CSSD Practice Exam 2026 – Complete Study Resource

As exercise intensity increases beyond a low level, carbohydrates become the primary fuel source for the body. At lower exercise intensities, fats are typically used for energy because they provide a more abundant source of fuel and are more efficient for lower-energy activities. However, as the demand for energy increases, such as during high-intensity activities, the body requires a faster-acting source of energy, which is found in carbohydrates.

Carbohydrates can be broken down quickly through glycolysis to provide ATP (adenosine triphosphate), the energy currency of the cell, allowing for sustained high-intensity performance. This shift to carbohydrates occurs because they can be metabolized more quickly than fats, which is crucial when the energy demands of the muscles rise sharply during intense exercise. While proteins can also serve as an energy source, they primarily contribute to energy production under conditions of prolonged endurance or starvation, not during the initial transition to higher intensity exercise.

Vitamins do not serve as direct fuel sources; instead, they play supportive roles in metabolic processes but do not contribute energy in the same way carbohydrates, fats, and proteins do. Therefore, as exercise intensity increases, carbohydrates are the dominant fuel source that supports the body’s energy needs.