[Research Review] Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Advances in Exercise Physiology Research (No. 685)

Reference Journal Source: International Journal of Sport Nutrition and Exercise Metabolism • International Scientific Research Findings Review Series

This article explores the balance 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 maximize muscle and liver glycogen stores by increasing carbohydrate intake ratios (while maintaining or reducing training volume) in the days before competition, thereby delaying the onset of hitting the wall.

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 g per hour), and 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 also the physiological basis for the growing popularity of “dual/multiple sugar source” sports nutrition products (glucose + fructose combinations)—with common ratios falling around 2:1 (glucose:fructose), which can raise the hourly carbohydrate intake ceiling to approximately 90 g.

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

Key Scientific Conclusions and Practical Recommendations

• Pre-race loading timeline: It is recommended to gradually increase carbohydrate intake ratios (above 70% of total calories) 2-3 days before the race 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 allows the gut to adapt to high-volume carbohydrate absorption loads, which can significantly reduce the risk of gastrointestinal discomfort during the actual race
• Regularize fueling timing: Rather than waiting until thirsty/hungry to fuel, it is recommended to set fixed time intervals (e.g., every 20-30 minutes) for small, frequent feedings to maintain stable blood glucose levels
• Personalized testing: Osmolality and taste tolerance of different fueling products vary from person to person; before major races, be sure to actually test fueling strategies during training or simulation races, 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), which 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 blended products for in-race fueling?

A: Not necessary, but for longer events (e.g., marathons over 4 hours) requiring higher carbohydrate intake, blended 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 on absorption rates of multiple-sugar-source carbohydrate fueling.
2. Sports Medicine — Literature on gut training and gastrointestinal tolerance in endurance sports.

Related Topic Reading

• Research Review: Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Advances in Exercise Physiology Research (No. 730)
• Research Review: Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Advances in Exercise Physiology Research (No. 883)
• Research Review: Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Advances in Exercise Physiology Research (No. 121)
• Research Review: Latest Report on Carbohydrate Loading Ratios and Gastrointestinal Tolerance in Long-Distance Endurance Running: Advances in Exercise Physiology Research (No. 115)