Vitamin C

Vitamin C or L-ascorbate is an essential nutrient required to allow a range of essential metabolic reactions in animals and plants, and widely known as the vitamin that prevents scurvy in humans.[1][2][3]

The pharmacophore of vitamin C is the ascorbate ion. In living organisms, ascorbate is an antioxidant, as it protects the body against oxidative stress,[4] and a cofactor in several vital enzymatic reactions.[5]

As a nutrient, its uses and requirements are matters of on-going debate. The North American Dietary Reference Intake recommends 90 milligrams per day and no more than 2 grams per day (2000 milligrams per day).[6] Other related species sharing the same inability to produce vitamin C and requiring exogenous vitamin C consume 20 to 80 times this reference intake.

Daily requirements
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.[36] It is generally agreed that a balanced diet without supplementation contains enough vitamin C to prevent acute scurvy in an average healthy adult, while those who are pregnant, smoke tobacco, or are under stress require slightly more.[6]

Vitamin C is recognized to be one of the least toxic substances known to medicine,[6] with the LD50 being 11,900 milligrams per kilogram.[32][37] High doses (thousands of milligrams) may result in diarrhoea, which is harmless if the dose is reduced immediately. Some researchers[38] claim the onset of diarrhoea to be an indication of where the body’s true vitamin C requirement lies. Both Cathcart[38] and Cameron have demonstrated that very sick patients with cancer or influenza do not display any evidence of diarrhoea at all until ascorbate intake reaches levels as high as 200 grams (half a pound).

Independent recommended intakes
Some independent researchers have calculated the amount needed for an adult human to achieve similar blood serum levels as vitamin C synthesising mammals as follows:

400 milligrams per day — the Linus Pauling Institute and the US National Institutes of Health
500 milligrams per 12 hours — Professor Roc Ordman, from research into biological free radicals[40]
3,000 milligrams per day (or up to 300,000 mg during illness or pregnancy) — the Vitamin C Foundation[41]
6,000–12,000 milligrams per day — Thomas E. Levy, Colorado Integrative Medical Centre
6,000–18,000 milligrams per day — Linus Pauling's personal use[42]
3,000–200,000 milligrams per day — Robert Cathcart's protocol known as a "vitamin C flush" wherein escalating doses of vitamin C are given until diarrhoea develops, then choosing the highest dose that does not cause diarrhoea (the bowel tolerance threshold)[38]

Common side-effects
Relatively large doses of vitamin C may cause indigestion, particularly when taken on an empty stomach. This unpleasant but harmless side-effect can be avoided by taking the vitamin along with meals or by offsetting its acidity by taking an antacid such as baking soda or calcium carbonate.

When taken in huge doses, vitamin C causes diarrhea. The minimum dose that brings about this effect varies with the individual. Robert Cathcart has called this limit the "bowel tolerance threshold" and observed that it is higher in people with serious illness than those in good health.[38] It ranges from 5 to 25 grams per day in healthy individuals to 300 grams per day in those that are severely ill. Diarrhea is not harmful, as long as the dose is reduced quickly.
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Natural and artificial dietary sources
Rose hips are a particularly rich source of vitamin C
The richest natural sources are fruits and vegetables, and of those, the camu camu fruit and the billygoat plum contain the highest concentration of the vitamin. It is also present in some cuts of meat, especially liver. Vitamin C is the most widely taken nutritional supplement and is available in a variety of forms, including tablets, drink mixes, crystals in capsules or naked crystals.