[Research Review] Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Recent Academic Literature Review and Training Practice (Article 1375)

Reference Journal Source: 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 for 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 intestinal glucose transporter (SGLT1) has a physiological absorption 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 the physiological basis for the growing popularity of “dual/multiple carbohydrate 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 grams.

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

Key Research Conclusions and Practical Recommendations

• Pre-race loading timeline: It is recommended to gradually increase carbohydrate intake (to over 70% of total calories) in the 2–3 days before competition while significantly reducing training volume, allowing sufficient time for glycogen storage
• Gut training: Gradually increasing the amount and frequency of in-race carbohydrate intake during long-term training helps the gut adapt to high-volume carbohydrate absorption loads, significantly reducing the risk of gastrointestinal discomfort during the actual race
• Regularize fueling timing: Rather than waiting until thirsty or 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
• Personalized testing: The osmolality and taste tolerance of different fueling products vary from person to person; be sure to actually 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 weight gain from short-term loading is mainly water (glycogen storage simultaneously retains water); it will naturally subside after the race as glycogen is depleted. It is not fat accumulation, so there is no need for excessive concern.

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 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 related to absorption rates of multiple-carbohydrate-source fueling.
2. Sports Medicine — Literature related to 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: Recent Academic Literature Review and Training Practice (Article 1258)
• Research Review: Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: International Research Literature Compilation and Review Report (Article 1372)
• Research Review: Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Recent Academic Literature Review and Training Practice (Article 175)
• Research Review: Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Recent Academic Literature Review and Training Practice (Article 844)