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The nutrient primarily involved in energy metabolism is iron. Iron plays a vital role in the production of hemoglobin, which carries oxygen in the blood to cells where it is needed for aerobic metabolism. This process is crucial for converting food into energy, particularly during physical activity, as muscles require a continuous supply of oxygen to effectively oxidize nutrients and produce ATP (adenosine triphosphate), the energy currency of the cell.

In the context of energy metabolism, iron is particularly important for athletes and active individuals since a deficiency can lead to decreased oxygen transport and endurance, significantly impacting performance. The other options, while important for various physiological functions, do not directly play this central role in energy metabolism. Calcium is primarily involved in bone health and muscle contraction, selenium has antioxidant properties but is not directly related to energy production, and vitamin C is important for collagen synthesis and immune function, but it does not participate in energy metabolism in the same fundamental way that iron does.