Why Combining Sugars Matters for Runners
For a long time, sports nutrition focused primarily on glucose (or maltodextrin, which rapidly converts to glucose) as the primary fuel source for runners. However, the body's intestinal transport proteins for glucose, specifically SGLT1, can become saturated at intakes exceeding approximately 60 grams per hour. Once this happens, additional glucose sits in the gut, potentially causing gastrointestinal (GI) issues and providing no extra energy.
The game-changing discovery for endurance athletes was that fructose uses a different transporter, GLUT5. By consuming both glucose and fructose simultaneously, runners can use these separate pathways to absorb significantly more carbohydrates per hour—up to 90 grams or even 120 grams in some cases. This strategy of using "multiple transportable carbohydrates" ensures a higher rate of energy delivery to the muscles, delaying fatigue and improving endurance performance.
The Standard 2:1 Ratio
The 2:1 glucose-to-fructose ratio has long been considered the standard recommendation for endurance sports. This ratio is proven effective for carbohydrate intakes up to 90 grams per hour during prolonged exercise, such as marathons or other events lasting over 2.5 hours. The rationale behind it is simple: it provides enough glucose to saturate the primary SGLT1 transporters while adding fructose via the GLUT5 transporters for extra absorption. This offers an excellent balance of high energy delivery, performance benefits, and comfort for most athletes, and often utilizes maltodextrin as the glucose source, which is less sweet and palatable during long events.
The Advanced 1:0.8 Ratio
For elite athletes or those competing in ultra-endurance events lasting over 2.5 hours who push their carbohydrate intake beyond 90 grams per hour (sometimes up to 120 grams), recent research suggests that a ratio closer to 1:0.8 (glucose:fructose) may be even more effective. Some studies have shown that this slightly higher proportion of fructose can further increase carbohydrate oxidation and potentially reduce GI distress at extremely high intake rates. However, this strategy requires dedicated "gut training" to build tolerance, as such high volumes can be challenging for the digestive system. For the majority of runners, the well-established 2:1 ratio remains the most practical and comfortable option.
Comparison of Glucose:Fructose Ratios
| Feature | 2:1 Glucose:Fructose Ratio | 1:0.8 Glucose:Fructose Ratio (approx. 1:1) |
|---|---|---|
| Best For | Carbohydrate intake up to 90g/hr. Most endurance runners and events, including standard marathons. | Carbohydrate intake of 90-120g/hr and above. Elite athletes, ultra-endurance events. |
| Primary Benefit | Optimal balance of energy delivery, enhanced performance, and minimizing GI issues for typical high-intensity endurance efforts. | Potentially higher rates of carbohydrate absorption and oxidation for extreme, high-volume fueling. |
| Palatability | Often uses more maltodextrin, resulting in a less sweet and more neutral taste. | Contains more fructose, which is sweeter and can affect palatability for some athletes. |
| Gut Comfort | Well-tolerated by most athletes when trained properly. | Requires extensive gut training and adaptation for optimal tolerance at very high intake rates. |
| Practicality | Widely available in commercial products like gels and drinks, easy to implement for most runners. | Best for highly conditioned athletes who have practiced a high-carb strategy extensively. |
Practical Application for Runners
How to Fuel with Multiple Carbohydrates
- Assess your needs: For runs over 90 minutes, start with a fueling strategy. Moderate duration (1-2 hours) may need 30-60g/hr, while longer efforts (>2.5 hours) require 60-90g/hr or more.
- Practice
gut training: Just as you train your legs, you must train your gut. Start by testing different products and intake rates during long training runs to find what you tolerate best. Gradually increase your hourly carbohydrate intake over time to adapt your digestive system. - Choose the right products: Many commercial gels, drinks, and chews are formulated with a multi-carbohydrate blend. Look for products listing ingredients like maltodextrin (for glucose) and fructose. Read the labels to determine the ratio, which is often listed implicitly by the total glucose and fructose content. You can also use real foods, such as table sugar (sucrose, a 1:1 glucose:fructose ratio), for more affordable options, especially if your intake rate is lower.
- Stay hydrated: Always consume your gels and concentrated carbohydrate sources with adequate water. Hydration is key for proper digestion and absorption.
- Listen to your body: Personal tolerance is the most important factor. If a specific ratio or product causes GI distress, experiment with a different one. A product's palatability can also be a significant factor during long events.
Conclusion
For endurance runners, the science is clear: combining glucose and fructose, or multiple transportable carbohydrates, is more effective than relying on glucose alone for efforts over 90 minutes. This method significantly increases the rate of energy absorption and oxidation while mitigating GI issues. The optimal glucose to fructose ratio for most runners, targeting up to 90 grams of carbs per hour, is 2:1. For ultra-endurance athletes needing higher intake, a 1:0.8 ratio may be beneficial, but it requires greater gut adaptation and testing. The key to success is to train your gut, test products and ratios during training, and find a strategy that works best for your individual needs and race duration.
For more in-depth research on multiple transportable carbohydrates, see the review by Rowlands et al. in Sports Medicine.
