Final answer:
Carbohydrate is the preferred fuel for the body at altitude, both at rest and during exercise, due to its role in both anaerobic and aerobic metabolism.
Step-by-step explanation:
At altitude, carbohydrate is the body's preferred fuel both at rest and during exercise. During low-intensity activities, the body primarily uses fatty acids as an energy source due to aerobic metabolism being more efficient, but with higher intensity or when the body's glycogen stores are depleted, glucose becomes the primary source of energy. Glucose is derived from blood glucose or muscle glycogen, and is essential for both anaerobic and aerobic metabolism. While protein is not a major energy source, it may be used in situations of nutritional deficiency or long endurance activities. Overall, carbohydrates serve as a critical fuel source, especially when immediate energy is required or under high-intensity conditions.