Why Carbohydrate Type Matters in Endurance Fueling
Carbohydrates are the primary fuel source during prolonged exercise, but not all carbohydrates enter the bloodstream through the same pathway. Understanding how glucose and fructose are absorbed clarifies why certain fueling strategies feel sustainable while others lead to discomfort.
The Transporter System
Think of the intestine as having two main “entry doors” for carbohydrates during exercise.
Door 1: SGLT1 (Glucose Transporter)
- Transports glucose
- Also transports maltodextrin once broken down
- Has a maximum transport capacity
- Saturates at moderate-to-high intake levels
When glucose intake exceeds SGLT1 capacity, additional carbohydrate may remain in the gut longer, increasing the risk of bloating or discomfort.
Research examining carbohydrate oxidation during prolonged exercise has shown that single-source glucose intake is limited by transporter availability.
Door 2: GLUT5 (Fructose Transporter)
- Transports fructose
- Uses a completely separate pathway
- Does not compete with glucose transport
Because glucose and fructose use different transporters, consuming them together allows the body to utilize both entry doors simultaneously.
This concept underpins what is often referred to as “multiple transportable carbohydrates,” which have been studied extensively in endurance contexts.
Why Blends Can Increase Total Carbohydrate Delivery
When both transporters are used:
- Total carbohydrate absorption may increase
- Carbohydrate oxidation rates may rise
- Energy delivery becomes more sustained
However, higher intake does not automatically equal better performance. Tolerance remains individual, and excessive intake can still overwhelm digestion.
What This Means for Athletes
The question is not “Which carbohydrate is better?”
It is:
- How much total carbohydrate can you tolerate?
- Does a blended approach improve comfort?
- Can your gut handle increased intake at race intensity?
Understanding transporters reframes fueling from marketing claims to physiology.