[Research Review] Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: An Updated Academic Literature Review and Training Practice (Article 343)

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

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

The 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 the proportion of carbohydrate intake (while maintaining or reducing training volume) in the days before competition.

Glucose-to-Fructose Ratios 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 grams per hour); exceeding this amount can easily 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/multiple carbohydrate source” sports nutrition products (glucose + fructose combinations)—a common ratio of approximately 2:1 (glucose:fructose) can raise the hourly carbohydrate intake ceiling to around 90 grams.

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

Key Research Conclusions and Practical Recommendations

• Pre-race loading timeline: It is recommended to gradually increase the proportion of carbohydrate intake (above 70% of total calories) 2-3 days before the race, while significantly reducing training volume, to allow sufficient time for glycogen storage
• Gut training: Gradually increasing the amount and frequency of in-race carbohydrate intake during long-term training allows the gut to adapt to high-volume carbohydrate absorption loads, which can significantly reduce the risk of gastrointestinal discomfort during the actual race
• Regularize fueling timing: Rather than waiting until thirsty/hungry to fuel, it is recommended to set fixed time intervals (e.g., every 20-30 minutes) and consume small amounts frequently to maintain stable blood glucose levels
• Individualized testing: The osmolality and taste tolerance of different fueling products vary from person to person. 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 you gain fat or weight?

A: The short-term weight gain from loading is mainly water (glycogen storage simultaneously retains water), which will naturally subside 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 glucose-fructose mixed products for in-race fueling?

A: Not necessary, but if the event is longer (e.g., a full marathon over 4 hours) and requires a higher carbohydrate intake, mixed sources 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 directions on absorption rates of multiple-carbohydrate-source sports nutrition.
2. Sports Medicine — Literature on gut training and gastrointestinal tolerance in endurance sports.

Further Reading

• Research Review: Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: An Updated Academic Literature Review and Training Practice (Article 844)
• Research Review: Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: An Updated Academic Literature Review and Training Practice (Article 1030)
• Research Review: Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: An Updated Academic Literature Review and Training Practice (Article 175)
• Research Review: Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: An Updated Academic Literature Review and Training Practice (Article 127)