[Research Review] Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Biomechanical Quantitative Experiment Report (No. 1006)

Reference Journal Source: International Journal of Sport Nutrition and Exercise Metabolism • International Research Findings Review Series

This article explores the balance strategy between carbohydrate loading ratios and gastrointestinal tolerance in long-distance endurance runners.

Physiological Basis of Carbohydrate Loading

Muscle and liver glycogen stores are limited, and prolonged endurance exercise (typically exceeding 90 minutes) can easily lead to “hitting the wall” due to glycogen depletion. Carbohydrate loading strategies aim to maximize muscle and liver glycogen stores—and delay the onset of hitting the wall—by increasing carbohydrate intake ratios (while maintaining or reducing training volume) in the days before competition.

Glucose-to-Fructose Ratio for In-Race Fueling

When using glucose-based carbohydrates alone, the absorption rate of the intestinal glucose transporter (SGLT1) has a physiological ceiling (approximately 60 g per hour); exceeding this amount tends to cause gastrointestinal discomfort. Research has found that pairing fructose (absorbed via a different transporter, GLUT5) can increase the total amount of carbohydrate absorbed. This is also the physiological basis for the growing popularity of “dual-source/multi-source” sports nutrition products (glucose + fructose combinations)—a common ratio is around 2:1 (glucose:fructose), which can raise the hourly carbohydrate intake ceiling to roughly 90 g.

Comparison of Gastrointestinal Discomfort Rates Across Different Fueling Strategies (Illustrative)

Key Research Conclusions and Practical Recommendations

• Pre-race loading timeline: Gradually increase carbohydrate intake (to 70%+ of total calories) in the 2-3 days before race day, while significantly reducing training volume, to allow ample time for glycogen storage
• Gut training: Progressively increase the amount and frequency of in-race carbohydrate intake during long-term training so the gut adapts to high carbohydrate absorption loads, which can significantly reduce the risk of gastrointestinal discomfort on race day
• Regularize fueling timing: Rather than waiting until thirsty/hungry, set fixed time intervals (e.g., every 20-30 minutes) for small, frequent feedings to maintain stable blood glucose
• Personalized testing: Osmolality and taste tolerance of different products vary by individual; be sure to test your fueling strategy during training or simulation races before major events, and avoid trying new products for the first time on race day

Common Research Q&A (FAQ)

Q: Will carbohydrate loading make me gain fat or weight?

A: The short-term weight gain from loading is mainly water (glycogen storage also retains water), which naturally subsides after the race as glycogen is depleted. It is not fat accumulation, so there is no need to worry excessively.

Q: Is it necessary to use a glucose-fructose blend for in-race fueling?

A: Not strictly necessary, but for longer events (e.g., marathons over 4 hours) requiring higher carbohydrate intake, a mixed-source product can indeed help raise the absorption ceiling and reduce gastrointestinal burden.

References and Academic Citations

1. International Journal of Sport Nutrition and Exercise Metabolism — Research on absorption rates of multi-source carbohydrate fueling.
2. Sports Medicine — Literature on gut training and gastrointestinal tolerance in endurance sports.

Related Reading

• 【Research Review】Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Biomechanical Quantitative Experiment Report (No. 1342)
• 【Research Review】Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Biomechanical Quantitative Experiment Report (No. 355)
• 【Research Review】Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Biomechanical Quantitative Experiment Report (No. 613)
• 【Research Review】Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Biomechanical Quantitative Experiment Report (No. 541)