What is the primary limitation of normal glycogen stores for marathon running?

The primary limitation of normal glycogen stores for marathon running is that they are inadequate for fueling a marathon. Glycogen is the body's key source of energy during prolonged exercise, particularly in endurance events like marathons. However, the amount of glycogen that can be stored in the muscles and liver is limited, typically enough to sustain around 90 to 120 minutes of moderate to high-intensity exercise.

A marathon, which is 26.2 miles long, generally takes much longer to complete for most runners, often exceeding two hours for recreational athletes. After depleting glycogen stores, the body must rely on fat as a source of energy, which is less efficient and leads to fatigue. This is why marathon runners focus on strategies such as carbo-loading before the race to maximize their glycogen reserves and may also consume carbohydrates during the run to maintain their energy levels.

Other choices highlight aspects of glycogen utilization, but they do not directly address the fundamental issue of insufficiency for the extended duration of a marathon. Hence, the inadequacy of normal glycogen stores for the demands of a marathon is a critical consideration for endurance athletes.