Vitamin C
Vitamin C, or L-ascorbic acid, is a water-soluble essential vitamin. It is a very popular dietary supplement due to its antioxidant properties, safety, and low price.
Vitamin C is often supplemented to reduce the symptoms of the common cold.
However, vitamin C is unable to reduce the frequency of colds in a healthy population. An athlete supplementing vitamin C, on the other hand, can expect to cut the risk of getting a cold in half. Supplemental vitamin C is able to reduce the duration of a cold by 8-14% in any population, when it is taken as a daily preventative measure, or at the beginning of a cold. Though superloading vitamin C (5-10g daily) is said to be more effective, further research is needed to determine the accuracy of this claim.
Vitamin C is capable of being both an antioxidant and pro-oxidant, depending on what the body needs. This mechanism allows it to serve a variety of functions in the body.
Vitamin C sequesters free radicals in the body. It is replenished by antioxidant enzymes, and is often used as a reference drug in antioxidant research. Vitamin C’s structure allows it to act on neurology and depression, as well as interact with the pancreas and modulate cortisol. Its antioxidant properties mean vitamin C provides neuroprotective effects and benefits for blood flow. By protecting the testes from oxidative stress, vitamin C can also preserve testosterone levels.
The role of vitamin C in cancer prevention is unclear. Studies indicate that it reduces oxidative stress. High plasma levels of vitamin C are associated with lower gastrointestinal cancer incidence probably due to its protective effects against dietary carcinogenic nitroso compounds. Intake of vitamins A, C, or E reduced risk of cervical cancer, but they do not appear to prevent gastrointestinal, prostate, lung cancers, or other cancer incidences, or affect cancer mortality.
High-dose vitamin C has been long been used as an alternative cancer treatment based on the hypothesis that the formation of new collagen resists malignant infiltration. Early reports using parenteral plus oral ascorbic acid showed promising results. But subsequent randomized, placebo-controlled trials in advanced cancer patients using 10 grams per day of oral vitamin C did not demonstrate significant benefits. This lack of effect is explained by the finding of a pharmacokinetic study that ascorbic acid can only reach a limited plasma concentration through oral administration. A higher pharmacologic concentration, achieved only via intravenous injection, had selective cytotoxic effects on cancer cell lines. Ascorbic acid is postulated to act as a carrier of hydrogen peroxide to the extracellular fluid where it generates free radicals against tumor cells (pro-oxidant). High-dose IV ascorbic acid up to 1.5 gm/kg/day, or used with chemotherapy appears to be well-tolerated. It may improve the quality of life of terminal cancer patients, and reduce chemotherapy-associated toxicity. This has led to a renewed interest in studying high-dose IV vitamin C as an anticancer treatment. However, it should be regarded as an investigational drug and used only in a clinical trial setting.
Dosage
The Recommended Daily Intake (RDI) of oral vitamin C is 100-200mg. This is easily attained through the diet, so supplementation of such low doses is usually unnecessary. Higher doses of vitamin C, up to 2,000mg, are used to support the immune system (for athletes) or reduce the duration of the common cold.
Most studies on vitamin C prescribe one dose per day. The claim that taking 2,000mg up to five times a day to optimally reduce cold symptoms has not yet been sufficiently tested and requires more evidence.
Safety
Adverse effects from vitamin C are rare but there are reports of gastrointestinal upset, kidney stone formation, and hemolytic anemia in patients with G6PDH deficiency. Patients with a history of oxalate kidney stones, renal insufficiency, hematochromatosis, or those undergoing chemotherapy should consult their physicians before taking vitamin C supplements.