Why Marathon Fueling Defines Your Finish Line
The human body stores roughly 400g to 500g of glycogen across the skeletal muscles and liver—representing approximately 1,600 to 2,000 calories of readily accessible carbohydrate fuel. In a standard marathon or ultra-endurance event, a runner burns between 2,500 and 3,800+ calories depending on body mass and tempo. Without systematic exogenous carbohydrate intake (gels, chews, or drink mix), glycogen stores deplete around the 30-kilometer (18-mile) mark, causing catastrophic fatigue commonly known as the wall.
Carbohydrate Absorption Limits
Standard glucose-only formulations saturate intestinal SGLT1 transport proteins around 60g/hour. By combining glucose and fructose in a 2:1 or 1:0.8 ratio, fructose utilizes GLUT5 transporters, enabling absorption rates up to 90g to 100g per hour without GI discomfort.
Sodium and Plasma Osmolality
Sweating expels not just water, but 500mg to 1,200mg of sodium per liter. Drinking pure water without electrolytes dilutes blood sodium, potentially causing dangerous exercise-associated hyponatremia. Matching sodium intake preserves blood volume and reduces cardiac drift.