Which energy source predominantly fuels cardio respiratory endurance activities?

The primary energy source that fuels cardiorespiratory endurance activities comes from a mix of fats and carbohydrates, with the body typically relying more heavily on fats as the duration of exercise increases. In endurance activities, such as running, cycling, or swimming, the body is engaged in prolonged activities that require consistent energy supply.

Carbohydrates serve as a quick-acting energy source; however, as endurance activities continue over longer periods, the body's reliance shifts toward fat, which provides a larger reserve of energy. The combination of approximately 60% fat and 40% carbohydrates reflects the body’s metabolic adjustments during prolonged exercise. This breakdown allows for sustained energy output while preserving glycogen stores for more intense bursts of activity or movement.

While carbohydrates are crucial for immediate energy, and protein does play a role in repairing and building muscle, protein is not a primary fuel source for endurance activities. Therefore, as duration and intensity of the exercise peak, the mixed utilization of fat and carbohydrates becomes critical, validating the answer as the most representative of the metabolic processes at play during cardiorespiratory endurance activities.