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Carbohydrates are primarily important for providing energy during extended physical activity due to their efficient conversion into glucose, which is the body's preferred energy source. During exercise, especially in endurance activities like running or cycling, the body relies heavily on stored glycogen (the stored form of carbohydrates) as the initial source of energy.

When carbohydrates are consumed, they are broken down into glucose and can quickly enter the bloodstream, making them readily available for energy production. In longer-duration activities, as glycogen stores become depleted, the body also taps into fats for energy. However, carbohydrates are crucial for sustaining performance, particularly in high-intensity activities, as they can be metabolized more quickly than fats.

Proteins serve primarily as building blocks for muscle repair and growth rather than a primary energy source, and vitamins are essential for various metabolic processes but do not provide energy directly. Therefore, carbohydrates are the key nutrient for maintaining energy levels during extended physical activity.