Definition:

Vitamin C, also known as L-ascorbic acid, is a water-soluble vitamin. Unlike most mammals and other animals, humans cannot synthesize vitamin C and must obtain it from their diet.

• Vitamin C is an essential cofactor in numerous enzymatic reactions, e.g., in the biosynthesis of collagen, carnitine, and neuropeptides, and the regulation of gene expression. It is also a potent antioxidant.
• Prospective cohort studies indicate that higher vitamin C status, assessed by measuring circulating vitamin C, is associated with lower risks of hypertension, coronary heart disease, and stroke.
• There is some evidence to suggest that vitamin C may be a useful adjunct to conventional medical practice to reduce myocardial injury and arrhythmia following a cardiac procedure or surgery in patients with cardiovascular disease
• There is insufficient data to suggest a link between vitamin C status and the risk of developing a given type of cancer. Current evidence of the efficacy of intravenous vitamin C in cancer patients is limited to observational studies, uncontrolled interventions, and case reports. There is a need for large, longer-duration phase II clinical trials that test the efficacy of intravenous vitamin C in cancer progression and overall survival.
• Overall, regular use of vitamin C supplements shortens the duration of the common cold but does not reduce the risk of becoming ill. Taking supplements once cold symptoms have already begun has no proven benefits.
• Some, but not all, studies have found that vitamin C supplementation increases urinary excretion of uric acid and lowers serum uric acid levels.
• Because vitamin C increases the absorption of nonheme iron, vitamin C supplementation could worsen iron overload in patients with increased body iron stores. Vitamin C may also increase iron-induced oxidative damage in patients with iron overload.
• Vitamin C and some pharmaceutical drug interactions have been reported.

Dosage and administration

• For adults, the most frequently used dosages of vitamin C are 100–3,000 mg/day.
• Vitamin C supplements are available in many forms, but there is little scientific evidence that any one form is better absorbed or more effective than another.
• Vitamin C is better tolerated when taken with food than when taken on an empty stomach. Taking vitamin C in 2 or 3 divided doses per day, as opposed to once a day, may increase the total amount absorbed and minimize the decline in serum vitamin C levels between doses. When administering large amounts of vitamin C, splitting it into multiple doses throughout the day may improve bowel tolerance and allow for a higher total daily dosage.
• There is no scientific evidence that large amounts of vitamin C (up to 10 grams [g]/day in adults) exert any adverse or toxic effects. An upper intake level of 2 g/day is recommended to prevent some adults from experiencing diarrhea and gastrointestinal disturbances.
• Supplemental vitamin C increases urinary oxalate concentrations, but whether an increase in urinary oxalate elevates the risk for kidney stones is not yet known. Those predisposed to kidney stone formation may consider avoiding high-dose (≥1 g/day) vitamin C supplementation.