Understanding carb types for endurance performance

For many endurance athletes, fueling starts with a simple goal: eat enough to avoid running out of energy. As training volume or intensity increases and race performance becomes the focus, fueling becomes less about preventing an epic bonk and more about facilitating performance. The conversation shifts from simply eating more carbohydrates to understanding which carbohydrates work best, how much can realistically be absorbed, and why carbohydrate type becomes increasingly important as during-exercise intake rates increase.

Athletes training for longer races and harder training loads are increasingly targeting 75 g/h, 90 g/h, and even higher carbohydrate intakes per hour based on duration and intensity. Some athletes tolerate intakes above 100 grams g/h whereas others are still cautious or hesitant to increase carbohydrate intake beyond 30-60 g/h, often because of concerns around gastrointestinal discomfort. However, success with increasing carbohydrate intake rates during harder sessions and in competition depends on knowledge and preparation, product selection, and gut training. 

Carbs as performance fuel

Carbohydrate serves as the primary fuel source during sustained moderate exercise and during high-intensity efforts, as it supplies energy rapidly enough to support sustained output. Muscle glycogen and blood glucose become critical contributors to maintaining power, pace, and even decision making as fatigue builds. Therefore, when carbohydrate availability falls, performance often declines. 

Because the amount of stored carbohydrate (glycogen) is limited in the body, it is often advantageous to support energy levels with exogenous carbohydrate intake (e.g., drinks, gels, chews) at hourly rates between 30-120 g/h during sustained exercise, but the intestines absorb carbohydrates through transport systems that have limits. Once those transporters begin approaching capacity, consuming additional carbohydrate from the same source does not necessarily improve fuel delivery and may lead to GI discomfort. This is why carbohydrate composition becomes increasingly important once athletes move to higher intake rates. 

Glucose and dextrose

Glucose is basically the backbone of sports nutrition, because it is rapidly absorbed and readily used by working muscle. Most sports drinks, gels, chews, and fueling products rely heavily on glucose or dextrose (the equivalent monosaccharide to glucose) as a core energy source. The challenge is that glucose absorption depends primarily on a transporter in the intestine called SGLT1; this transporter begins approaching saturation around approximately 60-70 grams per hour, creating a potential bottleneck.

Maltodextrin

Maltodextrin has become one of the most common ingredients in endurance fueling products for a reason. Derived from starch (e.g. corn or wheat), maltodextrin consists of glucose chains that digest rapidly into usable fuel. It provides substantial carbohydrate delivery without excessive sweetness and allows products to provide larger carbohydrate amounts without making drinks excessively concentrated, potentially improving gastrointestinal comfort.

For athletes trying to move toward higher carbohydrate intake targets, maltodextrin often becomes an important source, but because it is still glucose based, there continue to be limitations with intestinal transport. 

Fructose

Fructose has transformed modern endurance fueling, because it unlocks a parallel mechanism for intestinal transport, allowing for higher intake, absorption, and utilization rates. Rather than relying on SGLT1, fructose uses a separate intestinal transporter called GLUT5. Because glucose and fructose use different absorption pathways, combining them allows athletes to absorb and utilize more total carbohydrate than either source alone. This combination is often referred to as multiple transportable carbohydrates.

This is why many fueling products combine glucose and fructose or maltodextrin and fructose. Individuals will prefer varying ratios of glucose to fructose between 1:0.5 and 1:0.8. The result can be greater carbohydrate oxidation, improved carbohydrate availability, and potentially better tolerance during longer or harder sessions.

Sucrose

Sucrose, commonly known as table sugar, contains both glucose and fructose naturally bonded together in equal ratio. Once digestion occurs, sucrose provides access to both intestinal transport pathways. However, there may also be taste and tolerance issues at higher concentrations due to sweetness and more small particles and increasing osmolality of the solution. 

Highly branched cluster dextrin

Highly branched cluster dextrin represents is increasingly appearing in high-carb endurance products. Its structure creates lower osmolality solutions, influencing how carbohydrate and fluid move through the gastrointestinal system. Some athletes report improved comfort and steadier energy availability during longer sessions when using highly branched cluster dextrin, particularly when higher carbohydrate intakes of more than 60-90 g/h are involved.

Carbohydrate hydrogels

Despite a higher price tag, carbohydrate hydrogels have generated substantial interest in the endurance sports fueling market. These products combine carbohydrate with ingredients designed to create a gel structure in the stomach. One of the most widely recognized examples is Maurten, whose hydrogel technology centers on sodium alginate and pectin as the primary hydrogel forming ingredients in their drink products. The goal is to encapsulate carbohydrate and transport it through the stomach into the intestine where carbohydrate absorption occurs.

While they may not directly improve performance compared to other carbohydrate delivery practices, the hydrogel format may be helpful for some athletes looking to better tolerate high rates of carb intake, such as more than 90-120 g/h, for example. 

Slow vs. fast carbs

One persistent misconception is the idea that slower-digesting carbohydrates provide better sustained energy during exercise. Outside of training, slower digestion is generally preferred for many health reasons; however, during exercise, muscles are continuously demanding carbohydrate. Rapid carbohydrate availability often supports performance more effectively, because it narrows the gap between what muscles are using and what is being replaced.

This is also why athletes targeting very high carbohydrate intake strategies often emphasize simpler carbohydrate sources rather than high-fiber foods during demanding race preparation and competition nutrition. Simpler carbohydrate sources are typically easier to tolerate and absorb easier when intake rates are higher and environmental conditions and intensity demands pose a further challenge to gut absorption.

Prepare and practice 

Most athletes do not move directly from 30 grams per hour to 90 grams per hour without practice. Progressively exposing the gastrointestinal system to higher carbohydrate intakes with intentional choices and combinations (such as sports drinks and gels) during training can improve tolerance and may support adaptations that enhance carbohydrate handling.

Combining carbohydrate sources is okay and beneficial. However, it is important to not neglect fluid intake and ensure sodium intake is appropriate for the amount of fluid being consumed, especially for longer duration sessions and events. Once you find products and combinations that work for you, continue to practice using them and, as part of your training process, advance the carbohydrate intake rates higher as your training session and event goals warrant.

---

At BaseCamp, we believe that every cyclist has the potential to achieve greatness, no matter where they start. Our mission is to create a community-driven training environment where cyclists and triathletes of all levels can train together, support each other, and grow stronger, faster, and more confident in their abilities. Our cycling training programs are expert driven and tailored to your needs. Whether you're a seasoned pro or just getting started, BaseCamp is where you belong.