What is Vitamin C?Vitamin C or L-ascorbic acid, or simply ascorbate (the anion of ascorbic acid), is an essential nutrient for humans and certain other animal species. Vitamin C refers to a number of vitamers that have vitamin C activity in animals, including ascorbic acid and its salts, and some oxidized forms of the molecule like dehydroascorbic acid. Ascorbate and ascorbic acid are both naturally present in the body when either of these is introduced into cells, since the forms interconvert according to pH.
Vitamin C is 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 may also act as an antioxidant against oxidative stress. However, the fact that the enantiomer D-ascorbate (not found in nature) has identical antioxidant activity to L-ascorbate, yet far less vitamin activity, underscores the fact that most of the function of L-ascorbate as a vitamin relies not on its antioxidant properties, but upon enzymic reactions that are stereospecific. "Ascorbate" without the letter for the enantiomeric form is always presumed to be the chemical L-ascorbate.
Ascorbate (the anion 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; the main exceptions are bats, guinea pigs, capybaras, and the Anthropoidea (i.e., Haplorrhini, one of the two major primate suborders, consisting of tarsiers, monkeys, humans and other apes). Ascorbate 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.
Ascorbic acid is also widely used as a food additive, to prevent oxidation.
The North American Dietary Reference Intake recommends 90 milligrams per day and no more than 2 grams (2,000 milligrams) per day. Other related species sharing the same inability to produce vitamin C require exogenous vitamin C consumption 20 to 80 times this reference intake. 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. 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. 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.
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 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.
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