Vitamin C
Also Known As: Vitamin C, Ascorbic acid
Vitamin C or L-ascorbic acid or L-ascorbate is an essential nutrient for humans and certain other animal species. In living organisms ascorbate acts as an antioxidant by protecting the body against oxidative stress. It is also a cofactor in at least eight enzymatic reactions including several collagen synthesis reactions that, when dysfunctional, cause the most severe symptoms of scurvy. In animals these reactions are especially important in wound-healing and in preventing bleeding from capillaries.
Ascorbate (an ion of ascorbic acid) is required for a range of essential metabolic reactions in all animals and plants. It is made internally by almost all organisms although notable mammalian group exceptions are most or all of the order chiroptera (bats), guinea pigs, capybaras, and one of the two major primate suborders, the Anthropoidea (Haplorrhini) (tarsiers, monkeys and apes, including human beings). Ascorbic acid is also not synthesized by some species of birds and fish. All species that do not synthesize ascorbate require it in the diet. Deficiency in this vitamin causes the disease scurvy in humans. It is also widely used as a food additive.
The full extent of the effects and recommended daily intake of vitamin C are matters of ongoing debate, with RDI ranging from 45 mg to 400 mg or more per day.
Pro-oxidant
Ascorbic acid behaves not only as an antioxidant but also as a pro-oxidant.[63] Ascorbic acid has been shown to reduce transition metals, such as cupric ions (Cu2+), to cuprous (Cu1+), and ferric ions (Fe3+) to ferrous (Fe2+) during conversion from ascorbate to dehydroascorbate in vitro.[75] This reaction can generate superoxide and other ROS. However, in the body, free transition elements are unlikely to be present while iron and copper are bound to diverse proteins[63] and the intravenous use of vitamin C does not appear to increase pro-oxidant activity.[76] Thus, ascorbate as a pro-oxidant is unlikely to convert metals to create ROS in vivo. However, vitamin C supplementation has been associated with increased DNA damage in the lymphocytes of healthy volunteers.[77]
Immune system
Vitamin C is found in high concentrations in immune cells, and is consumed quickly during infections. It is not certain how vitamin C interacts with the immune system; it has been hypothesized to modulate the activities of phagocytes, the production of cytokines and lymphocytes, and the number of cell adhesion molecules in monocytes.[78]
Antihistamine
Vitamin C is a natural antihistamine. It both prevents histamine release and increases the detoxification of histamine. A 1992 study found that taking 2 grams vitamin C daily lowered blood histamine levels 38 percent in healthy adults in just one week.[79] It has also been noted that low concentrations of serum vitamin C has been correlated with increased serum histamine levels.[80][81]
Physiologic function in plants
Ascorbic acid is associated with chloroplasts and apparently plays a role in ameliorating the oxidative stress of photosynthesis. In addition, it has a number of other roles in cell division and protein modification. Plants appear to be able to make ascorbate by at least one other biochemical route that is different from the major route in animals, although precise details remain unknown.[82]
Daily requirements
The North American Dietary Reference Intake recommends 90 milligrams per day and no more than 2 grams (2,000 milligrams) per day.[83] Other related species sharing the same inability to produce vitamin C and requiring exogenous vitamin C consume 20 to 80 times this reference intake.[84] There is continuing debate within the scientific community over the best dose schedule (the amount and frequency of intake) of vitamin C for maintaining optimal health in humans.[85] A balanced diet without supplementation usually contains enough vitamin C to prevent scurvy in an average healthy adult, while those who are pregnant, smoke tobacco, or are under stress require slightly more.[83] However, the amount of vitamin C necessary to prevent scurvy is less than the amount required for optimal health, as there are a number of other chronic diseases whose risk are increased by a low vitamin C intake, including cancer, heart disease, and cataracts. A 1999 review suggested a dose of 90–100 mg Vitamin C daily is required to optimally protect against these diseases, in contrast to the lower 45 mg daily required to prevent scurvy.[86]
High doses (thousands of milligrams) may result in diarrhea in healthy adults, as a result of the osmotic water-retaining effect of the unabsorbed portion in the gastrointestinal tract (similar to cathartic osmotic laxatives). Proponents of orthomolecular medicine[87] claim the onset of diarrhea to be an indication of where the body's true vitamin C requirement lies, though this has not been clinically verified.
| United States vitamin C recommendations[83] | |
|---|---|
| Recommended Dietary Allowance (adult male) | 90 mg per day |
| Recommended Dietary Allowance (adult female) | 75 mg per day |
| Tolerable Upper Intake Level (adult male) | 2,000 mg per day |
| Tolerable Upper Intake Level (adult female) | 2,000 mg per day |
Government recommended intakes
Recommendations for vitamin C intake have been set by various national agencies:
- 40 milligrams per day: the United Kingdom's Food Standards Agency[2]
- 45 milligrams per day: the World Health Organization[88]
- 90 mg/day (males) and 75 mg/day (females): Health Canada 2007[89]
- 60–95 milligrams per day: United States' National Academy of Sciences.[83]
The United States defined Tolerable Upper Intake Level for a 25-year-old male is 2,000 milligrams per day.
Therapeutic uses
Vitamin C functions as an antioxidant and is necessary for the treatment and prevention of scurvy, though in nearly all cases dietary intake is adequate to prevent deficiency and supplementation is not necessary.[90][91][92][93][94][95] Though vitamin C has been promoted as useful in the treatment of a variety of conditions, most of these uses are poorly supported by the evidence and sometimes contraindicated.[96][97][98][99] Vitamin C may be useful in lowering serum uric acid levels, resulting in a correspondingly lower incidence of gout.[100] Neither prophylactic nor therapeutic use is supported in the prevention or treatment of pneumonia.[101] People with the highest levels of ascorbic acid in their blood stream seem to be at a significantly reduced risk of having a stroke and low ascorbic acid has been suggested as a way of identifying those at high risk of stroke. [102]
Vitamin C's effect on the common cold has been extensively researched. It has not been shown effective in prevention or treatment of the common cold, except in limited circumstances (specifically, individuals exercising vigorously in cold environments).[103][104] Routine vitamin C supplementation does not reduce the incidence or severity of the common cold in the general population, though it may reduce the duration of illness.[103][105][106]
Vitamin C megadosage
Several individuals and organizations advocate large doses of vitamin C in excess of 10–100 times RDI in the form of oral or intravenous therapy.[107] In this sense vitamin C is employed not as a nutritional supplement but as a therapeutic agent, used to treat a vast number of health conditions—each with its own recommended protocol, of which there are often multiple megadose protocols for each condition. The therapeutic use of vitamin C through megadosing is extensive and different enough from the nutritional use of vitamin C as to require a unique treatment of the subject.