Vitamin C

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This article will describe the biochemistry of vitamin of ascorbic acid, please see Ascorbic acid. C. For the chemical properties

Vitamin C is only required by a few mammalian species, including humans and higher primates. It is the only water-soluble vitamin whose exact biochemical function remains unknown; although it has numerous physiological effects and, at present, eight different well characterized roles, it is not specifically required for any enzyme.[1] As research advances, it appears that its first name, ignose, meaning "I don't know", or "godnose," describes it best.[2]

Once described as the vitamin that prevents scurvy (hence its chemical name, ascorbic acid), vitamin C is now recognized as an important factor in the maintenance of good health and as a rationale for the consumption of more fruits and vegetables. It is the vitamin of many superlatives, as it is the most sold supplement in the world, the vitamin required for the maintenance of the most abundant protein in the body, the most "luminously controversial of all biological, alternative cancer therapies",[3] the vitamin which intake has declined the most drastically in the course of human evolution, and the vitamin which requirements have been debated for the most time and with the most intensity.

This article describes the debate on the vitamin's requirements (and, in some cases, lack thereof), presents the state-of-the-art in vitamin C therapeutics and presents the context in which knowledge on this nutrient has been produced. In this evolving field of medical research, epistemological considerations provide clues to the future developments in vitamin C.

Vitamin C

Description

Vitamin C is produced from glucose in the liver of most mammals and in the kidneys of most birds. The fact that most vertebrate species produce it endogenously as well as the fact that this production is massive (see Vitamin C in evolution, below) disqualify it as a vitamin, but it continues to be known as such. In contrast, other vitamins are indeed required in small amounts in the diet by most mammals, including humans. The molecule is also known as ascorbic acid, which suggests that vitamin C is to scurvy what vitamin B1 is to beri-beri, for instance, which is inexact as well (for an estimation of the discrepancy, see daily requirements, below).

Role as enzyme cofactor

Vitamin C is required by some enzymes called hydroxylases to add hydroxyl radicals (O-H) to specific molecules.

Collagen synthesis

Collagen hydroxylase uses vitamin C to make the long collagen fibers hold together. Collagen is the most abundant protein in the human body. (in progress)

Norepinephrine synthesis

Norepineprine, (aka noradrenaline), is obtained from dopamine by the action of the enzyme dopamine beta-hydroxylase. Dopamine and norepinephrine are neurotransmitters which have different functions but which are closely involved in mood, learning, and movement. Many antidepressants raise dopamine and norepinephrine concentrations, by different mechanisms.

Carnitine synthesis

Carnitine is the molecule that allows most fat molecules to be carried in the mitochondria where they will be transformed into energy. Carnitine is also required to carry excess organic acids out of mitochondria, where they would otherwise impair energy production. The metabolic pathway that leads from the amino acid lysine to the conditionnaly essential vitamin carnitine requires vitamin C twice. The steps are the enzymes gamma-butyrobetaine hydroxylase and epsilon-N-trimethyl-lysine hydroxylase. Low vitamin C causes a decreases in carnitine production, which contributes to fat deposition and overweight. At present, whether low levels of vitamin C might contribute to obesity is not known, but the normalisation of vitamin C levels in people with low vitamin C status was shown to raise their ability to burn fat 4-fold during submaximal exercise.[4]

Antioxidant functions

(in progress)

Vitamin C is also a major water phase low-molecular weight antioxidant.

In oxidation process the molecule of vitamin C step by step oxidazed with built up some active prooxidant substances. [5].

Biosynthesis

Yeasts do not synthesize vitamin C, but produce another antioxidant, erythorbic acid.[6] However, metabolic engineering of yeasts such as Saccharomyces cerevisiae can be used for the industrial production of vitamin C.[7]

Plants, humans' main source of vitamin C, produce it in large amounts. Plants use vitamin C in such great amounts as a defense to survive to viruses, bacteria and other environmental challenges and to cope with the internal challenges associated with photosynthesis.[8]

In animals, vitamin C is synthesised through four enzyme-driven steps, which convert glucose to ascorbic acid. It is carried out either in the kidneys, in reptiles and birds, or the liver, in mammals and perching birds. The last enzyme in the process, l-gulonolactone oxidase, cannot be made by humans because the gene for this enzyme is defective (Pseudogene ΨGULO). The loss of an enzyme concerned with ascorbic acid synthesis has occurred quite frequently in evolution and has affected most fish; many birds; some bats; guinea pigs; and most primates, including humans. The mutations have not been lethal because ascorbic acid is so prevalent in the environement or because oxidative stress is less prevalent in the environment (as for fishes, who are not exposed to high oxygen concentrations).

In addition to those species who lost vitamin C synthesis during evolution, it is worth mentioning the Shionogi rat, which is used in laboratories (much like the guinea pig) to study the inability to produce vitamin C and its consequences.

Evolution

The evolution of all vertebrate species can be viewed as the history of how they responded to the "the call for oxygen"[9] -- for "the fire of life".[10] Most important is the need to use this fire without being "burnt" by it.[11] The development of antioxidant machineries is closely intertwined with the development of species. An analysis of the evolutionary record reveals that the aquatic animals that were ancestors of amphibians did not significantly increase their concentrations of superoxide dismutase (SOD), the first line of defense against oxygen toxicity, but developed a highly functioning machinery transforming glucose into ascorbic acid, in order to cope with the sharp, 30-fold, increase in oxygen exposure.[12] The further evolution of heavier four-legged animals, from reptiles to mammals, was marked by a gradual increase in SOD, which was favoured to the expense of the vitamin C-producing machinery. This trend led, in exceptional cases, to the complete loss of vitamin C production: anthropoideans afforded to do without endogenous vitamin C by living in an environment providing great amounts of it, and expressed roughly twice as much SOD as other mammals. Amongst those species, humans have the best SOD defense.[12]

According to the Online Mendeleian Inheritance in Man database, hypoascorbemia is a "public" inborn error of metabolism, as it affects all members of the human race.[13]

It is thought that the loss of the ability to produce vitamin C, due to a mutation in the L-gulono-gamma-lactone oxidase gene, occurred some 25 to 45 million years ago, at a time when the natural environment of the common ancestor of primates provided great amounts of vitamin C.[13] Primates who still live in vitamin-C rich environments, consume 2000 to 6000 mg of vitamin C per day,[14] much more than the recommended doses for modern man, which are at least 20 times lower.

The ODS rat

The newly developed model of hypoascorbemia, the Osteogenic Disorder Shionogi rat (ODS rat), provides a unique occasion to analyze the early adaptative changes occuring when a species loses endogenous vitamin C synthesis. Contrary to the long-held belief that the high vitamin C intake of early anthropoideans was alone sufficient to compensate for the mutation,[13] ODS rats compensate this metabolic disease through several different mechanisms, some of which are not well characterized yet.

Uric acid

It was noted in 1970 that the inability of higher primates to break down uric acid, due to a mutation in the enzyme uricase, strikingly parallels the well-known metabolic disease of higher primates.[15] Uric acid and ascorbate are both strong reducing agents (electron-donors). The hypothesis by Proctor that uric acid has taken over some of the functions of ascorbic acid received experimental support thirty years later, when it was shown that ODS rats spontaneously develop high plasma uric acid (without the help of a mutation in the uricase gene), amongst many other compensatory mechanisms. In further support of this hypothesis, uric acid was shown to protect different superoxide dismutases against peroxide-mediated inactivation.[16][17] Hence, uric acid further improves the expression of SODs, that already tend to greater expression with the evolution of heavier animals.

(in progress)

Linus Pauling specified that the machinery for producing vitamin C was a burden that handicapped vitamin C-synthesizing individuals. In times of stress, the synthesis of vitamin C from glycogen can raise sharply: an adult goat, who manufactures more than 13,000 mg of vitamin C per day in normal health, will produce as much as 100,000 mg daily when faced with life-threatening disease, trauma or stress.[18]

When vitamin C-synthecizing species are exposed to high dietary levels of vitamin C, vitamin C concentrations decrease disproportionately in various organs, suggesting that endogenous synthesis of the vitamin is downregulated (it responds by decreasing) and/or that catabolism (destruction) or elimination of the vitamin are increased.[19] Whether this "overreaction", in an environment providing large amounts of vitamin C, contributed to the selection of individuals with low or absent vitamin C synthesis is an open question.

Another possible compensatory mechanism is the synthesis of lipoprotein(a). Lipoprotein(a), which is almost exclusively present in primates, might strengthen the extracellular matrix and compensate to some extent the relative lack of collagen and elastin synthesis. In addition, evidence suggests that, in some circumstances, lp(a), like vitamin C, delays lipid oxidation (peroxidation).[20]

Transport

Vitamin C, being a water soluble molecule that exists as an ion in body fluids (the ascorbate anion), does not cross lipid-rich membranes easily: it has to follow specific paths through plasma membranes to enter and leave cells. It is thus important to understand the transport of vitamin C in the different types of cells of the body to comprehend its role in health and disease. When describing the movements of vitamin C in body compartments, two different molecules must be taken into account: vitamin C and dehydroascorbic acid (DHAA; vitamin C which has undergone oxidation).

Active transport requires energy. Two transporters with extreme specificity for vitamin C, sodium-dependent vitamin C transporters 1 and 2 (SVCT1 and SVCT2) have been characterized. Recently, the sodium dependence of SVCT2 has been questioned. It appears that at least this transporter subtype is calcium/magnesium dependent.[21] Intracellular and extracellular concentrations of both divalent ions thus condition the transport of vitamin C through these transporters. The presence of sodium at a certain threshold makes SVCT2 more efficient: vitamin C and sodium work cooperatively to achieve a high rate of transport of both molecules. The SVCTs have limited capacities, as they tend to decrease in number the more vitamin C is accumulated in cells, and with increasing concentrations of the vitamin in circulation.[22] SVCT1 is mostly found in the liver and the kidneys (worthy of note, these are the two sites for vitamin C synthesis in the animal kingdom); SVCT2 dominates in the brain, skeletal muscles, and the spleen.[23]

A lesser known, but important, mode of transport of vitamin C is exocytosis. In this process, vesicles filled with vitamin C are secreted from cells, allowing vitamin C to influence neighboring cells. The secretion of vitamin C appears to be coordinated with the secretion of biologically active polypeptides from various glands, notably the pituitary gland; the metabolism of those polypeptides requires vitamin C as a cofactor (peptidyl-glycine α-amidating mono-oxygenase, vitamin C-requiring).[24]

Facilitated diffusion is the process whereby molecules move from the compartment where there is more of the molecule to the compartment where there is less of it. Facilitated diffusion lets DHAA enter cells, but not vitamin C, and lets vitamin C, but not DHAA, leave cells. The latter process is less understood than the former, but is essential in cells which deliver and keep vitamin C in the blood, i.e. the enterocytes (intestinal cells) and renal tubular cells (kidney cells), respectively. Once DHAA has entered a given cell, it is recycled back to vitamin C.

The fact that glucose transporters also transport the glucose derivative DHAA explains a paradoxical finding made my James Lind in his Treatise of the Scurvy:

(Victims of scurvy had) ravaged bodies (but) what was very surprising, the brains of those poor creatures were always sound and entire (...)[25]

It thus appears that the glucose transporters, by transporting oxidized vitamin C, allow organs to quickly store vitamin C at times of increased oxidative stress.[26] Once dehydroascorbic acid has crossed the blood-brain barrier and is in the brain, it is recycled (reduced) back to vitamin C, and retained in this compartment.[26] Conversely, conditions associated with low insulin, insulin resistance, high glucose and/or inflammation (diabetes, type 1 and 2, trauma, sepsis) impact on DHAA uptake and intracellular vitamin C status (also see Therapeutic uses). Adipocytes, astrocytes, endothelial cells, erythrocytes, granulosa cells, hepatocytes, neutrophils, osteoblasts and smooth muscle cells are known to accumulate DHAA for the accumulation of vitamin C.

Distribution

In the blood

Vitamin C concentrations in the blood are usually between 10 and 160 micromol/L,[27] and seldom exceed 80 micromol/L after most meals[28] Oral supplementation can raise levels to 220 micromol/L, while intravenous infusion can raise concentrations to 13 400 micromol/L.[29]

Leukocytes (white blood cells) use oxidants to destroy microbes.[30] and have mechanisms to tolerate great levels of oxidative stress and have transport systems that allow for a quick and ample mobilization of vitamin C (concentrations of the vitamin can reach 50 times those found in the blood).[31] Although lymphocytes are presently used to evaluate the body's need for vitamin C, they are not viewed as especially representative of the needs of organs and tissues.

In urine and feces

(in progress)

Determining the concentrations of vitamin C in urine and feces allows for a basic evaluation of the amounts that were absorbed by the body. It is known, however, that vitamin C-synthecizing species continually urinate vitamin C. The mere urinary excretion of vitamin is a normal part of its metabolism and cannot be taken as a sign of excess consumption, and the relationship between the intake of the vitamin and its fecal excretion varies very widely.

In organs and tissues

Some glands, organs and tissues contain 100 times more vitamin C than the blood, including the adrenal glands, pituitary gland, thymus, retina, corpus luteum, and various types of neurons.[27]

High concentrations of vitamin C are required for the adequate synthesis of catecholamines and steroids in the adrenal gland (adrenal cortex and adrenal medulla).[32] In response to stress, the adrenals secrete vitamin C locally, creating high concentrations acting in a paracrine manner.[28]

The corpus luteum produces the steroid hormone progesterone, which is particularly important for maintaining pregnancy. Different enzymes involved in progesterone synthesis are enhanced by vitamin C at concentrations of 100 micromol/L (in the higher nutritional range).[33] Also see Therapeutic uses - Pregnancy. Conversely, prostaglandin PGF2 alpha, which is important for the initiation of parturition at the end of a normal pregnancy, increases the secretion of vitamin C by the corpus luteum.[34]

The brain contains on average 10 times more vitamin C than the blood. Species that are exceptionally tolerant to oxygen deprivation and to reoxygenation concentrate even higher amounts of vitamin C.[35] In animal models of diabetes, where blood glucose levels are abnormally high, a drastic inhibition of vitamin C transport to the brain (through its oxidized form) is observed.[36]

The retina, like the brain, accumulates high concentrations of vitamin C using GLUT1 glucose transporters, which are distributed on the blood-retinal barrier. An experimental model of diabetes showed vitamin C concentrations in the retina to be drastically reduced by the high concentrations of glucose seen in diabetes, as a result of the competition of glucose with dehydroascorbic acid for entry in the retina (in this study, the transport of DHA was decreased by two thirds).[36]

Food sources

The richest natural sources are fruits and vegetables, and of those, the camu camu fruit , the billygoat plum and the Indian gooseberry or amla (Emblica officinalis) contain the highest concentration of the vitamin (about 30 times more than oranges). Vitamin C is the most widely taken nutritional supplement.

Plants

There is an enormous difference in vitamin C content between cultivated fruits and fruits found in the wild, especially those that Human's ancestors consumed when they got rid of endogenous capacity. Amongst fruits commonly found on the market, citrus fruits and small fruits (such as strawberries or blueberries) are relatively good sources of vitamin C. The amount of vitamin C in foods of plant origin depends on the variety of the plant, the soil condition and the climate in which it grew, the length of time since it was picked and the storage conditions, and the method of preparation. Cooking in particular is often said to destroy vitamin C — but see Food preparation, below.

With the gradual recognition that vitamin C prevents more than the sailor's disease, and in response to the general trends in consumer demands, the biotechnological industry has realized the commercial possibilities of new, patented, plant species with an enhanced ability to make vitamin C.[37]


Animals

Some cuts of meat are sources of vitamin C for humans. The muscle and fat that make up the modern western diet are, however, poor sources. As with fruit and vegetables, cooking degrades the vitamin C content.

Vitamin C is present in mother's milk and in less amounts in raw cow's milk (but pasteurized milk contains only trace amounts of the vitamin). [38]

Food preparation

Recent observations suggest that the impact of temperature and cooking on vitamin C may have been overestimated, since it decomposes around 190–192°C, well above the boiling point of water:

  • Since it is water soluble, vitamin C will strongly leach into the cooking water, but this doesn't necessarily mean the vitamin is destroyed.
  • Contrary to what is commonly assumed, it can take much longer than 2-3 min to destroy vitamin C at boiling point.
  • Cooking doesn't leach vitamin C in all vegetables at the same rate; for instance, it has been suggested that the vitamin is not destroyed when boiling broccoli.[39] This may be a result of vitamin C leaching into the cooking water at a slower rate from this vegetable.

Consistent with the interaction of vitamin C with copper metals in physiology, pots made with alloys of this metal will destroy the vitamin.[40]

Fresh-cut fruit may not lose much of its nutrients when stored in the refrigerator for a few days.[41]

Supplements

Vitamin C is the most widely taken dietary supplement.[42] It is available in many forms including caplets, tablets, capsules, drink mix packets, in multi-vitamin formulations, in multiple anti-oxidant formulations, as chemically pure crystalline powder, time release versions, and also including bioflavonoids such as quercetin, hesperidin and rutin. Tablet and capsule sizes range from 25 mg to 1500 mg. Vitamin C (ascorbic acid) crystals are typically available in bottles containing 300 g to 1 kg of powder (a teaspoon of vitamin C crystals equals 5,000 mg). Other forms of Vitamin C as sodium ascorbate, magnesium ascorbate, calcium ascorbate, mixed mineral ascorbates (e.g. Na, K, Mg, Ca, Zn), and Ester-C are also available, though less popular.

Vitamin C-enriched teas and infusions are increasingly appearing in markets. If boiling temperatures did indeed destroy vitamin C at the rate that had previously been suggested, using such products would be nonsensical. As note above, boiling is not as potently detrimental to the integrity of the vitamin C as was previously assumed.

History

For more information, see: History of vitamin C.

Vitamin C was first isolated in 1928, and in 1932 it was shown to prevent scurvy. Both Charles Glen King at the University of Pittsburgh and Albert Szent-Györgyi (working with ex-Pittsburgh researcher Joseph Svirbely) came to discover what is now known as vitamin C around April of 1932. Although Szent-Györgyi was awarded the 1937 Nobel Prize in Medicine, many feel King is as responsible for its development. [43]

Recommended daily requirements

The Food and Nutrition Board at the Institute of Medicine advise that he best way to get the daily requirement of essential vitamins, including vitamin C, is to eat a balanced diet. A healthy diet should contain the following amounts of vitamin C:

Infants and Children

  • 0 - 6 months: 40 mg/day
  • 7 - 12 months: 50 mg/day
  • 1 - 3 years: 15 mg/day
  • 4 - 8 years: 25 mg/day
  • 9 - 13 years: 45 mg/day

Adolescents

  • Girls 14 - 18 years: 65 mg/day
  • Boys 14 - 18 years: 75 mg/day

Adults

  • Men age 19 and older: 90 mg/day
  • Women age 19 year and older: 75 mg/day
  • Women who are pregnant or breastfeeding and those who smoke need higher amounts.
 )

Different health advisory bodies offer different advice regarding the daily requirement for vitamin C, and the USA and Canada recommend about twice the amount that the World Health Organization (WHO)recommends. The Linus Pauling Institute recommends more than four times the amount that the USA and Canada recommend, or ten times what the WHO recommends. However, after the death of Pauling, the Linus Pauling Institute came to diverge from Linus Pauling himself, who recommended doses in the same range as what other primates consume in the wild (also see Biosynthesis, above).

Guinea pigs United Kingdom United States World Health Organization Linus Pauling Institute Vitamin C Foundation Linus Pauling Other primates
Daily vitamin C intake (in mg) 10-30[44] 40[45] 95[46] 45[47] 400 3000 [48] 6000-18000 2000-6000[49]

The antiscorbutic range

Scurvy is a potentially serious condition that results from inadequate consumption of fresh fruit and vegetables, usually because of ignorance about proper nutrition, psychiatric disorders, alcoholism, or social isolation; it was once a common disease of sailors on long voyages. The symptoms of scurvy first appear only after many weeks of low intake. The first symptom is fatigue, followed by a wide variety of cutaneous symptoms, including follicular hyperkeratosis, perifollicular hemorrhages, ecchymoses, xerosis, leg edema, and bent or coiled body hairs. Scurvy is associated with generally poor wound healing. Gum abnormalities include gingival swelling, purplish discoloration, and hemorrhages. The patient with scurvy commonly reports pain in the back and joints, that is sometimes accompanied by hemorrhage into the soft tissue and joints. Anemia is a common symptom, and leukopenia an occasional symptom. Scurvy is life-threatening; syncope and sudden death may occur. However, treatment with vitamin C results in rapid, often dramatic, improvement. [2]

While Hippocrates described a case of scurvy in about 400 BCE, the cause of this disease was first clearly established by a surgeon in the British Royal Navy, James Lind, in 1747. In what was one of the earliest "controlled experiments", Lind gave some of the crew two oranges and one lemon per day, in addition to normal rations, while others (the control group) continued with their normal rations. The results showed that citrus fruits prevented scurvy, and Lind published his work in 1753 as his Treatise on the Scurvy.

This disease is still the basis of some recommended dietary allowances throughout the world. In 1999, as 10 mg of vitamin C is effective in preventing scurvy, the WHO and the UK recommended 30 mg as a safeguard for most of the population.[2] RDAs have been slightly raised since.

In 1974, Linus Pauling pointed out that amounts of recommended vitamin C in the range of 45 mg per day (for adults) should be renamed Minimum Dietary Allowances to reflect the fact that they were only intended to prevent a deficiency disease.[50] Although this suggestion was not accepted by health authorities, more recent recommendations reflect the notion that vitamin C not only prevents scurvy but contributes to the attainment of the "best of health".

Based on pharmacokinetics

Some authors have argued that many studies of vitamin C are methodologically flawed, for a variety of reasons, and argue that more studies are needed to determine physiological daily requirements [51] In line with Pauling's suggestion, Levine et al. pioneered the use of pharmacokinetic studies to base recommended dietary allowances on physiological requirements.[52] This approach gave solid support to the 5 servings of fruits and vegetables a day recommandation[52][53] made by the World Health Organization.

Based on evolutionary biology

The notion that the genome of Man has not evolved as rapidly as his methods to produce food is commonly recognized, in particular in evolutionary biology and evolutionary medicine. The thrifty gene hypothesis is an example of an evolutionary biology theory that is based on the discrepancies between genetic evolution and historical evolution.

As early as 1949, Bourne[54] pointed out the magnitude of the decrease in vitamin C intake that occurred as the human lineage left the environment in which the vitamin C machinery had been lost. Most recent data confirm the initial statements by Bourne, Stone[55] and Pauling[56] that the environment in which vitamin C production was lost provided (and still provides) gram amounts of vitamin C (between 2000 mg and 6000mg).[14]

Stone called hypoascorbemia, the inability to produce vitamin C, an inborn error of metabolism, comparable to lactose intolerance, for example. The Online Mendeleian Inheritance in Man database (National Center for Biotechnology Information)[13] considers this analysis to be valid, and adds that it could be called a "public inborn error of metabolism".

Vitamin C intake recommandations are now set to levels necessary to attain the "best state of physical and mental health,"[57]. The international consensus is that increasing fruit and vegetable consumption is an essential part of the prevention and management of chronic diseases (cardiovascular diseases, cancer, diabetes and obesity)[58]

Future research will tell if Bourne, Stone, Pauling and Milton were right to suggest, in Milton's terms, that our closest living relatives "have lessons for us."[14]

Therapeutic uses

Viral diseases

Vitamin C has a very interesting therapeutic index in viral infections, and has been claimed to be effective against a very broad variety of viruses, including herpes simplex, vaccinia, rabies, herpes zoster (shingles), measles , influenza, foot-and-mouth, hepatitis, HIV, polio virus and so forth [59].

Vitamin C acts in conjunction with copper (and perhaps other transition metals such as iron) and oxygen to produce hydroxyl radicals, which are the most toxic free radicals.[60] As emphacized above (see Description -- Antioxidant properties of vitamin C), most of the enzymatic and non-enzymatic effects of vitamin C are due to its antioxidant properties, and in particular to its ability to reduce iron and copper. In viruses, transition metal chemistry is not regulated in the same way as in mammalian cells. Concentrations of vitamin C that will lead to viral DNA damage (through copper reduction and subsequent generation of hydroxyl radical from hydrogen peroxide) can be attained in the body through supplementation.[61]


(WP content under revision: Ascorbate usage in studies of up to several grams per day, have been associated with decreased cold duration and severity of symptoms, possibly as a result of an antihistamine effect [62].

In 2002 a meta-study into all the published research on effectiveness of ascorbic acid in the treatment of infectious disease and toxins was conducted, by Thomas Levy, Medical Director of the Colorado Integrative Medical Centre in Denver. He claimed that evidence exists for its therapeutic role in a wide range of viral infections and for the treatment of snake bites.


Colds A recent 55-study review [63] found little positive effect of a vitamin C intake on common cold at low doses, but indication of prophylaxis benefits at higher doses especially where the subjects were in stressful situations.

At least 29 controlled clinical trials (many double-blind and placebo-controlled) involving a total of over 11,000 participants have been conducted into vitamin C and the Common cold. These trials were reviewed in the 1990s[64][65] and again more recently.[66] The trials show that vitamin C reduces the duration and severity of colds but not the frequency. The data indicate that there is a normal dose-response relationship. Vitamin C is more effective the higher the dose. [67] The controlled trials and clinical experience show that vitamin C in doses ranging from 0.1 to 2 g/day have little effect. The vast majority of the trials were limited to doses below 1 g/day. As doses rise, it becomes increasingly difficult to keep the trials double blind because of the obvious gastro-intestinal side effects of heavy doses of vitamin C. So, the most effective trials at doses between 2 and 10 g/day are generally met with skepticism.


Hepatitis C virus infection A phase I clinical trial was conducted to determine whether antioxidants could be beneficial in hepatitis C virus infection (HCV infection). This infection leads to a lack of antiviral defenses and to oxidative stress in the liver. Ultimately, oxidative stress, notably lipid-mediated oxidative stress (lipid peroxidation), causes liver cells to degenerate and die. Vitamin C was part of the protocol. The trial yielded favourable changes : normalization of liver enzymes (ALT returned to normal in 44 % of those who had abnormal ALT); decrease in viral load (25 % of patients); tissue changes (36.1 % had improvements histologic parameters); and 58 % of patients saw their quality of life improve with the antioxidant treatment (increase in the SF-36[68] score).[69]

Toxics

Lead

(in progress)

There is also evidence that vitamin C is useful in preventing lead poisoning, possibly helping to chelate the toxic heavy metal from the body. [5]

Common pesticides and contaminants

There exists great concern about the impact of pesticides and other contaminants on the reproductive capabilities on animals, including humans.[70] The toxicity of pesticides and contaminants can occur, notably, through endocrine disruption and/or oxidative stress.

The oxidative toxicity of bisphenol A to the epididymis and its effect on sperm motility and sperm count have been shown to be lessened by vitamin C.[71] The oxidative toxicities of endosulfan, phosphamidon,mancozeb and PCB (Aroclor 1254) were also neutralized by vitamin C.[72][73] It is important to note that the protective effects occurred irrespective of the chemical structure of the toxics, but rather addressed a common pathway of injury, i.e. oxidative stress, considering the very broad variety of chemical properties of toxics commonly encountered in the environment and in humans.

Medications (reduction of adverse effects)

Reduction of gentamicin nephrotoxicity

Vitamin C has been found to be effective in reducing or protecting against nephrotoxicity caused by the aminoglycoside antibiotic gentamicin.[74]

Heart disease

After a high-fat meal, triglycerides raise and the flow of blood through the arteries is impaired. Two grams of vitamin C largely suppress the impairment in flow-mediated dilatation in people with coronary heart disease as well as in healhty persons.[75] This finding implies that studies on the consumption of vitamin C (and possibly other nutrients and foods) must be reinterpreted in function of the timing of the supplementation and in function of the amount of fat consumed.

(Under revision: Nobel laureate chemist Linus Pauling stated that "chronic scurvy" or "subclinical scurvy" is a condition of vitamin C deficiency which is not as easily noticeable as acute scurvy (because chronic scurvy is mostly internal), characterized by micro lesions of tissues (such as that caused by blood pulsing through arteries, which stretches the arterial walls causing them to tear slightly), due to suboptimal collagen synthesis (see Collagen synthesis, above). Pauling and Rath stated that cardiovascular disease is primarily a collagen defect in the vasculature, and that plaque deposits were consequences. In support of this notion, the Proceedings of the National Academy of Sciences published in 2000 evidence that Shionogi rats (see Biosynthesis, above), a scurvy-prone species like Man, had a tendency to develop damage to the aorta, low HDL cholesterol and high total cholesterol, in a manner akin to typical human heart disease, under suboptimal vitamin C nutriture.[76]

Vitamin C is the main component of the three ingredients in Pauling and Rath's patented preventive cure for Lp(a)[77] related heart disease, the other two being the amino acid lysine and nicotinic acid (a form of Vitamin B3). Lp(a) as an atherosclerotic, evolutionary substitute for ascorbate[78] is still discussed as a hypothesis by mainstream medical science[79] and the Rath-Pauling related protocols[80] have not been rigorously tested and evaluated as conventional medical treatment by the FDA. )

Cancer

As noted above, vitamin C at physiologically attainable concentrations limits or suppresses the deleterious effects resulting from oxidative stress. As oxidative stress is thought to increase the risk of several cancers, it has been suggested that vitamin C might help prevent such cancers(see Toxics -- Common pesticides and contaminants). There is evidence that a diet rich in fresh fruit and vegetables - prominant sources of antioxidants - can reduce the risk of some cancers, but there is no specific, authoratative evidence that taking vitamin C supplements is protective.

In 1979 and 1985, two placebo-controlled trials[81][82] could not show any positive effect of vitamin C in cancer patients, and as a result, caused a marked decline in interest for vitamin C in cancer, although there have been occasional case reports since then suggesting that there might be benefits in some cases [83]


In 2005 in vitro (test tube) research by the National Institutes of Health indicated that vitamin C at high concentrations was preferentially toxic to several strains of cancer cells. The authors noted: "These findings give plausibility to intravenous ascorbic acid in cancer treatment, and have unexpected implications for treatment of infections where H2O2 may be beneficial." This research appeared to support Linus Pauling's claims that vitamin C can be used to fight cancer.[84]

Vitamin C inhibits key pathways in the proliferation of cancer cells as well. The PI3K/AKT pathway is a central mechanism of cancer proliferation that raises intense interest in the field of cancer research.[85] Vitamin C inhibits this pathway in vitro as well as in vivo.[86] The form of vitamin C used to demonstrate these effects is ascorbyl stearate, a lipophilic, vitamin C derivative, which is termed a nutraceutical.

Hypoxia-inducible factor-1 (HIF-1) is another well known protein involved in carcinogenis. Vitamin C inhibits its expression, a fact that lead researchers to challenge the hypothesis that it is the antioxidant and DNA-protective effect of vitamin C that explain its anticancer effects.[87]

Cataracts

A decrease in lens vitamin C concentrations in the course of cataract progression was shown.[88]

The Jean Mayer USDA Human Nutrition Research Center on Aging showed that, in the Nurses' Health Study cohort, practically all older women who consumed vitamin C supplements for more than 10 years were protected from lense opacities,[89] thus confirming earlier epidemiological evidence on the benefits of vitamin C supplementation in the prevention of cataracts.[90]

The finding, made in 1998, that cataract is associated with lens vitamin C deficiency[88] received support in 2004. While concentrations of vitamin C in the healhty aqueous humour are between 60 to 85 mg/dL, about 20 to 30 times those found in plasma, they average 4.29 mg/dL in persons suffering from cataract, or 0,06 % of normals.[91] This finding, added to the fact that the transport of the vitamin from the aqueous humour to the lens appears to be rather slow in humans,[92] confirm that the lens is a tissue that benefits high intakes of vitamin C.

Obstetrics and gynaecology

Recent studies into the use of a combination of Vitamin E ("natural" source isomer moiety, d-alpha tocopheryl ester) and vitamin C in preventing oxidative stress leading to pre-eclampsia failed to show significant benefit at the dosage tested, [93] In another study the same dosage did decrease average gestational time resulting in a higher incidence of low birthweight babies in one study.[94] Studies into antioxidants for pre-eclampsia are continuing.[95]

Side effects and contraindications

Contraindications A Contraindication is a condition which makes an individual more likely to be harmed by a dose of vitamin C than an average person.

  • A primary concern is people with unusual or unaddressed iron overload conditions, including hemochromatosis. Vitamin C enhances iron absorption. If sufferers of iron overload conditions take gram sized doses of vitamin C, they may worsen the iron overload due to enhanced iron absorption.
  • Inadequate Glucose-6-phosphate dehydrogenase enzyme (G6PD) levels, a genetic condition, may predispose some individuals to hemolytic anemia after intake of specific oxidizing substances present in some food and drugs. This includes repeated, very large intravenous or oral dosages of vitamin C. There is a test available for G6PD deficiency [6].

Side-effects

  • Vitamin C causes diarrhea if taken in quantities beyond a certain limit, which varies by individual. The diarrhea will cease as soon as the dose is reduced.
  • Large doses of vitamin C may cause acid indigestion, particularly when taken on an empty stomach.

Toxicity

Vitamin C exhibits remarkably low toxicity. For example, in a rat, the LD50 (the dose that will kill 50% of a population) has been reported as 11900 mg/kg,[96] or, for a 70 kg (155 pound) human, 833 grams of vitamin C would need to be ingested to stand a 50% chance of killing the person.

Harmful effects

Reports of harmful effects of vitamin C tend to receive prominent media coverage. As such, these reports tend to generate much debate and more research into vitamin C. Some of the harmful effects described below were proven invalid in later studies, while other effects are still being analyzed.

  • In April 1998, the journal Nature reported carcinogenic and teratogenic effects of excessive doses of vitamin C. The effects were noted in test tube experiments. [97]

The authors later clarified their position, stating that their results "show a definite increase in 8-oxoadenine after supplementation with vitamin C. This lesion is at least ten times less mutagenic than 8-oxoguanine, and hence our study shows an overall profound protective effect of this vitamin".[98]

  • "Rebound scurvy" is a theoretical condition that could occur when daily intake of vitamin C is rapidly reduced from a very large amount to a relatively low amount. Advocates suggest this is an exaggeration of the rebound effect which occurs because ascorbate-dependent enzyme reactions continue for 24–48 hours after intake is lowered, and use up vitamin C which 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 or 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 large 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 gradually reduce dosage rather than quit taking vitamin C suddenly.
  • Some writers[101] have identified a theoretical risk of poor copper absorption from high doses of vitamin C. However, ceruloplasmin levels seem specifically lowered by high vitamin C intake. In one study, 600 mg of vitamin C daily did not decrease copper absorption or overall body copper status in young men, but led to lower ceruloplasmin levels similar to those caused by copper deficiency.[102] In another, ceruloplasmin levels were significantly reduced.[103]

Conflicts with prescription drugs

Pharmaceuticals designed to reduce stomach acid such as the proton pump inhibitors (PPIs), are among the most widely-sold drugs in the world. One PPI, omeprazole, lowers the bioavailability of vitamin C by 12%, independent of dietary intake. This means that one would have to consume 14% more vitamin C to counteract the use of 40 mg/day of omeprazole. The probable mechanism of vitamin C reduction, intragastric pH elevated into alkalinity, would apply to all other PPI drugs, though not necessarily to doses of PPIs low enough to keep the stomach slightly acidic. [104]

References

  1. ↑ Barja, G (1996) Ascorbic acid and aging. In Harris, James W. (1996). Ascorbic acid: biochemistry and biomedical cell biology. New York: Plenum Press. ISBN 0-306-45148-4. 
  2. ↑ 2.0 2.1 2.2 Hirschmann JV & al. (1999). "Adult scurvy". J Am Acad Dermatol 41: 895–906; quiz 907–10. PMID 10570371.
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  97. ↑ Oregon State University - Vitamin C and cancer
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  99. ↑ FAQ provided by The Vitamin C Foundation.
  100. ↑ Hokama S et al. (2000) Ascorbate conversion to oxalate in alkaline milieu and Proteus mirabilis culture. Mol Urol 4:321–8. Massey LK et al. (2005) Ascorbate increases human oxaluria and kidney stone risk J Nutr 135:1673–7.
  101. ↑ acu-cell
  102. ↑ NCBI
  103. ↑ NCBI
  104. ↑ E. B. Henry, and others Proton pump inhibitors reduce the bioavailability of dietary vitamin C "The gastric juice concentration of vitamin C is reduced in subjects with elevated intragastric pH. This is probably because of the fact that the vitamin is unstable at non-acidic pH and undergoes irreversible denaturation.
    .... After 28 days of 40 mg/day of omeprazole the mean plasma vitamin C level had fallen by 12.3% (P = 0.04)." Alimentary Pharmacology & Therapeutics Volume 22 Page 539 - September 2005 doi:10.1111/j.1365-2036.2005.02568.x Accessed Nov 2006
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