Vitamin C (ascorbic acid) is a water-soluble antioxidant involved in immune function, collagen synthesis, and protection against oxidative damage. Levels largely reflect dietary intake from fruits and vegetables. In an optimization context, vitamin C can serve as a marker of nutritional quality and antioxidant capacity. Low levels may suggest inadequate intake or increased oxidative demand, while very high levels are typically from supplementation and offer limited additional benefit beyond sufficiency.