[Research Review] Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: A Study of Elite Athletes’ Physiological Characteristics (No. 586)

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 grams per hour); exceeding this amount readily causes gastrointestinal discomfort. Research has found that pairing fructose (absorbed via a different transporter, GLUT5) can raise the total amount of carbohydrate absorbed. This is the physiological rationale behind 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 Rates Across Different Fueling Strategies (Illustrative)

Core Research Conclusions and Practical Recommendations

• Pre-race loading timeline: Begin gradually increasing carbohydrate intake ratio (to over 70% of total calories) 2–3 days before the race, while substantially reducing training volume, to allow sufficient time for glycogen storage
• Gut training: Gradually increase the amount and frequency of in-race carbohydrate intake during long-term training so the gut adapts to high-volume carbohydrate absorption loads, which can significantly reduce the risk of gastrointestinal discomfort on race day
• Regularize fueling timing: Rather than waiting until thirsty or hungry, 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 of different fueling products vary by individual; always 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); 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 blended products for in-race fueling?

A: Not necessary, but for longer events (e.g., a marathon lasting 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 direction on absorption rates of multiple-carbohydrate-source fueling.
2. Sports Medicine — Literature on 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 Athletes’ Physiological Characteristics (No. 1042)
• 【Research Review】Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: A Study of Elite Athletes’ Physiological Characteristics (No. 1324)
• 【Research Review】Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: A Study of Elite Athletes’ Physiological Characteristics (No. 904)
• 【Research Review】Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: A Study of Elite Athletes’ Physiological Characteristics (No. 352)