Which nutrient increases in use in the body as activity intensity increases?

Glucose is the primary source of energy for the body, especially during high-intensity activities. As the intensity of physical activity increases, the demand for immediate energy sources also rises. In such situations, the body primarily relies on glucose for quick energy through anaerobic and aerobic pathways.

During moderate to high-intensity exercise, the body preferentially uses carbohydrates, stored as glycogen in the muscles and liver, which can be rapidly mobilized to provide energy. This is in contrast to fats, which become more prominent during lower intensity, prolonged activities where oxygen supply is sufficient for fat oxidation. While proteins play a role in energy metabolism, particularly during prolonged exercise or starvation, they are not a primary fuel source for most activities. Vitamins are essential for various metabolic processes but do not serve as direct energy sources.

Thus, as activity intensity escalates, the reliance on glucose for energy becomes significantly greater, making it the correct answer.