Unlike most other mammals, humans and other primates don't synthesize vitamin C because we lack the enzyme glunolactone oxidase (1). The enzyme was lost long ago without affecting our survival due to frequent intake of high-vitamin C fruits and vegetables.
Thus, we must continue to get vitamin C from our diet by the same manner (fruits or veggies) or otherwise, lest we succumb to scurvy as British sailors did in the early 1800s before they adopted rationing limes on naval vessels (1).
Vitamin C deficiency leading to scurvy is now rare (1) and in the developed world, but studies on North American and European populations have found that many people who do not eat enough fruits and high-vitamin C vegetables continue to have inadequate levels of vitamin C (2;3).
Supplementation or dietary change would serve the majority of these patients because—when taken along with other vitamins—vitamin C may lead to reduced risk of chronic diseases such as cancer, cardiovascular disease and cataracts (1). The antioxidant vitamin is thought to possibly have a role in counteracting and detoxifying carcinogens, preventing myocardial lipid peroxidation and LDL oxidation, and prevent oxidative damage to lens in the eye (1).
Note that the vitamin has not been found to have any effect on reducing risk of colds (1). This is in contrast to what's marketed on many dietary supplements.
Large intakes of vitamin C (above 2g daily) can cause diarrhea (1). The vitamin C competes with uric acid inhibiting renal absorption of uric acid that can lead to increases of uric acid excretion, urine acidification and precipitation of uric acid crystals (1). This may increase risk of urate kidney stones if high doses are taken chronically (1). Chronic high doses may not be appropriate also for those with iron metabolism disorders since vitamin C increases iron absorption (1).
Recommended dietary intake levels are 75mg for women and 90 mg for men (1). If pregnant, elderly, smoking or afflicted with chronic disease, a little more may be needed (4-6). Supplementation with a multivitamin may be sufficient for oxidative stress protection, however, findings on reducing risk of chronic disease are mostly related to antioxidant vitamin intake from fruit and vegetables (6-10).
Based on the above rationale, I would recommend patients first attempt to increase fruit and vegetable consumption to meet desired plasma levels and with optimal synergistic effects of other vitamins. If there’s any doubt of the patient’s ability or motivation to eat fruits and vegetables, then vitamin C in form of a multivitamin would be the next step.
1. Gropper SS, Smith JL, Groff JL. Advanced Nutrition and Human Metabolism. Belmont, CA: Thomson Wadsworth, 2009.
2. Hampl JS, Taylor CA, Johnston CS. Vitamin C deficiency and depletion in the United States: the Third National Health and Nutrition Examination Survey, 1988 to 1994. Am J Public Health 2004;94:870-5.
3. Taylor CA, Hampl JS, Johnston CS. Low intakes of vegetables and fruits, especially citrus fruits, lead to inadequate vitamin C intakes among adults. Eur J Clin Nutr 2000;54:573-8.
4. Valdes F. [Vitamin C]. Actas Dermosifiliogr 2006;97:557-68.
5. Brubacher D, Moser U, Jordan P. Vitamin C concentrations in plasma as a function of intake: a meta-analysis. Int J Vitam Nutr Res 2000;70:226-37.
6. Goodwin JS, Brodwick M. Diet, aging, and cancer. Clin Geriatr Med 1995;11:577-89.
7. Genkinger JM, Platz EA, Hoffman SC, Comstock GW, Helzlsouer KJ. Fruit, vegetable, and antioxidant intake and all-cause, cancer, and cardiovascular disease mortality in a community-dwelling population in Washington County, Maryland. Am J Epidemiol 2004;160:1223-33.
8. Nagyova A, Krajcovicova-Kudlackova M, Horska A et al. Lipid peroxidation in men after dietary supplementation with a mixture of antioxidant nutrients. Bratisl Lek Listy 2004;105:277-80.
9. Broekmans WM, Klopping-Ketelaars IA, Schuurman CR et al. Fruits and vegetables increase plasma carotenoids and vitamins and decrease homocysteine in humans. J Nutr 2000;130:1578-83.
10. Zino S, Skeaff M, Williams S, Mann J. Randomised controlled trial of effect of fruit and vegetable consumption on plasma concentrations of lipids and antioxidants. BMJ 1997;314:1787-91.