complex.”

      Simple Carbohydrates:

      These are the most basic types of carbohydrates. They consist of just one molecule (monosaccharide) or two molecules (disaccharide) of sugar. Because they are so simple, the body breaks them down and absorbs them quickly.

      Complex Carbohydrates:

      These consist of longer chains of monosaccharides linked together. The more complex structure means they take longer for the body to digest, resulting in a more sustained release of energy and don’t cause blood sugar levels to spike.

      As a general rule, simple carbohydrates are “fast” and complex carbohydrates are “slow.”

      Because of this, you will often hear these terms being used interchangeably. However, this generalization isn’t completely accurate, since many fast carbs (like white bread, rice, and maltodextrin) are technically complex carbohydrates because they contain more than two molecules of sugar.

      What about the Glycemic Index?

      High glycemic foods quickly raise blood sugar (glucose) levels. Low glycemic foods release energy more slowly. Because of this, you may hear the terms being used interchangeably with fast and slow carbohydrates. However, when it comes to digestion, they aren’t always the same.

      For example, fructose is considered a low glycemic carbohydrate, but the body uses it quickly, so it is a fast carbohydrate for athletes. And fried potatoes are high glycemic, but the fat content slows digestion, so it is a slow carbohydrate for athletes. Because of this:

      high glycemic ≠ fast and low glycemic ≠ slow

      While it is important to understand glycemic index, athletes can ignore glycemic index when choosing their carbohydrate sources for loading, fueling, and recovery.

      Aren’t Fast Carbs Bad?

      Carbohydrates which typically fall on the “healthy” list–such as whole grains, beans, and vegetables–are slow carbs. Fast carbs like sweetened juices and sugar get put on the “bad” list.

      The problem is that these lists are generally made for sedentary people. It is very easy to over-eat simple carbohydrates, and they cause big fluctuations in blood glucose levels.

      However, when doing high-intensity exercise, athletes need fast carbohydrates because they quickly provide energy. 

      The Fiber Problem

      Another problem with slow carbohydrates for endurance athletes is that the fiber they contain isn’t fully digestible. Instead, fiber typically acts as a prebiotic, fermenting in the large intestine. To reduce the chance of GI distress, endurance athletes should avoid fiber before a training session or event.

      Read more in our guide to training the gut.

      How the Body Utilizes Carbohydrates for Energy

      Regardless of whether they are fast or slow, the body breaks all carbohydrates down into simple sugar molecules (monosaccharides). The difference is in how quickly they get broken down into the monosaccharides. These sugar molecules are glucose, fructose, and galactose.

      Of the three monosaccharides, glucose can be used immediately and directly by the body. It goes straight into the bloodstream, where it can then fuel cells.

      Fructose and galactose first must go to the liver. The liver converts them into glucose and lactate, which is then sent to the bloodstream so it can be used for energy. Because of this, fructose and galactose are not as “fast” as glucose. However, they get utilized at a very similar rate as glucose during exercise.

      Glycogen: The Body’s Way of Storing Glucose for Later

      The body stores glucose for later use in the liver and muscles. This stored form of glucose is called glycogen.

      • Liver glycogen: This acts as a reservoir of glucose for the blood. When blood sugar levels are low, the liver will convert its stored glycogen back into glucose and release it into the bloodstream.
      • Muscle glycogen: Muscle glycogen does not get released into the bloodstream. It is only used as fuel for the muscles where it is located. For example, glycogen in your arm muscles cannot be used to fuel your legs when cycling.

      Carb Loading = Filling Glycogen Stores

      The reason athletes consume large amounts of carbohydrates before a race or event, such as eating a massive pasta dinner, is to fill up their glycogen stores. This helps ensure their muscles and liver have enough fuel available for the event. Read more about this in our carb loading guide.

      Fueling = Maintaining a Constant Supply of Carbohydrates

      Since the body has a limited amount of stored glycogen, athletes need to consume carbohydrates during intense endurance events to provide their bodies with energy (and make sure the dreaded ‘bonking’ doesn’t occur). This is known as fueling.

      Unfortunately, the body is limited in how many carbohydrates it can absorb at once. To maximize carbohydrate absorption during fueling, it is important that athletes fuel with a combination of glucose and fructose. Read more about this in our fueling guide.

      Recovery = Replenishing Depleted Glycogen Stores

      During intense exercise, the body’s glycogen stores become depleted. Since it takes time for the liver and muscles to replenish these stores, athletes need to consume a significant amount of carbohydrates after a training session or event.

      It is important to start fueling with carbohydrates immediately after finishing a training session or event. If you wait too long to fuel, your body may have a difficult time replenishing glycogen stores, and you may have a harder time recovering. Read more about this in our recovery guide.

        Why Don’t Sports Supplements Contain Glucose?

        Glucose is the “fastest” carbohydrate, and all other carbohydrates eventually get broken down into glucose before they can be used for energy.

        So, why don’t we ever see glucose as an ingredient on sports supplements?

        Because “glucose” is the word we use for the sugar molecule when it’s in the bloodstream. It goes by other names in food. The two most common names you’ll see are maltodextrin and dextrose.

        Maltodextrin

        Maltodextrin is a chain of glucose molecules that is derived from vegetable starches. It is what you will typically find in high-quality sports supplements because:

        • It isn’t sweet: This is important so the supplements don’t become too sweet when combined with fructose.
        • Fast absorption: One molecule of maltodextrin contains multiple glucose molecules. This means it has a lower osmolality, which helps with absorption.
        • Less Gut Distress: The low osmolality of maltodextrin also means it is less likely to cause GI issues compared to other types of carbohydrates.

        Dextrose

        Dextrose is another name for D-glucose, which is the naturally occurring form of glucose used by the body for energy. Essentially, dextrose and glucose are the same sugar, but "dextrose" is the term often used in processed forms, such as in glucose tablets for diabetes or in sports supplements.

        Compared to maltodextrin, dextrose is made up of fewer glucose molecules and has a much sweeter taste. Because of this, Nduranz uses maltodextrin in its fueling products.

        Names for Simple Carbohydrates

        There are also many other names for simple carbohydrates that you might encounter. Here are some of the common ones in the world of endurance sports.

        Sugar Name Description Sources Maltodextrin Chain of glucose molecules Sports supplements Dextrose Monosaccharide Honey, fruit, glucose tablets Fructose Monosaccharide Honey, fruit, high-fructose corn syrup Galactose Monosaccharide Dairy; component of lactose Sucrose Equal parts glucose and sucrose Table sugar, candies, soda Lactose Disaccharide made of glucose and lactose Dairy products; not commonly used in sports products because of digestive issues Isomaltulose Similar to sucrose, but much slower to digest Sports supplements

        Carbohydrate Type Matters

        Choosing the right types of carbohydrate is crucial for endurance athletes who want to optimize their energy levels. Further, to ensure absorption, it’s important that athletes consume carbohydrates in the right ratio. For more information, we recommend you read our guides to Loading, Fueling, and Recovery.

        Ready to Take Your Performance to the Next Level?

        Check out Nduranz products. They are formulated with the ideal 1:0.8 ratio of glucose to fructose for maximum absorption without digestive discomfort.