Why Carbohydrate Intake Matters During Prolonged Intense Exercise
During intense exercise, your body relies heavily on stored carbohydrates, or glycogen, for fuel. Muscle glycogen is the most readily available fuel source for high-intensity work, but your body can only store a finite amount. Once these stores are significantly depleted, typically after 90 minutes or more of continuous high-intensity effort, your performance will drop dramatically—a phenomenon known as 'hitting the wall' or 'bonking'.
Consuming carbohydrates during exercise provides an external source of glucose, which is quickly absorbed into the bloodstream. This helps to maintain stable blood sugar levels, spares the remaining muscle glycogen, and provides a continuous fuel supply to working muscles, delaying the onset of fatigue and allowing you to sustain a higher workload for longer. For endurance events, this is a non-negotiable strategy for success.
Recommendations Based on Exercise Duration and Intensity
Nutritional recommendations for carbohydrate intake during exercise are not one-size-fits-all; they are dependent on the duration and intensity of your activity. The amount of carbohydrates you need increases with exercise duration. For efforts exceeding 2.5 hours, the use of multiple transportable carbohydrates becomes critical for maximizing absorption and minimizing gastric upset.
Exercise Under 75 Minutes
For shorter, high-intensity workouts lasting between 45 and 75 minutes, you may not need to consume a large amount of carbohydrates, especially if you start with adequate fuel stores. A carbohydrate mouth rinse or a small amount of an energy drink can sometimes provide a performance benefit through a central nervous system effect, without needing to be fully absorbed.
Exercise Lasting 1–2 Hours
During exercise lasting between 1 and 2 hours, supplementing with carbohydrates will improve performance by maintaining blood glucose levels. A recommended intake is approximately 30–60 grams of carbohydrates per hour. This can easily be achieved with a sports drink, a couple of energy gels, or an energy bar. For shorter durations within this window, a smaller amount closer to 30g/hour may suffice, while athletes working harder or for longer should aim closer to 60g/hour.
Exercise Lasting Over 2 Hours
As exercise duration extends beyond two hours, your body's ability to use exogenous (external) carbohydrates becomes more pronounced. The recommendation increases to 60–90 grams of carbohydrates per hour. This higher intake requires a specific fueling strategy involving multiple transportable carbohydrates to prevent intestinal transport limitations and stomach issues. Instead of relying solely on glucose, a mixture of glucose and fructose (often in a 2:1 ratio) is recommended to utilize different intestinal absorption pathways simultaneously.
Ultra-Endurance Events
For ultra-endurance athletes involved in events lasting longer than 4 hours, some research suggests intake could even go as high as 90–120 grams per hour, especially when using glucose-fructose mixtures. However, this requires significant 'gut training' during practice to improve tolerance. Higher intakes are typically easier to achieve during the cycling portion of a triathlon than the running portion due to less gastric jostling.
Strategies for Success: Training Your Gut
Simply consuming a high amount of carbohydrates on race day without prior practice can lead to gastrointestinal distress. The gut is adaptable, much like your muscles, and can be trained to tolerate higher intake levels.
To effectively train your gut, follow these tips:
- Practice with race-day products: Use the same gels, chews, or drinks during long training sessions to assess tolerance and get your system accustomed to processing the fuel.
- Start with smaller amounts: Gradually increase your hourly carbohydrate intake over time. Start with 30-40g/hour and build up towards your target based on event duration.
- Don't wait until you're hungry: Begin fueling early in your workout, typically within the first 30-45 minutes, and continue with consistent, frequent intake every 20-30 minutes.
- Mix up your fuel sources: Try a combination of liquids, gels, and solid foods to find what works best for your digestive system and personal preference.
- Consider fluid and electrolytes: Remember that carbohydrate intake is only one part of the equation. Balancing your intake with adequate fluid and electrolytes, especially sodium, is also vital for performance and preventing dehydration.
Comparison of Carbohydrate Fueling for Endurance
| Exercise Duration | Recommended Carb Intake (per hour) | Carbohydrate Type Recommendation | Primary Goal |
|---|---|---|---|
| Under 60 min | 0–30 grams (mouth rinse optional for 45–75 min) | Any high oxidation carb (glucose, sucrose) | Optimize CNS response, not glycogen replacement |
| 60–120 min | 30–60 grams | Any high oxidation carb (gels, sports drinks) | Maintain blood glucose, spare glycogen |
| >120 min (Endurance) | 60–90 grams | Multiple transportable carbohydrates (glucose + fructose) | Maximize exogenous carb oxidation, spare glycogen |
| >240 min (Ultra) | 90–120 grams (with gut training) | Multiple transportable carbohydrates (glucose + fructose) | Sustain energy over extremely long durations |
The Role of Multiple Transportable Carbohydrates
Research in the early 2000s demonstrated that the intestine's capacity to absorb a single type of carbohydrate, like glucose, is limited to approximately 60 grams per hour. By using a combination of carbohydrates that are absorbed via different transporters in the small intestine, such as glucose (using SGLT1) and fructose (using GLUT5), athletes can increase their absorption and oxidation rate to 90 grams per hour and beyond. This allows for a much higher total carbohydrate intake, which directly correlates with improved endurance performance in longer events. A common and effective combination is a 2:1 ratio of glucose to fructose.
Conclusion: Fueling is a Skill
Determining how much carbohydrates per hour are recommended during prolonged intense exercise is crucial for unlocking your full athletic potential. The amount needed scales with the duration and intensity of the activity, ranging from 30–60g/hr for shorter endurance efforts to 60–90g/hr or more for ultra-endurance events. Optimizing your fueling strategy involves using the right combination of carbohydrates, especially glucose-fructose blends for high intake rates, and consistently practicing your nutrition plan during training to adapt your gut. By treating your fueling as a key aspect of your training, you can minimize fatigue, prevent gastrointestinal distress, and sustain your performance to reach your goals.
