Vitamin C is a generic descriptor for all compounds exhibiting the biological activity of ascorbic acid, and the vitamin is often called ascorbic acid. Vitamin C has three main functions: to provide reducing equivalents for biochemical reactions, to serve as a cofactor for reactions requiring reduced metal ions, and to serve as a protective antioxidant. Vitamin C is a reductive cofactor in the hydroxylation of the amino acids proline and lysine during the formation of collagen. It also has an influence on other connective tissue components, elastin, fibronectin, and bone matrix. As an antioxidant, vitamin C can donate electrons to decrease free radicals, and can easily return to its reduced state. Vitamin C protects against the peroxidation of plasma lipid and low-density lipid protein (LDL), provides antioxidant protection in the eye, and protects DNA from oxidative damage.
Vitamin C has many other functions in the body. It is involved in the neurotransmitter synthesis. Vitamin C is involved in the regulation of iron metabolism. Dietary vitamin C enhances the absorption of nonheme iron, but can also interact with iron to promote oxidative damage. Vitamin C and iron play a role in the synthesis of carnitine. Vitamin C enhances vasodilatory and anticlotting effects.
The absorption of vitamin C into the intestinal tract is an active process requiring energy. Absorption is dose dependent, being greater with low dietary intake of vitamin C. Only 50% of vitamin C is absorbed when intakes are as high as 1 to 1.5 grams. Vitamin C is found in the body in the pituitary and adrenal glands, leukocytes, eye lenses, and the brain. As plasma ascorbic acid increases, the ability of the renal tubules to absorb it maximizes (called renal threshold). Unabsorbed excess is excreted in the urine.
Vitamin C deficiency results in a disease called scurvy. Many symptoms of the disease are due to defects in connective tissue formation. Symptoms include inflammed and bleeding gums, bleeding into joints and peritoneal cavity, arthralgia, impaired wound healing, weakness, fatigue, depression, and vasomotor instability. Scurvy is rare in developed countries but it is seen in cases of alcohol and drug abuse, which usually coincide with a poor diet.
Approximately 5 to 10 mg of vitamin C is required daily to prevent scurvy. The RDA is set higher to ensure an adequate body pool to prevent scurvy after four weeks of low vitamin C intake. The 2000 recommendations are 15 to 45 mg for children, 65 to 90 mg for adolescents, 75 mg for adult females, 90 mg for adult males, 80-85 mg for pregnant women and 115-120 mg for lactating women.
Vitamin C is found most abundantly in citrus fruits and juices, green vegetables, tomatoes, tomato juice, and potatoes.
The effects of vitamin C supplementation on disease states has been studied extensively but still remain controversial. Vitamin C alters the immune system, specifically lymphocyte proliferation and natural killer cell activities. The vitamin inhibits and inactivates viruses, but no clinical efficacy has been proven. Vitamin C supplementation trials have not shown that it reduces the incidence of colds, but some studies have seen a decrease in the duration and severity of colds with supplementation.
Vitamin C blocks carcinogenic processes through antioxidant activity. Epidemiological studies have shown a strong association of vitamin C supplementation with decreased risk of cancer of the oral cavity, esophagus, stomach, and pancreas. Less strong associations were found with lung, cervix, rectum, and breast cancer.
Epidemiological studies have shown mixed results of the efficacy of vitamin C for heart disease. It has been suggested that vitamin C may decrease risk of heart disease due to the inhibition of plasma LDL oxidation and vasodilatory and anticlotting activity.