[Research Review] Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Physiological Characteristics of Elite Athletes (Article 247)

Reference Journal Source: International Journal of Sport Nutrition and Exercise Metabolism • International Scientific 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 intestinal glucose transporter (SGLT1) has a physiological absorption ceiling (approximately 60 grams per hour); exceeding this amount readily causes 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 approximately 2:1 (glucose:fructose), which can raise the hourly carbohydrate intake ceiling to around 90 grams.

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

Key Scientific Conclusions and Practical Recommendations

• Pre-race loading timeline: Gradually increase carbohydrate intake (to over 70% of total calories) in the 2–3 days before competition, while substantially reducing training volume, to allow sufficient time for glycogen storage
• Gut training: Gradually increasing in-race carbohydrate intake and frequency during long-term training allows the gut to adapt to high carbohydrate absorption loads, significantly reducing the risk of gastrointestinal discomfort during actual competition
• Regularize fueling timing: Rather than waiting for thirst or hunger, 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 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 me gain fat or weight?

A: The short-term weight gain from loading is primarily water (glycogen storage simultaneously retains water), which 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 blended products for in-race fueling?

A: Not strictly necessary, but for longer events (e.g., marathons over 4 hours) requiring higher carbohydrate intake, blended 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 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: Physiological Characteristics of Elite Athletes (Article 1135)
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