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The Science of Sugars in Endurance Sports

BY Elizabeth Inpyn

Understand how the body breaks down carbs into sugars and explore the various types of sugars used in endurance sports.

When looking through the sports nutrition aisle these days, it’s no wonder there are high levels of confusion about which products are best. While many athletes question the need for, benefits, and added cost of new solutions, most just want some relief from GI distress. In order to compare the different options and find the best solution, we need to dig into the science of sugars and their relationship with gut issues in endurance sports. 

Laying foundational definitions and providing examples of each, we’ll examine the various sugar sources, i.e. multiple transportable carbohydrates, hydrogels and superfuels (Cluster Dextrin), allowing your athletes to formulate a race day plan that works best for them

Breaking Down Sugars

In the simplest of terms, sugar is a carbohydrate. Carbohydrate molecules (CHO) are built with carbon, hydrogen and oxygen in a 1:2:1 ratio. Carbohydrates come in many forms: simple sugars (glucose, fructose, and galactose, as well as maltose, lactose and sucrose) and complex sugars (glycogen, maltodextrin, starches and fibers). When we consume carbohydrates, our bodies break them down into simple sugars, which are absorbed into the bloodstream and utilized for energy. 

Glucose is our primary energy source and is used/consumed by almost every cell in the body. Of the other two simple sugars, fructose is metabolized primarily in the liver, while galactose is converted into glucose and used for energy. This is especially important in long-distance endurance events (like Ironman, stage racing and ultramarathons) that require higher levels of carbohydrate consumption.

Triathletes, ultrarunners and cyclists need carbohydrates for energy. These ‘sugars’ help them maintain energy and performance throughout training and racing. During exercise, the body uses primarily glucose as its primary fuel source. An article published in the Journal of Sports Sciences states, “Muscle glycogen and blood glucose are the most important substrates for the contracting muscle. Fatigue during prolonged exercise is often associated with muscle glycogen depletion and reduced blood glucose concentrations.” 

Recent research indicates that the ideal amount of carbohydrates to consume during these events for peak performance is 90-120g CHO/hour. The challenge lies in how/what to consume to achieve that high level of carbohydrate intake, especially while running. Intestinal absorption can be the limiting factor that has resulted in the development of products like hydrogels and powder formulations using Cluster-Dextrin. Before we dive into those, let’s review what happens in the gut during endurance events. 

It’s a Gut Feeling

The athlete’s gut undergoes significant changes during longer-distance endurance events due to the physical demands of the activity. As the body works to supply the muscles with the necessary energy to keep swimming-biking-running, blood flow is redirected away from the digestive system, leading to a decrease in digestive function. 

Those system demands can result in less-than-desirable GI symptoms: nausea, abdominal cramping, vomiting, and diarrhea. While the exact causes are not entirely mapped out, it is believed to be a combination of factors, including dehydration, ‘nutrition’ ingestion, decreased blood flow to the gut, changes in hormone levels and increased stress on the gut from all the jostling. 

Good news; the gut is extremely adaptable, allowing athletes to manipulate the amount, timing and type of fuel consumed. Gut training is challenging the gut with a high content and volume of carbohydrates and testing different types of carbohydrates. This gut training has been shown to improve carbohydrate malabsorption, improve glucose availability in the blood, and substantially reduce gut discomfort, which translates into improved endurance performance. 

The Different Types of Sugars Used in Fuels

What options do athletes have then to meet these higher carbohydrate recommendations? Enter multiple transportable carbs, hydrogels and superfuels. According to research, “Among the nutritional factors, a high intake of carbohydrate and hyperosmolar solutions increases GI problems. Several nutritional manipulations have been proposed to minimize gastrointestinal symptoms, including using multiple transportable carbohydrates. This type of CHO intake increases the oxidation rates and can prevent accumulation of carbohydrate in the intestine.” 

Traditional Gels

Traditional gels such as Precision Hydration/GU/Clif use a combination of maltodextrin and fructose or sucrose. They are examples of multiple transportable carbohydrates and are highly effective at allowing athletes to reach the desired 90-120 g/hour with minimal GI distress. 
We know that carbohydrate nutrition dosing during exercise allows up to a maximum of ~60 g.h-1 of glucose/maltodextrin in order to be absorbed and used for oxidation. Still, the addition of fructose to this dose of glucose allows greater rates of carbohydrate ingestion, absorption, and oxidation because fructose uses a different transport protein to move from the gut into the bloodstream. The point being, when ingesting glucose and fructose together, we can absorb carbohydrates at rates exceeding the absorption limit for glucose alone.

Modern Hydrogels & Gastric Emptying 

Carbohydrate hydrogels are regular carbohydrate-containing sports drinks with pectin and sodium alginate added. Pectin and sodium alginate form a gel-like mixture when met with the stomach’s high acidity. The gel-like mixture allows for a faster pass through the stomach into the intestine — what is called ‘gastric emptying’ — and speeds the rate at which the ingested carbohydrate is available for absorption across the intestine and utilization as a fuel source by the working muscles. 

The big question with any new “engineered” product is ‘does it work’? Earlier studies (2020) looking at the gastrointestinal and metabolic effects of carbohydrate hydrogels were inconclusive and the research did not demonstrate positive effects on exogenous carbohydrate availability or gastrointestinal symptoms during exercise. However, current research supports the ingestion of hydrogels. “The ingestion of glucose and fructose in hydrogel form during running benefited endurance performance, exogenous CHO oxidation, and GI symptoms compared with a standard CHO solution.” 

In January 2022, a study published in Medicine and Science in Sports and Exercise compared 90 g/hr of glucose and fructose consumption in a 2:1 ratio as a hydrogel or standard carbohydrate product (or a placebo drink with no carbohydrates at all) during 120-min of steady running at 70% VO2max, followed by a 5-km time-trial. “The ingestion of glucose and fructose in hydrogel form during running benefited endurance performance, exogenous CHO oxidation, and GI symptoms compared with a standard CHO solution.” 

Carbohydrate consumption improved performance versus placebo, and hydrogel consumption was shown to improve performance compared to the standard carbohydrate drink by ~2.1%. From a metabolic standpoint, exogenous carbohydrate oxidation rates were greater with the hydrogel. However, it’s important to note that absolute liver and muscle glycogen oxidation rates were similar in the hydrogel and standard carbohydrate conditions. Important to also note gastrointestinal distress was worse with the standard carbohydrate compared to the hydrogel.

Another hydrogel study looked at participants ingesting 70 g/hr of maltodextrin and fructose as a standard carbohydrate solution or hydrogel, a massive 180 g./hr of maltodextrin and fructose as a hydrogel, or water during 105 min of running at 70% VO2max. Focusing on the 70 g/hr data, hydrogel ingestion did not enhance exogenous carbohydrate oxidation or impact reports of gastrointestinal symptoms during exercise. 
Coupling these data with those of the previous study supports the contention that hydrogels are more likely to be of use during challenging scenarios where the dose of carbohydrate ingested during exercise is notably high (> 90 g/hr). Maurten is an example of a hydrogel. They claim that their “biopolymer matrix,” with a 0.8:1 ratio of fructose and glucose, allows an athlete to take up to 100 g of carbohydrates an hour.

Corn Starches or Cluster Dextrin

Last up, we have highly branched cluster dextrin (HBCD), a type of carbohydrate derived from corn starch. Due to its high molecular weight and low osmolality, HBCD is thought to provide an ergogenic advantage over other carbohydrate sources via faster gastric emptying and faster absorption. In theory, faster gastric emptying means ‘easier to digest,’ reducing GI distress risk. 

HBCD also provides a sustained release of energy due to its slow breakdown over a prolonged period in the body. Much of this is explained by cluster dextrin’s large size. A single molecule contains between 60 to 70 glucose units, at an average molecular weight of 10,548 grams per mole, with a dextrose equivalent of 1.7.

In contrast, a long-chain amylose starch like maltodextrin only contains between 3 to 20 glucose units, at an average molecular weight of 2,000 grams per mole, with a Dextrose Equivalent of 8-10 [8/9]. Skratch Labs (Super-High Carb Sport Drink Mix) is an example of HBCD. They claim that their ‘Super High-Carb Drink Mix’ is an extremely flexible fuel (and hydration) source that can be used at lower or higher concentrations depending on the workload and caloric need.

What’s the Sweet Takeaway?

In summary, athletes have many options when it comes to carbohydrate fueling during long-distance endurance events. Multiple transportable carbs, hydrogels and superfuels each have unique formulations, benefits, and palatability. If you are trying to increase carb consumption during training and racing or need to alleviate GI issues, any of these options, alone or in combination, should do the trick. 

Key Takeaways:

  1. Various products (hydrogels, superfuels, glucose-fructose combo gels) can provide the necessary carbohydrates for performance while reducing the negative GI symptoms endurance athletes experience. 
  2. Regardless of which product you choose, the gut is trainable and should be tested/pushed to enhance tolerance and effectiveness.
  3. Hydrogels and superfuels (Cluster-Dextrin) have solid research supporting their use to ease GI symptoms which may or may not enhance performance.  What matters is that each athlete practices race-day nutrition well ahead of the big day to find the right combination that works for them. 


Jeukendrup, A. (2017, March). Training the Gut for Athletes. Retrieved from  https://pubmed.ncbi.nlm.nih.gov/28332114/

Jeukendrup, A. (2011). Nutrition for endurance sports: marathon, triathlon, and road cycling. Retrieved from https://pubmed.ncbi.nlm.nih.gov/21916794/

Osmolality measures made using an EliTech Vapro® Vapor Pressure
Osmometer at the Applied Exercise Science Laboratory at the University of
Colorado at Boulder, 2020.

Prado de Oliveira, E. and Burini, R. (2014, October). Carbohydrate-dependent, exercise-induced gastrointestinal distress. Retrieved from https://pubmed.ncbi.nlm.nih.gov/25314645/

Rowe, J. et al. (2022, January). Glucose and Fructose Hydrogel Enhances Running Performance, Exogenous Carbohydrate Oxidation, and Gastrointestinal Tolerance. Retrieved from https://pubmed.ncbi.nlm.nih.gov/34334720/

Sutehall S. et al. (2022, January 20). The impact of sodium alginate hydrogel on exogenous glucose oxidation rate and gastrointestinal comfort in well-trained runners. Retrieved from https://pubmed.ncbi.nlm.nih.gov/35127792/

Viribay, A. et al. (2020, May 11). Effects of 120 g/h of Carbohydrates Intake during a Mountain Marathon on Exercise-Induced Muscle Damage in Elite Runners. Retrieved from https://pubmed.ncbi.nlm.nih.gov/32403259/

Wilburn D., Machek, S. and Ismaeel A. (2021, August 1.) Highly branched cyclic dextrin and its ergogenic effects in athletes: A brief review. Retrieved from https://www.journalofexerciseandnutrition.com/index.php/JEN/article/view/100

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About Elizabeth Inpyn

Elizabeth is a former NCAA Division 1 swimmer and water polo player, multiple podium triathlete (Olympic and 70.3 distance) and a USAT National Qualifier. She’s a certified sports nutritionist, holds an ISSN and Plant-based nutrition certification, a Masters of Science in Applied Exercise Science: Sports Nutrition and a Bachelor of Science in Biochemistry from the University of California Santa Barbara. As a performance nutrition coach at Fuelin, Elizabeth helps professional and elite competitors and teams, recreational athletes and coaches achieve their health, performance and wellness goals.

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