The Science of Carbohydrate Absorption
To understand the maximum carb absorption per hour, it's essential to look at how carbohydrates are transported from the gut into the bloodstream. The small intestine uses specific transporter proteins to absorb different types of sugars. The primary transporters for glucose (including maltodextrin, which is made of glucose units) can become saturated at an absorption rate of approximately 60 grams per hour. In the early 2000s, this was widely considered the ceiling for effective carbohydrate intake during exercise.
However, a key breakthrough came with the discovery that fructose uses a different transporter protein (GLUT5), which has its own absorption capacity, estimated to be around 30 grams per hour. By combining glucose and fructose, athletes can effectively use both transport pathways simultaneously, thereby increasing the total amount of carbohydrates that can be absorbed and utilized by the body.
The 2:1 Glucose-to-Fructose Ratio
This discovery led to the development of a 2:1 ratio of glucose to fructose for carbohydrate fueling, which became the new standard for athletes aiming for higher intake rates. This strategy allows for a combined absorption of up to 90 grams of carbohydrates per hour (60g from glucose + 30g from fructose) with minimal risk of intestinal issues. This has become the go-to fueling strategy for many endurance athletes in events lasting longer than 2.5 hours.
Pushing Beyond 90g/hr: The New Frontier
For highly trained and elite endurance athletes, recent research suggests that even higher absorption rates may be possible. Some studies have shown athletes successfully tolerating and absorbing up to 120 grams per hour or more, often by adjusting the glucose-to-fructose ratio closer to 1:0.8 or even 1:1. The mechanisms behind this heightened capacity are still being explored, but theories include adaptations to the gut and a potentially higher capacity for fructose absorption than previously thought. These very high intake rates are typically reserved for the most intense and prolonged efforts, such as ultra-endurance events, and are only possible with extensive training.
The Role of Gut Training
Just as athletes train their muscles and cardiovascular system, they can also train their digestive system to be more efficient at absorbing carbohydrates. This process, known as 'gut training,' involves gradually and consistently increasing carbohydrate intake during training sessions to expand the gut's capacity for absorption. For a non-gut-trained athlete, attempting to consume 90 grams per hour or more might lead to significant gastrointestinal distress, such as bloating, cramping, and diarrhea. By introducing higher carbohydrate loads slowly over several weeks, the body adapts, increasing the number and function of intestinal transporters.
Comparison of Fueling Strategies
To illustrate the different approaches, here is a comparison of standard and advanced fueling strategies based on exercise duration and intensity.
| Exercise Duration & Intensity | Strategy | Maximum Carb Absorption Rate | Recommended Fuel Sources | Common Applications |
|---|---|---|---|---|
| Short (<1 hr), High Intensity | Mouth Rinse or Small Amounts | Not applicable (minimal needed) | Sports drink, gel | Interval training, short races |
| Moderate (1-2 hrs) | Standard Intake (30-60g/hr) | ~60g/hr (single source) | Simple carbs (gels, sports drinks) | Mid-distance running, cycling |
| Long (2-3 hrs) | Multi-Transportable Carbs (60-90g/hr) | ~90g/hr (glucose + fructose) | Gels, chews, drinks with 2:1 ratio | Marathons, long-distance cycling |
| Ultra (>3 hrs), Elite | High Intake (>90g/hr) | ~120g/hr or more | Gels, drinks with 1:0.8 or 1:1 ratio | Ultra-marathons, Ironman triathlon |
| Recovery | Post-Exercise Fueling (1.0-1.2g/kg/hr) | Highest possible post-exercise | High GI carbs, often with protein | Glycogen replenishment after a race |
Optimizing Your Intake Strategy
- Start Low and Go Slow: If you are new to fueling during exercise, begin with a conservative intake of 30-40 grams of simple carbs per hour and gradually increase the amount over several weeks.
- Experiment with Ratios: As you increase your intake, experiment with adding a fructose source to your glucose-based fuel to see if you can comfortably tolerate and absorb more.
- Choose Appropriate Fuel Sources: For shorter, higher-intensity efforts, liquids and gels are often easier to consume. For longer, steadier efforts, bars, chews, or even real foods can be incorporated.
- Train with Your Race Fuel: Never try a new fueling strategy on race day. Practice with the exact products and quantities you plan to use in competition during your long training sessions.
- Listen to Your Gut: Pay attention to how your body responds. Signs of exceeding your absorption limit include bloating, nausea, and stomach cramps.
Conclusion
The maximum carb absorption per hour is not a static number but a dynamic, trainable capacity that depends on the type of carbohydrates consumed and the athlete's preparation. While a single source like glucose caps out around 60 grams per hour due to saturated transporters, combining it with fructose allows for higher absorption rates of up to 90 grams per hour with a 2:1 ratio. Elite athletes, through specific gut training protocols, may even push beyond this to 120 grams per hour or more, especially in ultra-endurance events. By understanding these mechanisms and practicing a personalized fueling strategy, athletes can optimize their energy intake, sustain performance, and prevent gastrointestinal issues during prolonged exercise. More research is still needed to fully understand the outer limits of absorption and the benefits of higher intakes for all athletes.
The Absorption Mechanism
Carbohydrates must first be broken down into their most basic units (monosaccharides) before they can be absorbed. Glucose is absorbed via the SGLT1 transporter and is the rate-limiting step when only one type of sugar is consumed. Fructose, using the GLUT5 transporter, provides an additional pathway, allowing for a higher total absorption rate when both are present. This dual-transporter mechanism is what enables intakes of 90g/hr and beyond. The efficiency of this process is individual and can be improved through targeted training.
Fueling Different Types of Athletes
The optimal carbohydrate intake varies not just with duration but also with the athlete's level of training. For example, a less-trained athlete might find 60g/hr sufficient for a long race, whereas a top professional might need 90-120g/hr to sustain a higher absolute intensity. The focus for all athletes should be on finding their individual tolerance and optimizing their intake accordingly, rather than blindly following elite-level recommendations.
The Future of Fueling
Research continues to explore the limits of carbohydrate absorption and oxidation. New technologies, such as hydrogel formulations, aim to further enhance delivery and minimize stomach upset at high intake rates. Personalization of nutrition strategies, considering individual physiology and gut function, is the next frontier in maximizing athletic performance through fueling. As knowledge evolves, so too will the recommendations for what constitutes the maximum carb absorption per hour.
