[Research Review] Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: A Study of Elite Athlete Physiological Characteristics (Article 1135)

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 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 in the days before competition (while maintaining or reducing training volume).

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 distress. Research has found that combining 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 sugar source” sports nutrition products (glucose + fructose combinations). The common ratio falls around 2:1 (glucose:fructose), which can raise the hourly carbohydrate intake ceiling to approximately 90 grams.

Comparison of Gastrointestinal Distress 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, 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, significantly reducing the risk of gastrointestinal distress on race day
• 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
• Individualized testing: The osmolality and taste tolerance of different 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), which will naturally subside as glycogen is depleted after the race. 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 for longer events (e.g., a full marathon over 4 hours) requiring 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 related to absorption rates of multi-source carbohydrate 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: A Study of Elite Athlete Physiological Characteristics (Article 643)
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