Vitamin C, a.k.a. Ascorbic Acid, is the antiscorbutic vitamin. Although scurvy was first described during the Crusades and commonly plagued early explorers and voyagers, the specific relationship between scurvy, citrus foods, and ascorbic acid was not established until the 20th century. English sailors have been nicknamed "limeys" since the days when ships were required to carry citrus fruits (actually lemons) as scurvy preventative.

The antiscorbutic factor was isolated and named hexuronic acid in 1928 by Szent-Gyorgyi, who found it in adrenal tissue, orange, and cabbage. In 1932 both he and C. Glenn King demonstrated that hexuronic acid was vitamin C.

Ascorbic acid has multiple functions as either a coenzyme or cofactor. Its ability to lose and take on hydrogen gives it an essential role in metabolism. Its role in enhancing absorption of iron is well recognized. In addition, ascorbic acid blocks the degradation of ferritin to hemosiderin, from which iron is poorly mobilized, thus ensuring a more available supply inform of ferritin (Bridges, 1987).

Vitamin C is involved in the hydroxylation of proline to from hydroxyproline in the synthesis of collagen, a protein substance on which the integrity of cellular structure in all fibrous tissues depends. These include connective tissue, cartilage, bone matrix, tooth dentin, skin, and tendon. It is thus involved in healing wounds, Fractures, bruises, pinpoint hemorrhages, and bleeding gums. It also reduces liability to infections.

Vitamin C is essential for the oxidation of phenylalanine and tyrosine, the conversion of folacin to tetrahydrofolic acid, the conversion of tryptophan to 5-hydroxytrytophan and the neurotransmitter serotonin, and the formation of norepinephrine from dopamine. It also reduces ferric to ferrous iron in the intestinal tract to facilitate absorption and is involved in the transfer of iron from plasma transferrin to liver ferritin.

Ascorbic acid participates in hydroxylation of certain steroids synthesized in adrenal tissue. Concentration is decreased under stress when adrenal cortical hormone activity is high. Injection of ACTH causes considerable loss of ascorbic acid from the adrenal cortex. During periods of emotional, psychologic, or physiologic stress, the urinary excretion of ascorbic acid is increased.

Vitamin C promotes resistance to infection through the immunologic activity of leukocytes, the production of interferon, and the process of inflammatory reaction, or the integrity of the mucous membranes. The value of large amounts of ascorbic acid to prevent and cure the common cold has been reported.

The minimal daily intake of vitamin C needed to prevent scurvy is approximately 10 mg; however, this does not provide acceptable reserves of the vitamin. The RDA of 60 mg for adults is based on the amount needed to prevent the onset of scorbutic symptoms for 4 weeks and provide a margin of safety.

Increased intakes of vitamin C are required to maintain normal plasma levels under acute emotional or environmental stress such as trauma, fever, infection, or elevated environmental temperatures. Because of the lower concentrations of ascorbic acid in the serum of cigarette smokers, it is recommended that smokers increase their intake to at lease 100 mg/day (Food and Nutrition Board, 1989).

Ascorbic acid is easily destroyed by oxidation, particularly in the presence of heat and alkalinity, and because it is highly soluble in water, it is often discarded in cooking water. Although the vitamin occurs in small amounts in animal tissues, it is usually destroyed either by exposure to air or by processing before it reaches the table. Therefore the best sources are fruits and vegetables, preferably acidic, fresh, and when necessary, rapidly cooked in very little water and served immediately. Sodium bicarbonate added to preserve and improve the color of cooked vegetables is highly destructive of vitamin C.

Cumulative losses when vegetables are prepared and held for 24 hours in a refrigerator can be as high as 45% for fresh products and 52% for frozen products. As consumers eat out more frequently, and as more foods are supplied to restaurants or institutions partially prepared (e.g. shredded lettuce, peeled and diced vegetable) or served from open salad bars, this loss must be considered when evaluating dietary in take (Carlson and Tabacchi, 1988).

The ascorbic acid content of fruits and vegetables varies with the conditions under which they are grown and the degree of ripeness when harvested.

Ascorbic acid is widely found in citrus fruits, raw leafy vegetables, and tomatoes. Strawberries, cantaloupe, cabbage, and green peppers, are good sources. When properly prepared, potatoes are a good source because of the quantity eaten. It is estimated that 38% of Americans obtain their ascorbic acid from citrus fruits and juices alone. One author supports the use of 2 to 4 fruits and 3 to 5 vegetables each day (Gershoff, 1993), as promoted by the Food Pyramid.

Because vitamin C is the most commonly used supplement in the United States, taken by 8% of young people and 44% of the elderly (Johnson and Luo, 1994), excessive intake may occur. The usual symptom from massive doses of ascorbic acid is diarrhea from the osmotic effect of the unabsorbed vitamin passing through the intestinal tract.

Excess ascorbic acid excreted in the urine gives a false-positive test for sugar. It has been implicated in the formation of urate and oxalate stones, but recent evidence shows that massive vitamin C ingestion (90 g/day) produces only a small increase in urinary oxalate concentration and no change in urate or inorganic phosphate.

A "rebound scurvy" has also been reported in those on massive intakes of vitamin C when the dosage is discontinued. Thus, supplements should be decreased gradually. With regard to excessive use of vitamin C and other antioxidants for prevention of cardiovascular disease and cancer, further studies are needed.

Vitamin C and bone density

Dr. Suzanne Leveille and colleagues at the Epidemiology, Demography, and Biometry Program, National Institute on Aging in Bethesda, Maryland, examined the relationship between dietary vitamin C and hip bone mineral density (BMD) in a group of postmenopausal women in Seattle, Washington. This was a cross-sectional study using retrospective diet and vitamin supplement data.

The average intake of vitamin C through the diet was 110 mg a day, and the average intake of vitamin C supplement in those who take pills was about 300 mg. Longer duration of vitamin C intake was associated in higher bone mineral density in women who had not used estrogen replacement. Benefits were not evident in women who had used estrogen in the past. Frequent intake of foods rich in vitamin C was not associated with bone mineral density.

The researchers conclude, "There was no evidence that vitamin C from the diet was associated with bone mineral density, although long-term use of vitamin C supplements was associated with a higher bone mineral density in the early postmenopausal years and among never-users of estrogen."

Comment: Guess it’s time to add vitamin C supplement to one’s anti-osteoporosis regimen.

Reference:

Leveille SG, LaCroix AZ, Koepsell TD, Beresford SA, Van Belle G, Buchner DM: Dietary vitamin C and bone mineral density in postmenopausal women in Washington State, USA. J Epidemiol Community Health 51:479-485, 1997.

Over a period of up to 12 years, 247 Boston area women, aged 56 to 71 years, took supplemental vitamin C and were observed for cataract development. These women were part of the Nurse’s Health Study. It was determined that vitamin C supplementation for more than 10 years was associated with a 77% lower rate of early cataracts, compared to thoses who did not take supplemental vitamin C.

The researchers state, "These data, together with data from earlier experimental and epidemiologic studies, suggest that long-term consumption of vitamin C supplements may substantially reduce the development of age-related lens opacities."

Reference:

Jacques PF, Taylor A, Hankinson SE, Willett WC, et al; Long-term vitamin C supplement use and prevalence of early age-related lens opacities. The American Journal of Clinical Nutrition 66:911-6, 1997.