Vitamin C (Ascorbic Acid)

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Chapter 47 Vitamin C (Ascorbic Acid)

Dheeraj Shah, H.P.S. Sachdev

Vitamin C is important for synthesis of collagen at the level of hydroxylation of lysine and proline in precollagen. It is also involved in neurotransmitter metabolism (conversion of dopamine to norepinephrine and tryptophan to serotonin), cholesterol metabolism (conversion of cholesterol to steroid hormones and bile acids), and the biosynthesis of carnitine. In these reactions, vitamin C functions to maintain the iron and copper atoms, cofactors of the metalloenzymes, in a reduced (active) state. Vitamin C is an important antioxidant (electron donor) in the aqueous milieu of the body. This function of ascorbic acid may be important in preventing degenerative diseases, cardiovascular diseases, and some cancers. Vitamin C enhances nonheme iron absorption, the transfer of iron from transferrin to ferritin, and the formation of tetrahydrofolic acid and thus can affect the cellular and immunologic functions of the hematopoietic system.

Dietary Needs and Sources

Humans depend on dietary sources for vitamin C. An adequate intake is 40 mg for age 0-6 mo and 50 mg for age 6-12 mo. For older children, the RDA is 15 mg for age 1-3 yr, 25 mg for age 4-8 yr, 45 mg for age 9-13 yr, and 65-75 mg for age 14-18 yr. The RDAs during pregnancy and lactation are 85 mg/day and 120 mg/day, respectively. The requirement for vitamin C is increased during infectious and diarrheal diseases. Children exposed to smoking or environmental tobacco smoke also require increased amounts of foods rich in vitamin C. The best food sources of vitamin C are citrus fruits and fruit juices, peppers, berries, melons, tomatoes, cauliflower, and green leafy vegetables. Vitamin C is easily destroyed by prolonged storage, overcooking, and processing of foods.

Absorption of vitamin C occurs in the upper small intestine by an active process or by simple diffusion when large amounts are ingested. Vitamin C is not stored in the body but is taken up by all tissues; the highest levels are found in the pituitary and adrenal glands. The brain ascorbate content in the fetus and neonate is many-fold higher than the content in the adult brain, a finding probably related to its function in neurotransmitter synthesis.

When a mother’s intake of vitamin C during pregnancy and lactation is adequate, the newborn will have adequate tissue levels of vitamin C related to active placental transfer, subsequently maintained by the vitamin C in breast milk or commercial infant formulas. Breast milk contains sufficient vitamin C to prevent deficiency throughout infancy. Infants consuming pasteurized or boiled animal milk are at significant risk of developing deficiency if the other sources of vitamin C are also lacking in the diet. Neonates whose feeding has been delayed because of clinical condition can also suffer from ascorbic acid deficiency. For patients on total parenteral nutrition (TPN), a parenteral dose of 80 mg/day is recommended for full-term infants and a parenteral dose of 25 mg/kg/day is recommended for preterm infants.

Deficiency

A deficiency of vitamin C results in the clinical presentation of scurvy, the oldest nutritional deficiency disease to be recognized. Children fed predominantly heat-treated (ultra-high-temperature or pasteurized) milk or unfortified formulas and not receiving fruits and fruit juices are at significant risk for symptomatic disease. In scurvy, there is defective formation of connective tissues and collagen in skin, cartilage, dentine, bone, and blood vessels, leading to their fragility. In the long bones, osteoid is not deposited by osteoblasts, cortex is thin, and the trabeculae become brittle and fracture easily.

Clinical Features

The early manifestations are irritability, loss of appetite, low-grade fever, and tenderness in the legs. These signs and symptoms are followed by leg swelling—most marked at the knees and the ankles—and pseudoparalysis. The infant might lie in the “pithed frog” position, with the hips and knees semiflexed and the feet rotated outward. Subperiosteal hemorrhages in the lower limb bones sometimes acutely increase the swelling and pain, and the condition might mimic acute osteomyelitis or arthritis. A “rosary” at the costochondral junctions and depression of the sternum are other typical features (Fig. 47-1

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