ascorbic acid

Ascorbic acid is a water-soluble vitamin obtained from dietary sources such as citrus fruits and vegetables, as the human body cannot synthesize it (unlike most other mammals, due to lack of the enzyme L-gulonolactone oxidase). Its major biochemical role is as a cofactor for prolyl hydroxylase and lysyl hydroxylase, enzymes that hydroxylate proline and lysine residues in procollagen, which is necessary for collagen cross-linking and stability. This makes vitamin C critical for maintaining connective tissue, blood vessel integrity, and wound healing. Ascorbic acid also functions as a powerful antioxidant by donating electrons to neutralize reactive oxygen species, and it helps regenerate other antioxidants such as vitamin E. Additionally, it facilitates dietary iron absorption in the gut by reducing ferric iron (Fe3+) to the more absorbable ferrous form (Fe2+), and it plays a role in the synthesis of catecholamines (dopamine beta-hydroxylase) and carnitine. Deficiency results in scurvy, classically presenting with poor wound healing, gingival bleeding and swelling, perifollicular hemorrhages, corkscrew hairs, easy bruising (due to weakened vessel walls), and in children, bone pain and subperiosteal hemorrhages. Populations at risk include alcoholics, smokers, and those with poor dietary intake (e.g., elderly, low fruit/vegetable consumption). Excess vitamin C intake can cause GI upset and increase risk of calcium oxalate kidney stones due to oxalate metabolism. On USMLE exams, ascorbic acid is high-yield for its role in collagen synthesis (commonly tested with Ehlers-Danlos syndrome comparisons), scurvy presentation, and its use in enhancing iron absorption in iron-deficiency anemia treatment.