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Up to date as of January 23, 2010

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< Metabolomics | Nutrition
Chemical make-up of vitamin C

Vitamin C is a water-soluble nutrient essential for life and is used by the human body for many purposes. To the best of scientific knowledge, all animals and plants synthesize their own vitamin C, except for a small number of animals, including humans.

Ambersweet oranges.jpg

Contents

Sources

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Plant sources

Rose hips are a particularly rich source of vitamin C

Citrus fruits (lime, lemon, orange, grapefruit), tomatoes, and potatoes are good common sources of vitamin C. Other foods that are good sources of vitamin C include papaya, broccoli, brussels sprouts, black currants, strawberries, cauliflower, spinach, cantaloupe, and kiwifruit. Also, cranberries and red peppers are good sources of the vitamin.

The amount of vitamin C in foods of plant origin depends on:

  • the precise variety of the plant,
  • the soil condition
  • the climate in which it grew,
  • the length of time since it was picked,
  • the storage conditions,
  • the method of preparation. Cooking in particular is often said to destroy vitamin C - but see the section on Food preparation.

The following table is approximate and shows the relative abundance in different raw plant sources. The amount is given in mg per 100 grams of fruit:

Plant source Amount
Camu Camu 2800
Tibetan Goji berry 2500
Rosehip 2000
Acerola 1600
Jujube 500
Baobab 400
Blackcurrant 200
Guava 100
Kiwifruit 90
Broccoli 90
Loganberry 80
Redcurrant 80
Brussels sprouts 80
Lychee 70
Persimmon 60
Papaya 60
Strawberry 50
Orange 50
Plant source Amount
Lemon 40
Melon, cantaloupe 40
Cauliflower 40
Grapefruit 30
Raspberry 30
Tangerine 30
Mandarin orange 30
Passion fruit 30
Spinach 30
Cabbage raw green 30
Lime 20
Mango 20
Melon, honeydew 20
Raspberry 20
Tomato 10
Blueberry 10
Pineapple 10
Pawpaw 10
Plant source Amount
Grape 10
Apricot 10
Plum 10
Watermelon 10
Banana 9
Carrot 9
Avocado 8
Crabapple 8
Peach 7
Apple 6
Blackberry 6
Beetroot 5
Pear 4
Lettuce 4
Cucumber 3
Eggplant 2
Fig 2
Bilberry 1


Animal sources

Goats and most animals make their own vitamin C

It was only realised in the 1920s that some cuts of meat and fish are also a source of vitamin C for humans. The muscle and fat which make up the modern western diet are however poor sources. As with fruit and vegetables cooking destroys the vitamin C content.

The following table shows the relative abundance of vitamin C in various foods of animal origin, given in mg of vitamin C per 100 grams of food:

Food Amount
Calf liver (raw) 36
Beef liver (raw) 31
Oysters (raw) 30
Cod roe (fried) 26
Pork liver (raw) 23
Lamb brain (boiled) 17
Chicken liver (fried) 13
Lamb liver (fried) 12
Lamb heart (roast) 11
Food Amount
Lamb tongue (stewed) 6
Human milk (fresh) 4
Goat milk (fresh) 2
Cow milk (fresh) 2
Beef steak (fried) 0
Hen's egg (raw) 0
Pork bacon (fried) 0
Calf veal cutlet (fried) 0
Chicken leg (roast) 0


Functions in the body

  • As a participant in hydroxylation, vitamin C is needed for the production of collagen in the connective tissue. These fibres are ubiquitous throughout the body; providing firm but flexible structure. Some tissues have a greater percentage of collagen, especially: skin, mucous membranes, teeth and bones.
  • Vitamin C is required for synthesis of dopamine, noradrenaline and adrenaline in the nervous system or in the adrenal glands.
  • Vitamin C is also needed to synthesise carnitine, important in the transfer of energy to the cell mitochondria.
  • It is a strong antioxidant.
  • The tissues with greatest percentage of vitamin C — over 100 times the level in blood plasma — are the adrenal glands, pituitary, thymus, corpus luteum, and retina.
  • The brain, spleen, lung, testicle, lymph nodes, liver, thyroid, small intestinal mucous membrane|mucosa, leukocytes, pancreas, kidney and salivary glands usually have 10 to 50 times the concentration present in plasma.

No bodily organ stores ascorbate as a primary function, and so the body soon depletes itself of ascorbate if fresh supplies do not continue to arrive though the digestive system, eventually leading to death if unresolved.

Vitamin C deficiency

Lack of ascorbic acid in the daily diet leads to a disease called scurvy, a form of avitaminosis that is characterized by:

  • loose teeth
  • superficial bleeding
  • fragility of blood vessels
  • poor healing
  • compromised immunity
  • mild anaemia.

Daily requirement

There is a continuing debate within the scientific community over the optimum amount of vitamin C for humans.2

A healthy person on a balanced western diet should be able to get all the vitamin C needed to prevent the symptoms of scurvy from their daily diet. However, a person who is just freed from a scorbutic condition with only a small amount of ascorbate (i.e. RDA quantities) is arguably a very unhealthy individual, and certainly not one in optimum health. People who smoke, those under stress and women in pregnancy have a slightly higher requirement.

The amount of vitamin C needed to avoid deficiency symptoms of scurvy has been set by variously national agencies as follows:

40 mg per day UK Food Standards Agency
60–95 mg per day US Food and Nutrition Board 2001 revision.

Some researchers have calculated the amount needed for an adult human to achieve similar blood serum levels as Vitamin C synthesising mammals as follows:

400 mg per day – Linus Pauling Institute & US National Institutes of Health (NIH) Recommendation.
3000 mg per day – Vitamin C Foundation's recommendation.
6000-12000 mg per day – Thomas Levy, Colorado Integrative Medical Centre recommendation.
6000-18000 mg per day – Linus Pauling's daily recommendation
from 3000 mg to 200,000 mg based on bowel tolerance levels (see Cathcart below)

High doses (thousands of mg) may result in diarrhoea, which is harmless if the dose is reduced immediately. Some researchers (Cathcart) claim the onset of diarrhoea to be an indication of where the body’s true vitamin C requirement lies. Both Cathcart 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 (1/2 pound).

The small size of the ascorbic acid molecule means the kidneys cannot retain it in the body. Some other explanation is required to explain why very sick persons retain such huge quantities of vitamin C without any discharge. Quite a low level in the blood serum will cause traces to be present in the urine. All vitamin C synthesising mammals have traces in the urine at all times. The fact that animals like rats pass ascorbate into their urinary tract, after expending valuable energy manufacturing it, implies that there is a benefit to having vitamin C passing through, and this does not represent waste.

The RDA is based on blood plasma and white blood cell saturation data. In 2004, the validity of the pharmacokinetic research from the National Institutes of Health (NIH) that gave this data was challenged by Hickey and Roberts 4. According to these authors, the doses required to achieve blood, tissue and body "saturation" are much larger than previously believed. They allege that the Institute of Medicine (IoM) and the NIH have ignored an open letter from Drs Steve Hickey, Hilary Roberts, Ian Brighthope, Robert Cathcart, Abram Hoffer, Archie Kalokerinos, Tom Levy, Richard Passwater, Hugh Riordan, Andrew Saul and Patrick Holford, calling for revision of the RDA, and have refused to reconsider their low-dose recommendations. Hickey and Roberts have issued their open scientific challenge in the form of a book, which was submitted to the IoM and NIH prior to publication.

Food preparation

It is important to choose a suitable method of food preparation. When cooking vegetables, one should seek to minimize or not discard the water used in their preparation, e.g. by frying the food – which unfortunately increases fat content, steam cooking or by making soup.

Recent observations suggest that the impact of temperature and cooking on vitamin C may have been overestimated:

  1. Since it is water soluble, vitamin C will strongly leach into the cooking water while cooking most vegetables — but this doesn't necessarily mean the vitamin is destroyed — it's still there, but it's in the cooking water. (This may also suggest how the apparent misconception about the extent to which boiling temperatures destroy vitamin C might have been the result of flawed research: If the vitamin C content of vegetables (and not of the water) was measured subsequent to cooking them, then that content would have been much lower, though the vitamin has not actually been destroyed.)
  2. Not only the temperature, but also the exposure time is significant. Contrary to what was previously and is still commonly assumed, it can take much longer than two or three minutes to destroy vitamin C at boiling point.

It also appears that cooking does not necessarily leach vitamin C in all vegetables at the same rate; it has been suggested that the vitamin is not destroyed when boiling broccoli.1, this may however just be a result of vitamin C leaching into the cooking water at a slower rate from this vegetable. Copper pots will destroy the vitamin.

Vitamin C enriched teas and infusions have increasingly appeared on supermarket shelves. Such products would be nonsense if boiling temperatures did indeed destroy vitamin C at the rate it had previously been suggested. It should be noted however that as of 2004 most academics not directly involved in vitamin C research still teach that boiling temperatures will destroy vitamin C very rapidly.

Reported potential harmful effects

Reports of harmful effects of vitamin C tend to receive great prominence in the world's media. Many such reports have never been published in peer reviewed journals, which casts some suspicion on their credibility.

  • In April 1998 Nature reported alleged carcinogenic and teratogenic effects of excessive doses of vitamin C. The effects were noted in test tube experiments and on only two of the 20 markers of free radical damage to DNA. They have not been supported by further evidence from living organisms. Almost all mammals manufacture their own vitamin C in amounts equivalent to human doses of thousands of milligrams per day.
  • University of Southern California researchers in April 2000, reported a thickening of the arteries of the neck in persons taking high vitamin C doses. It was pointed out by vitamin C advocates that vitamin C's collagen synthesising role would lead to thicker and stronger artery walls. This research did not measure the blood velocity, which after simple mathematics, gives a more precise estimate of cross sectional area of the blood vessel.
  • In June 2004, Duke University researchers reported an increased susceptibility to osteo-arthritis in guinea pigs fed a diet high in vitamin C. However, a 2003 study at Umeå University in Sweden, found that "the plasma levels of vitamin C, retinol and uric acid were inversely correlated to variables related to rheumatoid arthritis disease activity."
  • "Rebound scurvy" is an often quoted theoretical, and never observed, condition that occurs when the amount of daily intake of Vitamin C is rapidly reduced from a very large amount to a relatively lower amount. This is an exaggeration of the rebound effect that occurs because ascorbate-dependent enzyme reactions continue for 24-48 hours, and use up vitamin C in the blood that is not being replenished. The effect is to lower one's serum vitamin C blood concentration to less than normal for a short amount of time. During this period of time there is a slight risk of cold of flu infection through reduced resistance. Within a couple of days the enzyme reactions shut down and blood serum returns to the normal level of someone not taking high supplements. This is not scurvy, which takes weeks of zero vitamin c consumption to produce symptoms. It is something people who take large vitamin C supplements need to be aware of in order to manage phased rather than sudden changes to the amount taken.
  • Kidney stones are a much cited harmful side effect of taking vitamin C in larger than normal amounts. However research has shown this not to be the case 3

Therapeutic uses

Vitamin C is needed in the diet to prevent scurvy. It also has a reputation for being useful in the treatment of colds and flu. The evidence to support this idea, however, is ambiguous, unless the studies are divided by dose size and dosing regime. When that is done, it is remarkable that most of the studies showing little or no effect employ quite small doses of ascorbate such as 100mg to 500mg per day. ("small" according to the vitamin C advocates) The Vitamin C foundation (1) recommends 8 grams of vitamin C every half hour in order to show an effect on the symptoms of a cold infection that is in progress.

References

  • Pauling, Linus (1986), How to Live Longer and Feel Better, W. H. Freeman and Company, ISBN 0-380-70289-4
  • Thomas Levy (September 2002), Vitamin C, Infectious Diseases, and Toxins, Xlibris Corporation (Paperback). ISBN 1401069630 (Note: Xlibris is a print on demand self-publishing house.)
  • Hickey, Steve & Roberts, Hilary; (May, 2004), Ascorbate: The Science of Vitamin C, Lulu Press, Inc. ISBN 1411607244 (Note: Lulu is a print on demand self-publishing house.)

External links

Footnotes

1 Combs GF. The Vitamins, Fundamental Aspects in Nutrition and Health. 2nd ed. San Diego, CA: Academic Press, 2001:245-272.
2 British pharmacology professors debate with the US National Institutes of Health over the optimum vitamin c dose (from PR Newswire - 6th July 2004) [1]
3 The Linus Pauling Institute at Oregon State University, "What About Vitamin C and Kidney Stones? "[2]
4 Hickey, Steve & Roberts, Hilary; (March, 2005), Ridiculous Dietary Allowance, Lulu Press, Inc. ISBN 1411622219.(Note: Lulu is a print on demand publisher.)


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