Humans, other primates, and guinea pigs are unable to synthesize L-ascorbic acid (vitamin C); therefore, they require it in their diet. [7] The enzyme L-gluconolactone oxidase, which would usually catalyze the conversion of L-gluconogammalactone to L-ascorbic acid, is defective due to a mutation or inborn error in carbohydrate metabolism.

The total body pool of vitamin C is approximately 1500 mg. The absorbed vitamin is found ubiquitously in body tissues, with the highest concentrations in glandular tissue and the lowest concentrations in muscle and stored fat. Ascorbic acid is metabolized in the liver by oxidation and sulfation. The renal threshold for excretion by the kidney in urine is approximately 1.4 mg/100 mL plasma. Excess amounts of ascorbic acid are excreted unchanged or as metabolites. When body tissue or plasma concentrations of vitamin C are low, excretion of the vitamin is decreased. Scurvy occurs after vitamin C has been eliminated from the diet for at least 3 months and when the body pool falls below 350 mg.

One study identified a genetic polymorphism of the human plasma protein haptoglobin, Hp 2, which may be an important non-nutritional modifying factor in the pathogenesis of vitamin C deficiency. The Hp 2-2 polymers are less efficient inhibitors of hemoglobin-driven oxidative stress, leading to ascorbic acid depletion. The Hp 2-2 phenotype is present in 35% of whites and 50% of South Asians and East Asians and may help identify patients who are more prone to develop clinically significant vitamin C deficiency. [1]

Vitamin C functionality

Vitamin C is required as a redox agent, reducing metal ions in many enzymes and removing free radicals. In this capacity, it protects DNA, protein, and vessel walls from damage caused by free radicals.

Vitamin C is functionally most relevant for the triple-helix formation of collagen; a vitamin C deficiency results in impaired collagen synthesis. The typical pathologic manifestations of vitamin C deficiency, including poor wound healing, are noted in collagen-containing tissues and in organs and tissues such as skin, cartilage, dentine, osteoid, and capillary blood vessels. Pathologic changes in affected children and adults are a function of the rate of growth of the affected tissues; hence, the bone changes are often observed only in infants during periods of rapid bone growth. Defective collagen synthesis leads to defective dentine formation, hemorrhaging into the gums, and loss of teeth. Hemorrhaging is a hallmark feature of scurvy and can occur in any organ. Hair follicles are one of the common sites of cutaneous bleeding.

The bony changes occur at the junction between the end of the diaphysis and growth cartilage. Osteoblasts fail to form osteoid (bone matrix), resulting in cessation of endochondral bone formation. Calcification of the growth cartilage at the end of the long bones continues, leading to the thickening of the growth plate. The typical invasion of the growth cartilage by the capillaries does not occur.

Preexisting bone becomes brittle and undergoes resorption at a normal rate, resulting in microscopic fractures of the spicules between the shaft and calcified cartilage. With these fractures, the periosteum becomes loosened, resulting in the classic subperiosteal hemorrhage at the ends of the long bones. Guidelines for the evaluation of fractures in infants and young children have been established. [8] Intra-articular hemorrhage is rare because the periosteal attachment to the growth plate is very firm.

Although the clinical manifestations are unclear, vitamin C is a cofactor in the metabolism of tyrosine and cholesterol and the synthesis of carnitine, neurotransmitters (eg, norepinephrine), peptide hormones, corticosteroids, and aldosterone.

Vitamin C also affects hematopoiesis by enhancing the absorption of iron from the small intestine by reducing dietary iron from the ferric form to the ferrous form. This may contribute to the anemia seen with vitamin C deficiency, in which the availability of intracellular iron is reduced. Vitamin C is also necessary to convert folic acid to its active metabolite, folinic acid.