What is the effect of prolonged exercise on protein usage?

The correct answer highlights the role of proteins as an energy source during prolonged exercise, particularly once carbohydrate reserves become limited. Initially, the body relies heavily on carbohydrates, which are stored as glycogen in muscles and the liver, to meet energy demands. As exercise continues and glycogen stores are depleted, the body starts to utilize protein through a process called gluconeogenesis, where amino acids from proteins are converted into glucose for energy.

This transition reflects the body's adaptation to extended periods of physical exertion. While proteins are primarily known for their role in building and repairing tissues, they can serve as an alternative energy source when carbohydrate availability decreases significantly, thus contributing to energy production during prolonged exercise.

Other options misrepresent the relationship between exercise duration and protein usage. The idea that protein becomes the primary energy source immediately disregards the body's preference for carbohydrates in the early stages of exercise. Arguing that protein has no effect on energy production overlooks its role when carbohydrates are insufficient. Lastly, stating that it is only used for muscle repair ignores the metabolic flexibility of the body, which can adapt to use protein for energy under certain conditions.