The 60g/hour Barrier: The Glucose Absorption Limit
For decades, sports nutrition advice was based on the understanding that the body could absorb and utilize about 60 grams of carbohydrates per hour during exercise. This limit is tied to the sodium-glucose cotransporter (SGLT1), which is responsible for transporting glucose from the intestine into the bloodstream. When an athlete ingests more than 60 grams of glucose per hour, these SGLT1 transporters become saturated, creating a bottleneck. Undigested carbohydrates then sit in the gut, attracting water and often leading to uncomfortable gastrointestinal symptoms like bloating, gas, and diarrhea. This is why relying solely on glucose-based sports drinks or gels for prolonged, high-intensity efforts can cause stomach upset and hinder performance.
Breaking the Barrier: The Power of Multiple Transportable Carbohydrates
Scientific breakthroughs in the early 2000s demonstrated how to overcome the 60g/hour limit by using multiple transportable carbohydrates. By combining glucose (or maltodextrin, a glucose polymer) with fructose, athletes can leverage a separate intestinal transporter (GLUT5) for fructose. This dual-pathway approach significantly increases the total rate of carbohydrate absorption, leading to higher rates of exogenous carbohydrate oxidation—the use of consumed carbs for fuel. A 2:1 glucose-to-fructose ratio became the standard for achieving intake rates of up to 90 grams per hour, while newer research explores 1:0.8 ratios to potentially push the limits even further.
Maximizing Your Carbohydrate Intake and Absorption
Properly fueling your body during prolonged exercise is a strategic process that goes beyond simply consuming enough calories. It involves optimizing the rate at which your body can absorb and utilize fuel without causing digestive distress. Here are some critical factors and tips for maximizing your carbohydrate intake during exercise:
- Match your intake to duration and intensity: Your body's needs change based on the length and effort of your activity. A moderate 90-minute run requires less fuel per hour than a six-hour ultramarathon. Adjusting your intake accordingly prevents both under-fueling and over-fueling.
- Prioritize dual-carb sources: For activities lasting over 2.5 hours, opt for products containing a glucose and fructose blend (like a 2:1 or 1:0.8 ratio) to maximize absorption via different transporters and minimize gut issues.
- Start early and fuel consistently: Begin your fueling strategy within the first hour of exercise to stay ahead of glycogen depletion. Consuming smaller amounts of carbohydrates every 15-20 minutes, rather than large doses at once, is often easier on the stomach and provides a more constant energy supply.
- Train your gut: The gastrointestinal system is adaptable, similar to your muscles. Regularly practicing your race-day fueling strategy during training sessions can increase your gut's tolerance and absorptive capacity for high carbohydrate intake. Start with lower doses and gradually increase them over several weeks.
- Mix formats for variety: Flavor fatigue and gastrointestinal issues can arise from relying on just one type of product (e.g., all gels). Use a combination of drinks, gels, chews, and even some low-fat/low-fiber real foods to meet your hourly carb goals and maintain palatability.
Fueling Strategy Comparison Table
| Feature | Single Carb Strategy (Glucose) | Multiple Carb Strategy (Glucose + Fructose) |
|---|---|---|
| Carb Source | Glucose, Maltodextrin | Glucose/Maltodextrin + Fructose, Sucrose |
| Max Absorption Rate | ~60 grams per hour | 90+ grams per hour |
| Transporter(s) Used | SGLT1 (Saturated at ~60g/hr) | SGLT1 (for glucose) + GLUT5 (for fructose) |
| Event Suitability | Shorter duration exercise (~1-2 hours) | Long-distance endurance events (>2.5 hours) |
| Risk of GI Issues | Higher if intake exceeds 60g/hr | Lower, even with higher intake rates |
What Factors Influence Your Carb Absorption Rate?
Exercise Intensity
As exercise intensity increases, blood flow is diverted away from the digestive system and toward the working muscles. This can reduce the efficiency of carbohydrate absorption, making it harder to process larger quantities during high-intensity efforts. Practicing your fueling strategy at race pace allows your gut to adapt to the physiological stress, helping to mitigate these effects.
Gut Training
Consistent high carbohydrate intake during training periods can increase the number of carbohydrate transporters in your intestine, thereby boosting your absorptive capacity. This progressive overload for the gut is known as 'gut training' and is a key strategy for elite endurance athletes to tolerate very high hourly carb intakes with minimal distress.
Individual Tolerance
Every athlete's digestive system is unique. Factors like genetics, gut microbiome composition, and diet history can all influence tolerance to different carbohydrate types and quantities. What works perfectly for one athlete may cause severe discomfort for another. This underscores the importance of personalized testing during training to find your optimal fueling strategy.
Conclusion
The question of how many carbs can your body digest per hour has an evolving answer, heavily dependent on the type of carbohydrate consumed and an individual's specific physiology. While a single carb source like glucose is limited to around 60 grams per hour, using multiple transportable carbohydrates (e.g., glucose and fructose) can significantly raise this ceiling to 90 grams or more for prolonged endurance events. Maximizing absorption requires a strategic approach that involves gut training, matching intake to exercise demands, and consistent, well-timed fueling during long sessions. Through diligent practice, athletes can push their fueling limits to optimize performance and prevent gut issues on race day. For further reading, explore the Gatorade Sports Science Institute's article on multiple transportable carbohydrates and their benefits(https://www.gssiweb.org/en/sports-science-exchange/Article/sse-108-multiple-transportable-carbohydrates-and-their-benefits).
