Disclaimer: The products and knowledge discovered on this site aren't supposed to substitute skilled medical recommendation or treatment. They will supply advice and teach you methods to count carbohydrates and how to adjust your insulin. The first purpose that marathoners carbo-load (or eat a number of carbohydrates) before the race is to store up glycogen. It boils all the way down to stored power: glycogen and fatty acids. For those runners you see limping throughout the end line, you can assume their bodies have tapped into fatty acid reserves