Carbohydrate availability is one of the most well-studied determinants of endurance performance. Over the past two decades, research has substantially refined our understanding of how exogenous carbohydrates are absorbed, oxidised, and utilised during prolonged exercise. This article provides an evidence-based overview of high-carbohydrate fueling strategies, with particular attention to dual-source carbohydrate systems and their role in modern endurance nutrition.
The role of exogenous carbohydrates during endurance exercise
During prolonged endurance exercise, typically defined as continuous effort lasting 90 minutes or more, the body's endogenous carbohydrate stores (primarily muscle and liver glycogen) become progressively depleted. When glycogen stores are substantially reduced, the ability to maintain exercise intensity is compromised. This is a well-documented phenomenon in exercise physiology.
Exogenous carbohydrate ingestion during exercise has been shown to help maintain blood glucose availability and sustain carbohydrate oxidation rates when endogenous stores are limited. Research suggests that consuming carbohydrates during prolonged exercise is associated with delayed fatigue and the maintenance of work output (Burke et al., 2011).
However, the rate at which ingested carbohydrates can be absorbed and oxidised is not unlimited. It is constrained by the capacity of intestinal transporters — a critical consideration that has shaped the development of modern carbohydrate fueling strategies.
Intestinal carbohydrate transport: glucose and fructose pathways
Carbohydrate absorption in the small intestine relies on specific transport proteins. Glucose (and glucose polymers such as maltodextrin, which are rapidly hydrolysed to glucose) is absorbed primarily via the sodium-dependent glucose transporter 1 (SGLT1). Fructose, by contrast, is absorbed via a separate transporter, GLUT5.
When a single carbohydrate source — such as glucose or maltodextrin alone — is consumed during exercise, the rate of exogenous carbohydrate oxidation has been observed to plateau at approximately 60 grams per hour. This apparent ceiling is thought to reflect the saturation of the SGLT1 transporter (Jeukendrup & Jentjens, 2000).
The question that followed was straightforward: if glucose absorption is limited by SGLT1 capacity, could additional carbohydrate be delivered through a different transporter?
Multiple transportable carbohydrates: the dual-source model
The concept of multiple transportable carbohydrates (MTC) emerged from research demonstrating that the co-ingestion of glucose and fructose — each absorbed via distinct intestinal transporters — could increase total carbohydrate absorption and oxidation beyond the ~60 g/h limit observed with glucose alone.
In a landmark study, Currell and Jeukendrup (2008) demonstrated that a glucose–fructose beverage consumed during exercise resulted in significantly higher exogenous carbohydrate oxidation rates compared to glucose alone. In this and related studies, combined glucose–fructose ingestion has been associated with oxidation rates approaching or exceeding 90 grams per hour under controlled conditions.
The proposed mechanism is that glucose and fructose are absorbed through separate, non-competing intestinal pathways (SGLT1 and GLUT5, respectively), thereby increasing total carbohydrate delivery to the bloodstream. Once absorbed, fructose is metabolised in the liver and can be converted to glucose or lactate, both of which serve as oxidisable substrates during exercise.
The 2:1 ratio of maltodextrin to fructose
Research into the optimal ratio of glucose-source to fructose has suggested that a ratio of approximately 2:1 (by weight) tends to produce the highest rates of total exogenous carbohydrate oxidation, while remaining within the range that is well tolerated gastrointestinally in most individuals (Jeukendrup, 2004).
In practical terms, maltodextrin is commonly used as the glucose source in endurance fueling products because it is a glucose polymer that is rapidly hydrolysed, has low osmolality relative to free glucose, and is generally well tolerated during exercise.
A formulation based on maltodextrin and fructose in a 2:1 ratio therefore reflects the current body of research regarding carbohydrate delivery optimisation during prolonged exercise. This is the carbohydrate architecture used in HYDRAX CORE.
Gastrointestinal tolerance and practical considerations
One of the practical advantages associated with dual-source carbohydrate formulations is improved gastrointestinal comfort. Research suggests that beverages containing both glucose and fructose are associated with lower ratings of gastrointestinal distress compared to isocaloric glucose-only beverages at the same ingestion rate (Jeukendrup, 2004).
This is thought to be related to the reduced osmotic load in the intestine when carbohydrate absorption is distributed across two transporter systems. For endurance athletes who must consume significant carbohydrate volumes during competition, particularly in events lasting three hours or more, gastrointestinal tolerance is a non-trivial consideration.
It is worth noting that individual variability exists, and gut training (the practice of consuming carbohydrates during training sessions to improve tolerance) has been suggested as a useful strategy for athletes intending to fuel at higher rates during competition (Burke et al., 2015).
Carbohydrate availability across the performance timeline
While in-session carbohydrate delivery is critical, it is only one element of a broader fueling approach. Endogenous glycogen stores are influenced by pre-exercise nutrition, and the rate of glycogen resynthesis post-exercise is influenced by carbohydrate intake in the hours following a session.
Burke et al. (2011) have outlined a comprehensive framework for carbohydrate availability in endurance sport, emphasising that fueling strategy should be periodised and context-dependent — varying according to session duration, intensity, and the athlete's broader training objectives.
This underscores the value of a systems-based approach to endurance fueling, where each phase of the training or competition cycle — preparation, execution, and recovery — is addressed with appropriate nutritional inputs.
Summary
The science of high-carbohydrate fueling for endurance has evolved substantially over the past two decades. Key findings include:
- Exogenous carbohydrate intake during prolonged exercise is associated with maintained blood glucose availability and sustained work output.
- Glucose absorption via SGLT1 appears to saturate at approximately 60 g/h when consumed alone.
- The co-ingestion of glucose and fructose, absorbed via separate intestinal transporters, has been shown to increase total carbohydrate oxidation rates.
- A maltodextrin-to-fructose ratio of approximately 2:1 is supported by current research as an effective formulation for maximising carbohydrate delivery.
- Dual-source carbohydrate systems are associated with improved gastrointestinal tolerance compared to single-source alternatives at equivalent intake rates.
References
- Burke LM, Hawley JA, Wong SH, Jeukendrup AE. Carbohydrates for training and competition. Journal of Sports Sciences. 2011;29(sup1):S17–S27.
- Burke LM, et al. Toward a common understanding of diet-exercise strategies to manipulate fuel availability for training and competition support in endurance sport. International Journal of Sport Nutrition and Exercise Metabolism. 2015;25(2):178–182.
- Currell K, Jeukendrup AE. Superior endurance performance with ingestion of multiple transportable carbohydrates. Medicine & Science in Sports & Exercise. 2008;40(2):275–281.
- Jeukendrup AE, Jentjens R. Oxidation of carbohydrate feedings during prolonged exercise: current thoughts, guidelines and directions for future research. Sports Medicine. 2000;29(6):407–424.
- Jeukendrup AE. Carbohydrate intake during exercise and performance. Nutrition. 2004;20(7–8):669–677.