Final answer:
Carbohydrates contribute up to 15% of the body's energy needs during endurance exercise such as marathon training. Carbohydrate loading is a strategy used by athletes to maximize energy storage in the muscles. Athletes should consume enough carbohydrates to meet their increased energy needs during training or competitions.
Step-by-step explanation:
The energy-yielding nutrient that contributes up to 15% of the body's energy needs during extended periods of endurance exercise, such as training for a marathon, is carbohydrates.
Carbohydrate loading is a strategy used by endurance athletes to maximize the storage of energy in the form of glycogen in the muscles. Glycogen serves as an energy reserve that can be quickly mobilized to provide glucose, which is then converted into ATP through cellular respiration.
It is important for athletes to consume enough carbohydrates from their diet to meet their increased energy needs during training or competitions. Carbohydrate needs can vary depending on the intensity of exercise, but generally, athletes should aim to consume 3-10 grams of carbohydrates per kilogram of body weight per day.