[Research Review] Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Physiological Characteristics of Elite Athletes (Article 352)

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 carbohydrate intake ratios (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 g per hour); exceeding this amount readily causes gastrointestinal discomfort. Research has found that pairing fructose (absorbed via a different transporter, GLUT5) can increase total carbohydrate absorption. This is the physiological basis for the growing popularity of “dual/multiple carbohydrate source” sports nutrition products (glucose + fructose combinations)—with a common ratio around 2:1 (glucose:fructose)—which can raise the hourly carbohydrate intake ceiling to roughly 90 g.

Comparison of Gastrointestinal Discomfort Rates Across Fueling Strategies (Illustrative)

Key Research Conclusions and Practical Recommendations

• Pre-race loading timeline: Gradually increase carbohydrate intake ratio (to over 70% of total calories) in the 2–3 days before the race, while substantially reducing training volume, to allow sufficient time for glycogen storage
• Gut training: Gradually increasing in-race carbohydrate intake volume and frequency during long-term training allows the gut to adapt to high carbohydrate absorption loads, significantly reducing the risk of gastrointestinal discomfort during the actual race
• Regularize fueling timing: Rather than waiting until thirsty/hungry to fuel, set fixed time intervals (e.g., every 20–30 minutes) for small, frequent intakes to maintain stable blood glucose
• Personalized testing: Osmolality and taste tolerance vary between individuals; always test fueling strategies 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); it naturally subsides 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 blend products for in-race fueling?

A: Not strictly necessary, but for longer events (e.g., a marathon lasting over 4 hours) requiring higher carbohydrate intake, mixed sources do help raise the absorption ceiling and reduce gastrointestinal burden.

References and Academic Citations

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

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