It all comes down to glycogen depletion.
The age-old question for long-distance runners: When is the right time to eat and what should you eat?
The National Post recently ran an in-depth article on their Web site that tries to answer this very question. First: The basics on carbohydrates. Jennifer Sygo, a Canadian dietician, writes that “stored carbohydrate, known as glycogen, is one of the limiting factors for any endurance sport.” The average person can store roughly a pound of glycogen in their muscle tissue.
This amount is enough to sustain about two hours of running.
Sygo notes that since most half marathoners take about two hours to complete the race, running out of glycogen is a concern. For those runners who consume a lot of carbs, this usually isn’t’ a factor. However, with more and more people switching to “low-carb” diets, glycogen depletion can be a major concern.Simply put, glycogen depletion is the proverbial “wall” that some runners encounter.
“Wall-hitting types can push it back by adding more carbohydrates to their usual diet, which helps to top up glycogen stores for both training and racing,” writes Sygo.
For More: National Post