The Problem: One Carb Source Has a Ceiling
Carbohydrate that is absorbed in the small intestine relies on specific transporters. Glucose is absorbed via the SGLT1 transporter. SGLT1 has an absorption ceiling of roughly 60 grams of glucose per hour regardless of how much glucose is consumed, although there is some data that some may be able to absorb more. Regardless, pushing intake of a single, glucose-only carb source past that point doesn’t increase the carbohydrate the body can use; it mostly increases the carbohydrate sitting unabsorbed in the gut, which is a major driver of GI distress (bloating, cramping, nausea) during exercise.
The Solution: Multiple Transportable Carbohydrates
The case for including fructose in any endurance fuel is well established. Glucose is absorbed via SGLT1, an intestinal transporter that saturates at approximately 60g per hour, although early data suggests that some athletes may be able to have the capacity to absorb > 60 grams of glucose per hour. Fructose uses an entirely separate transporter, GLUT5, which operates independently. By combining both, athletes can absorb and oxidize more total carbohydrate than either transporter could deliver alone, while reducing the GI distress associated with single-source, high-dose carb intake. This is the science behind the “multiple transportable carbohydrates” (MTC) approach used across modern high-carb sports nutrition, and it’s why Carbs Fuel formulates with both glucose-based and fructose-based ingredients rather than glucose alone.
Why Carbs Fuel Chooses 2:1 and Not Other Ratios
At Carbs Fuel, our goal is to create fueling products that help the greatest number of athletes effectively fuel their training and racing. The 2:1 glucose-to-fructose ratio was selected because the evidence supports it as the formulation where absorption efficiency, GI tolerability, and performance benefit converge across the full range of carbohydrate intake rates encountered by recreational, elite age-group, and professional competitors.
To understand why we chose this ratio, it helps to understand how glucose and fructose differ in how they are absorbed and metabolized during exercise. Exogenous glucose is oxidized more rapidly than exogenous fructose, and this difference comes down to two factors. First, glucose is absorbed more quickly allowing it to enter systemic circulation and rapidly reach the working muscles. Fructose, by contrast, is absorbed more slowly and is processed liver, where it is converted to glucose, lactate, or pyruvate before it can be released into circulation and oxidized by the working muscle. Thus, the absorption and processing steps introduce delays that reduce the fructose oxidation rate relative to glucose.
The primary rationale for including fructose in a fuel source is to offer an additional pathway of carbohydrate absorption when glucose absorption threshold has been reached. Below this threshold, additional fructose does not confer an absorption advantage. This means that athletes currently fueling below 60 grams per hour do not require high fructose proportions for absorption benefits. Furthermore, the lower fructose load in a 2:1 formulation also reduces the risk of GI symptoms such as bloating and cramping for athletes with a fructose sensitivity.
While we considered formulating at 1:0.8, we were not entirely convinced this would be the best ratio for the greatest number of athletes, despite some research suggesting it may offer marginally higher exogenous carbohydrate oxidation rates above approximately 100g/hr. The primary evidence for this comes from O'Brien and Rowlands (2010), which found approximately 0.1 g/min higher exogenous oxidation with a 1:0.8 ratio versus 2:1 at 108g/hr, and a 3.6% ±3.5% improvement in incremental peak power. After careful consideration of all the aforementioned factors and evidence, we believe that the 2:1 is the ratio that is the most practical and functional ratio where absorption efficiency, GI tolerability, and performance benefit can be realized for athletes of all levels.
