Vitamin C in Human Health and Disease is Still a Mystery? An Overview

Abstract

Ascorbic acid is one of the important water soluble vitamins. It is essential for collagen, carnitine and neurotransmitters biosynthesis. Most plants and animals synthesize ascorbic acid for their own requirement. However, apes and humans can not synthesize ascorbic acid due to lack of an enzyme gulonolactone oxidase. Hence, ascorbic acid has to be supplemented mainly through fruits, vegetables and tablets. The current US recommended daily allowance (RDA) for ascorbic acid ranges between 100-120 mg/per day for adults. Many health benefits have been attributed to ascorbic acid such as antioxidant, anti-atherogenic, anti-carcinogenic, immunomodulator and prevents cold etc. However, lately the health benefits of ascorbic acid has been the subject of debate and controversies viz., Danger of mega doses of ascorbic acid? Does ascorbic acid act as a antioxidant or pro-oxidant? Does ascorbic acid cause cancer or may interfere with cancer therapy? However, the Panel on dietary antioxidants and related compounds stated that the in vivo data do not clearly show a relationship between excess ascorbic acid intake and kidney stone formation, pro-oxidant effects, excess iron absorption. A number of clinical and epidemiological studies on anti-carcinogenic effects of ascorbic acid in humans did not show any conclusive beneficial effects on various types of cancer except gastric cancer. Recently, a few derivatives of ascorbic acid were tested on cancer cells, among them ascorbic acid esters showed promising anticancer activity compared to ascorbic acid. Ascorbyl stearate was found to inhibit proliferation of human cancer cells by interfering with cell cycle progression, induced apoptosis by modulation of signal transduction pathways. However, more mechanistic and human in vivo studies are needed to understand and elucidate the molecular mechanism underlying the anti-carcinogenic property of ascorbic acid. Thus, though ascorbic acid was discovered in 17th century, the exact role of this vitamin/nutraceutical in human biology and health is still a mystery in view of many beneficial claims and controversies.

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Nutrition Journal
Open Access
Review
Vitamin C in human health and disease is still a mystery ? An
overview
K Akhilender Naidu*
Address: Department of Biochemistry and Nutrition, Central Food Technological Research Institute, Mysore 570 013, India
Email: K Akhilender Naidu* - kanaidu@mailcity.com
* Corresponding author
Abstract
Ascorbic acid is one of the important water soluble vitamins. It is essential for collagen, carnitine
and neurotransmitters biosynthesis. Most plants and animals synthesize ascorbic acid for their own
requirement. However, apes and humans can not synthesize ascorbic acid due to lack of an enzyme
gulonolactone oxidase. Hence, ascorbic acid has to be supplemented mainly through fruits,
vegetables and tablets. The current US recommended daily allowance (RDA) for ascorbic acid
ranges between 100–120 mg/per day for adults. Many health benefits have been attributed to
ascorbic acid such as antioxidant, anti-atherogenic, anti-carcinogenic, immunomodulator and
prevents cold etc. However, lately the health benefits of ascorbic acid has been the subject of
debate and controversies viz., Danger of mega doses of ascorbic acid? Does ascorbic acid act as a
antioxidant or pro-oxidant ? Does ascorbic acid cause cancer or may interfere with cancer therapy?
However, the Panel on dietary antioxidants and related compounds stated that the in vivo data do
not clearly show a relationship between excess ascorbic acid intake and kidney stone formation,
pro-oxidant effects, excess iron absorption. A number of clinical and epidemiological studies on
anti-carcinogenic effects of ascorbic acid in humans did not show any conclusive beneficial effects
on various types of cancer except gastric cancer. Recently, a few derivatives of ascorbic acid were
tested on cancer cells, among them ascorbic acid esters showed promising anticancer activity
compared to ascorbic acid. Ascorbyl stearate was found to inhibit proliferation of human cancer
cells by interfering with cell cycle progression, induced apoptosis by modulation of signal
transduction pathways. However, more mechanistic and human in vivo studies are needed to
understand and elucidate the molecular mechanism underlying the anti-carcinogenic property of
ascorbic acid. Thus, though ascorbic acid was discovered in 17
th
century, the exact role of this
vitamin/nutraceutical in human biology and health is still a mystery in view of many beneficial claims
and controversies.
Historical perspective
The sea voyager/sailors developed a peculiar disease called
scurvy when they were on sea. This was found to be due to
eating non-perishable items and lack of fresh fruits and
vegetables in their diet. A British naval Physician, Lind [1]
documented that there was some substance in citrus fruits
that can cure scurvy. He developed a method to concen-
trate and preserve citrus juice for use by sailors. British
Navy was given a daily ration of lime or lemon juice to
overcome ascorbic acid deficiency. Ascorbic acid was first
isolated from natural sources and structurally character-
Published: 21 August 2003
Nutrition Journal 2003, 2:7
Received: 13 May 2003
Accepted: 21 August 2003
This article is available from: http://www.nutritionj.com/content/2/1/7
© 2003 Naidu; licensee BioMed Central Ltd. This is an Open Access article: verbatim copying and redistribution of this article are permitted in all media
for any purpose, provided this notice is preserved along with the article's original URL.

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ized by Szent-Gyorgyi, Waugh and King [2,3]. This
vitamin was first synthesized by Haworth and Hirst [4].
Currently ascorbic acid is the most widely used vitamin
supplement through out the world.
Sources of Ascorbic acid
Ascorbic acid is widely distributed in fresh fruits and veg-
etables. It is present in fruits like orange, lemons, grape-
fruit, watermelon, papaya, strawberries, cantaloupe,
mango, pineapple, raspberries and cherries. It is also
found in green leafy vegetables, tomatoes, broccoli, green
and red peppers, cauliflower and cabbage.
Most of the plants and animals synthesize ascorbic acid
from D-glucose or D-galactose. A majority of animals
produce relatively high levels of ascorbic acid from glu-
cose in liver (Fig 1).
However, guinea pigs, fruit eating bats, apes and humans
can not synthesize ascorbic acid due to the absence of the
enzyme L-gulonolactone oxidase. Hence, in humans
ascorbic acid has to be supplemented through food and or
as tablets.
Ascorbic acid is a labile molecule, it may be lost from
foods during cooking/processing even though it has the
ability to preserve foods by virtue of its reducing property.
Synthetic ascorbic acid is available in a wide variety of
supplements viz., tablets, capsules, chewable tablets, crys-
talline powder, effervescent tablets and liquid form. Buff-
ered ascorbic acid and esterfied form of ascorbic acid as
ascorbyl palmitate is also available commercially. Both
natural and synthetic ascorbic acid are chemically identi-
cal and there are no known differences in their biological
activities or bio-availability.
Chemistry of ascorbic acid
L-ascorbic acid (C
6
H
8
O
6
) is the trivial name of Vitamin C.
The chemical name is 2-oxo-L-threo-hexono-1,4-lactone-
2,3-enediol. L-ascorbic and dehydroascorbic acid are the
major dietary forms of vitamin C [5]. Ascorbyl palmitate
is used in commercial antioxidant preparations. All com-
mercial forms of ascorbic acid except ascorbyl palmitate
are soluble in water. L-ascorbic acid and its fatty acid esters
are used as food additives, antioxidants, browning inhib-
itors, reducing agents, flavor stabilizers, dough modifiers
and color stabilizers. Ascorbyl palmitate has been used for
its greater lipid solubility in antioxidant preparations. In
foods, pH influences the stability of ascorbic acid. It
exhibits maximal stability between pH 4 and 6 [5]. Cook-
ing losses of ascorbic acid depend on degree of heating,
surface area exposed to water, oxygen, pH and presence of
transition metals.
Catabolism of Ascorbic acid
Ascorbic acid present in foods is readily available and eas-
ily absorbed by active transport in the intestine [6]. Most
of it (80–90%) will be absorbed when the in take is up to
100 mg/day, whereas at higher levels of intake (500 mg/
day) the efficiency of absorption of ascorbic acid rapidly
Biosynthesis of L-Ascorbic acid in animalsFigure 1
Biosynthesis of L-Ascorbic acid in animals
D-glucose D-galactose
Glucose-6-phosphate
Uridine diphosphate
glucose
Uridine diphosphate
glucuronic acid
D-glucuronic acid
D-glucoronalactone pentose
phosphate
pathway
L-gulono-
J
-lactone
L-gulonolactone
oxidase
2-keto-gulono-
J
-lactone
L-ascorbic acid

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declines. Ascorbic acid is sensitive to air, light, heat and
easily destroyed by prolonged storage and over processing
of food.
Ascorbic acid being a water soluble compound is easily
absorbed but it is not stored in the body. The average
adult has a body pool of 1.2–2.0 g of ascorbic acid that
may be maintained with 75 mg/d of ascorbic acid. About
140 mg/d of ascorbic acid will saturate the total body pool
of vitamin C [7]. The average half life of ascorbic acid in
adult human is about 10–20 days, with a turn over of 1
mg/kg body and a body pool of 22 mg/kg at plasma ascor-
bate concentration of 50 µmol/ L [8,9]. Hence ascorbic
acid has to regularly supplemented through diet or tablets
to maintain ascorbic acid pool in the body.
The major metabolites of ascorbic acid in human are
dehydroascorbic acid, 2,3-diketogulonic acid and oxalic
acid (Fig 2). The main route of elimination of ascorbic
acid and its metabolites is through urine. It is excreted
unchanged when high doses of ascorbic acid are con-
sumed. Ascorbic acid is generally non-toxic but at high
doses (2–6 g/day) it can cause gastrointestinal distur-
bances or diarrhea [10,11]. The side effects are generally
not serious and can be easily reversed by reducing intake
of ascorbic acid. Furthermore, there is no consistent and
compelling data on serious health effects of vitamin C in
humans [11].
A deficiency of ascorbic acid leads to scurvy. It is charac-
terized by spongy swollen bleeding gums, dry skin, open
sores on the skin, fatigue, impaired wound healing and
depression [13]. Scurvy is of rare occurrence nowadays
due to adequate intake ascorbic acid through fresh vegeta-
bles and fruits and or supplementation as tablets.
Dietary recommendations of Ascorbic acid
The new average daily intake level that is sufficient to meet
the nutritional requirement of ascorbic acid or recom-
mended dietary allowances (RDA) for adults (>19 yr) are
90 mg/day for men and 75 mg/day for women [14]. Con-
sumption of 100 mg/day of ascorbic acid is found to be
sufficient to saturate the body pools (neutrophils, leuko-
cytes and other tissues) in healthy individuals. Based on
clinical and epidemiological studies it has been suggested
that a dietary intake of 100 mg/day of ascorbic acid is
associated with reduced incidence of mortality from heart
diseases, stroke and cancer [15]. However, stress, smok-
ing, alcoholism, fever, viral infections cause a rapid
decline in blood levels of ascorbic acid.
Smoking is known to increase the metabolic turnover of
ascorbic acid due to its oxidation by free radicals and reac-
tive oxygen species generated by cigarette smoking [16]. It
has been suggested that a daily intake of at least 140 mg/
day is required for smokers to maintain a total body pool
similar to that of non-smokers consuming 100 mg/day
[17]. Based on latest literature reports, it has been recom-
mended that the RDA for ascorbic acid should be 100–
120 mg/day to maintain cellular saturation and optimum
risk reduction of heart disease, stroke and cancer in
healthy individuals [18]. There is no scientific evidence to
show that even very large doses of vitamin C are toxic or
exert serious adverse health effects [11,19]. Furthermore,
the panel on dietary antioxidants and related compounds
suggested that in vivo data do not clearly show a relation-
ship between excess vitamin C intake and kidney stone
formation, pro-oxidant effects, excess iron absorption
[20].
Physiological functions of Ascorbic acid
The physiological functions of ascorbic acid are largely
dependent on the oxido-reduction properties of this vita-
min. L-ascorbic acid is a co-factor for hydroxylases and
monooxygenase enzymes involved in the synthesis of col-
lagen, carnitine and neurotransmitters [21]. Ascorbic acid
accelerates hydroxylation reactions by maintaining the
active center of metal ions in a reduced state for optimal
activity of enzymes hydroxylase and oxygenase.
Catabolism of Ascorbic acidFigure 2
Catabolism of Ascorbic acid
L-ascorbic acid
Ascorbyl radical L-ascorbate
sulphate
Dehydroascrobate CO
2
2,3-diketogulonic acid
Oxalic acid

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Ascorbic acid plays an important role in the maintenance
of collagen which represents about one third of the total
body protein. It constitutes the principal protein of skin,
bones, teeth, cartilage, tendons, blood vessels, heart
valves, inter vertebral discs, cornea and eye lens. Ascorbic
acid is essential to maintain the enzyme prolyl and lysyl
hydroxylase in an active form. The hydroxylation of pro-
line and lysine is carried out by the enzyme prolyl hydrox-
ylase using ascorbic acid as co-factor. Ascorbic acid
deficiency results in reduced hydroxylation of proline and
lysine, thus affecting collagen synthesis.
Ascorbic acid is essential for the synthesis of muscle carni-
tine (β-hydroxy butyric acid). [22]. Carnitine is required
for transport and transfer of fatty acids into mitochondria
where it can be used for energy production. Ascorbic acid
acts as co-factor for hydroxylations involved in carnitine
synthesis. Further, ascorbic acid acts as co-factorfor the
enzyme dopamine-β-hydroxylase, which catalyzes the
conversion of neurotransmitter dopamine to norepine-
phrine. Thus ascorbic acid is essential for synthesis of cat-
echolamines. In addition, ascorbic acid catalyzes other
enzymatic reactions involving amidation necessary for
maximal activity of hormones oxytocin, vasopressin,
cholecystokinin and alpha-melanotripin [23].
Ascorbic acid is also necessary for the transformation of
cholesterol to bile acids as it modulates the microsomal 7
α-hydroxylation, the rate limiting reaction of cholesterol
catabolism in liver. In ascorbic acid deficiency, this reac-
tion becomes slowed down thus, resulting in an accumu-
lation of cholesterol in liver, hypercholesterolemia,
formation of cholesterol gall stones etc [24].
Ascorbic acid and iron
Ascorbic acid is known to enhance the availability and
absorption of iron from non-heme iron sources [25].
Ascorbic acid supplementation is found to facilitate the
dietary absorption of iron. The reduction of iron by ascor-
bic acid has been suggested to increase dietary absorption
of non-heme iron [26]. It is well known that in the pres-
ence of redox-active iron, ascorbic acid acts as a pro-oxi-
dant in vitro and might contribute to the formation of
hydroxyl radical, which eventually may lead to lipid, DNA
or protein oxidation [27]. Thus, ascorbic acid supplemen-
tation in individuals with high iron and or bleomycin-
detectable iron (BDI) in some preterm infants could be
deleterious because it may cause oxidative damage to
biomolecules [28–31]. However, no pro-oxidant effect
was observed on ascorbic acid supplementation on DNA
damage in presence or absence of iron [32].
Ascorbic acid in health and disease
Ascorbic acid and common cold
The most widely known health beneficial effect of ascor-
bic acid is for the prevention or relief of common cold.
Pauling [33] suggested that ingestion of 1–2 g of ascorbic
acid effectively prevents/ ameliorate common cold. The
role of oral vitamin C in the prevention and treatment of
colds remains controversial despite many controlled tri-
als. Several clinical trails with varying doses of ascorbic
acid showed that ascorbic acid does not have significant
prophylactic effect, but reduced the severity and duration
of symptoms of cold during the period of infection. Ran-
domized and non-randomized trials on vitamin C to pre-
vent or treat the common cold showed that consumption
of ascorbic acid as high as 1.0 g/day for several winter
months, had no consistent beneficial effect on the inci-
dence of common cold. For both preventive and thera-
peutic trials, there was a consistent beneficial but
generally modest therapeutic effect on duration of cold
symptoms. There was no clear indication of the relative
benefits of different regimes of vitamin C doses. However,
in trials that tested vitamin C after cold symptoms
occurred, there was some evidence of greater benefits with
large dose than with lower doses [34].
There has been a long-standing debate concerning the role
of ascorbic acid in boosting immunity during cold infec-
tions. Ascorbic acid has been shown to stimulate immune
system by enhancing T-cell proliferation in response to
infection. These cells are capable of lysing infected targets
by producing large quantities of cytokines and by helping
B cells to synthesize immunoglobulins to control inflam-
matory reactions. Further, it has been shown that ascorbic
acid blocks pathways that lead to apoptosis of T-cells and
thus stimulate or maintain T cell proliferation to attack
the infection. This mechanism has been proposed for the
enhanced immune response observed after administra-
tion of vitamin C during cold infections [35].
Ascorbic acid and wound healing
Ascorbic acid plays a critical role in wound repair and
healing/regeneration process as it stimulates collagen syn-
thesis. Adequate supplies of ascorbic acid are necessary for
normal healing process especially for post-operative
patients. It has been suggested that there will be rapid uti-
lization of ascorbic acid for the synthesis of collagen at the
site of wound/ burns during post-operative period [36].
Hence, administration of 500 mg to 1.0 g/day of ascorbic
acid are recommended to accelerate the healing process
[8].
Ascorbic acid and atherosclerosis
Lipid peroxidation and oxidative modification of low
density lipoproteins (LDL) are implicated in development
of atherosclerosis [37]. Vitamin C protects against oxida-

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tion of isolated LDL by different types of oxidative stress,
including metal ion dependent and independent proc-
esses [38]. Addition of iron to plasma devoid of ascorbic
acid resulted in lipid peroxidation, whereas endogenous
and exogenous ascorbic acid was found to inhibit the
lipid oxidation in iron-over loaded human plasma [39].
Similarly, when ascorbic acid was added to human serum
supplemented with Cu
2+
, antioxidant activity rather than
pro-oxidant effects were observed [40].
Ascorbic acid is known to prevent the oxidation of LDL
primarily by scavenging the free radicals and other reac-
tive oxygen species in the aqueous milieu [41]. In addi-
tion, in vitro studies have shown that physiological
concentrations of ascorbic acid strongly inhibit LDL oxi-
dation by vascular endothelial cells [42]. Adhesion of leu-
kocytes to the endothelium is an important step in
initiating atherosclerosis. In vivo studies have demon-
strated that ascorbic acid inhibits leukocyte-endothelial
cell interactions induced by cigarette smoke [43,44] or
oxidized LDL [45]. Further, lipophilic derivatives of ascor-
bic acid showed protective effect on lipid-peroxide
induced endothelial injury [46].
A number of studies have been carried out in humans to
determine the protective effect of ascorbic acid supple-
mentation (500–100 mg/day) on in vivo and ex vivo lipid
peroxidation in healthy individuals and smoker. The find-
ings are inconclusive as ascorbic acid supplementation
showed a reduction or no change in lipid peroxidation
products [10,47–50]. In this context, it is important to
note that during ex vivo LDL oxidation studies, water sol-
uble ascorbic acid is removed during initial LDL isolation
step itself. Therefore, no change in ex vivo would be
expected [15]. Overall, both in vitro and in vivo experi-
ments showed that ascorbic acid protects isolated LDL
and plasma lipid peroxidation induced by various radical
or oxidant generating systems. However, a recent report
demonstrated that large doses of exogenous iron (200
mg) and ascorbic acid (75 mg) promoted the release of
iron from iron binding proteins and also enhanced in vitro
lipid peroxidation in serum of guinea pigs. This finding
supports the hypothesis that high intake of iron along
with ascorbic acid could increase in vivo lipid peroxidation
of LDL and therefore could increase risk of atherosclerosis
[51]. However, Chen et al., [52] demonstrated that ascor-
bic acts as an antioxidant towards lipids even in presence
of iron over load in in vivo systems.
Numerous studies have looked at the association between
ascorbic acid intake and the risk of developing
cardiovascular disease (CHD). A large prospective epide-
miological study in Finnish men and women suggested
that high intake of ascorbic acid was associated with a
reduced risk of death from CHD in women and not in
men [53]. Similarly, another study showed that high
intake of ascorbic acid in American men and women
appeared to benefit only women [54,55]. A third Ameri-
can cohort study suggested that cardiovascular mortality
was reduced in both sexes by vitamin C [56]. In the UK, a
study showed that the risk of stroke in those with highest
intake of vitamin C was only half that of subjects with the
lowest intake and no evidence suggestive of lower rate of
CHD in those with high vitamin C intake [57]. However,
a recent meta analysis on the role of ascorbic acid and
antioxidant vitamins showed no evidence of significant
benefit in prevention of CHD [58]. Thus, no conclusive
evidence is available on the possible protective effect of
ascorbic acid supplementation on cardiovascular disease.
Ascorbic acid and Cancer
Nobel laureate Pauling and Cameron advocated use of
high doses of ascorbic acid (> 10 g/day) to cure and pre-
vent cold infections and in the treatment of cancer
[34,59]. The benefits included were increased sense of
well being/ much improved quality of life, prolongation
of survival times in terminal patients and complete regres-
sion in some cases [60–62]. However, clinical studies on
cancer patients carried out at Mayo Clinic showed no sig-
nificant differences between vitamin C and placebo
groups in regard to survival time [63]. Cameron and Paul-
ing [23] believed that ascorbic acid combats cancer by
promoting collagen synthesis and thus prevents tumors
from invading other tissues. However, researchers now
believe that ascorbic acid prevents cancer by neutralizing
free radicals before they can damage DNA and initiate
tumor growth and or may act as a pro-oxidant helping
body's own free radicals to destroy tumors in their early
stages [64–66].
Extensive animal, clinical and epidemiological studies
were carried out on the role of ascorbic acid in the preven-
tion of different types of cancers. A mixture of ascorbic
acid and cupric sulfate significantly inhibited human
mammary tumor growth in mice, while administered
orally [67]. Ascorbic acid decreased the incidence of kid-
ney tumors by estradiol or diethylstilbesterol in hamsters
due to decrease in the formation of genotoxic metabolites
viz., diethylstilbesterol-4'-4"-qunione [68]. Ascorbic acid
and its derivatives were shown to be cytotoxic and inhib-
ited the growth of a number of malignant and non-malig-
nant cell lines in vitro and in vivo [69–72]. Ascorbic acid
has been reported to be cytotoxic to some human tumor
cells viz., neuorblastoma [73], osteosarcoma and retino-
blastoma [74]. A number of ascorbic acid isomers/ deriv-
atives were synthesized and tested on tumor cell lines.
Roomi et al., 1998 [75] demonstrated that substitution at
2- or 6- and both at 2,6-positions in ascorbic acid have
marked cytotoxicity on malignant cells. Ascorbate-6-
palmitate and ascorbate-6-stearate, the fatty acid esters of

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ascorbic acid were found to be more potent inhibitors of
growth of murine leukemia cells compared to ascorbate 2-
phosphate, ascorbate 6-phosphate and or ascorbate 6-sul-
fate respectively [75].
Among ascorbic acid derivatives, fatty acid esters of ascor-
bic acid viz., ascorbyl palmitate and ascorbyl stearate have
attracted considerable interest as anticancer compounds
in view of their lipophilic nature as they can easily cross
cell membranes and blood brain barrier [76]. Ascorbic
acid and ascorbyl esters have been shown to inhibit the
proliferation of mouse glioma and human brain tumor
cells viz., glioma (U-373) and glioblastoma (T98G) cells
and renal carcinoma cells [77–79]. Ascorbyl stearate was
found to be more potent than sodium ascorbate in inhib-
iting proliferation of human glioblastoma cells [80].
Ascorbyl-6-O-palmitate and ascorbyl-2-O-phosphate-6-
O-palmitate also showed anti-metastatic effect by inhibit-
ing invasion of human fibrosarcoma HT-1080 cells
through matrigel and pulmonary metastasis of mouse
melanoma model systems [81].
Numerous reports are available in literature on cytotoxic
and anti-carcinogenic effect of ascorbic acid and its deriv-
atives in different tumor model systems. However, the
molecular mechanisms underlying the anti-carcinogenic
potential of ascorbic acid are not completely elucidated.
Recently, Naidu et al [80] demonstrated that ascorbyl
stearate inhibited cell proliferation by interfering with cell
cycle, reversed the phenotype and induced apoptosis by
modulation of insulin-like growth factor 1-receptor
expression in human brain tumor glioblastoma (T98G)
cells. They also studied the effect of ascorbyl stearate on
cell proliferation, cell cycle, apoptosis and signal trans-
duction in a panel of human ovarian and pancreatic can-
cer cells. Treatment with ascorbyl stearate resulted in
concentration-dependent inhibition of cell proliferation
and also clonogenicity of ovarian/ pancreatic cancer cells
[82,83]. The anti-proliferative effect was found to be due
to the arrest of cells in S/G2-M phase of cell cycle, with
increased fraction of apoptotic cells. The cell cycle pertur-
bations were found to be associated with ascorbyl stearate
induced reduction in the expression and phosphorylation
of IGF-I receptor, while the expression of EGFR and
PDGFR remained unchanged. These changes were also
associated with activated ERK1/2 but late reduction in
AKT phosphorylation. Overexpression of IGF-I receptor in
OVCAR-3 cells had no protective effect, however ectopic
expression of a constitutively active AKT2 did offer protec-
tion from the cytotoxic effects of ascorbyl stearate. In con-
clusion, ascorbyl stearate-induced anti-proliferative and
apoptotic effects in ovarian cancer were found to be medi-
ated through cell cycle arrest and modulation of the IGF-
IR and PI3K/AKT2 survival pathways [83].
A plethora of epidemiological studies were carried out to
find out the association of ascorbic acid with various types
of cancers including breast, esophageal, lung, gastric, pan-
creatic, colorectal, prostate, cervical and ovarian cancer
etc. The results were found to be inconclusive in most
types of cancers except gastric cancer [84]. One of the most
consistent epidemiological findings on vitamin C has
been an association with high intake of ascorbic acid or
vitamin C rich foods and reduced risk of stomach cancer.
Considerable biochemical and physiological evidence
suggests that ascorbic acid functions as a free radical scav-
enger and inhibit the formation of potentially carcino-
genic N-nitroso compounds from nitrates, nitrite in
stomach and thus offer protection against stomach cancer
[85–87].
Low intake of ascorbic acid and other vitamins was asso-
ciated with an increased risk of cervical cancer in two of
three studies reported [88–91]. This relationship needs
further study because the results suggest that other nutri-
ents including vitamin E, carotenoids, retinoic acid either
individually or in synergy with ascorbic acid may impart a
protective effect against various cancers. Current evi-
dences suggest that vitamin C alone may not be sufficient
as an intervention in the treatment of most active cancers,
as it appears to be preventive than curative. However, vita-
min C supplementation has shown to improve the quality
of life and extend longevity in cancer patients, hence it
could be considered as an adjuvant in cancer therapy.
Dehydroascorbic acid, the oxidized form of ascorbic acid
was shown to cross the blood brain barrier by means of
facilitative transport and was suggested to offer neuropro-
tection against cerebral ischemia by augmenting antioxi-
dant levels of brain [92].
Controversies on health benefits of Ascorbic
acid
Does ascorbic acid acts as antioxidant or pro-oxidant?
Vitamin C is an important dietary antioxidant, it signifi-
cantly decreases the adverse effect of reactive species such
as reactive oxygen and nitrogen species that can cause oxi-
dative damage to macromolecules such as lipids, DNA
and proteins which are implicated in chronic diseases
including cardiovascular disease, stroke, cancer, neurode-
generative diseases and cataractogenesis [93].
As shown in Table 2, ascorbic acid is a potent water solu-
ble antioxidant capable of scavenging/ neutralizing an
array of reactive oxygen species viz., hydroxyl, alkoxyl,
peroxyl, superoxide anion, hydroperoxyl radicals and
reactive nitrogen radicals such as nitrogen dioxide, nitrox-
ide, peroxynitrite at very low concentrations [15]. In addi-
tion ascorbic acid can regenerate other antioxidants such
as α-tocopheroxyl, urate and β-carotene radical cation

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from their radical species [94]. Thus, ascorbic acid acts as
co-antioxidant for α-tocopherol by converting α-tocophe-
roxyl radical to α-tocopherol and helps to prevent the α-
tocopheroxyl radical mediated peroxidation reactions
[95].
AH
-
+ Fe
3+
- - - → A
• -
+ Fe
2+
+ H
+
AH
-
+ Cu
2+
- - - → A
• -
+ Cu
+
+ H
+
H
2
O
2
+ Fe
2+
- - - → HO
•
+ Fe
3+
+
-
OH
H
2
O
2
+ Cu
+
- - - → HO
•
+ Cu
2+
+
-
OH
LOOH + Fe
2+
- - - → LO• + Fe
3+
+
-
OH
LOOH + Cu
+
- - - → LO + Cu
+
+
-
OH
HO
•
, LO
•
- - - → Lipid peroxidation
Adapted from Carr and Frei [15]
These radical species are highly reactive and can trigger
lipid peroxidation reactions. Thus the question arises
whether vitamin C acts as a pro-oxidant in in vivo condi-
tions? The answer appears to be "no" as though these reac-
tions occur readily in vitro, its relevance in in vivo has been
a matter of debate concerning ready availability of catalyt-
ically active free metal ions in vivo [94]. In biological sys-
tems, iron is not freely available, but it is bound to
proteins like transferrin, hemoglobin and ferretin. Mobi-
lization of iron from these biomolecules may be required
before it can catalyze lipid peroxidation. Further, the con-
centration of free metal ions in in vivo is thought to be very
low as iron and other metals are sequestered by various
metal binding proteins [94]. Another factor that may
affect pro-oxidant vs antioxidant property of ascorbic acid
is its concentration. The in vitro data suggest that at low
concentrations ascorbic acid act as a pro-oxidant, but as
an antioxidant at higher levels [96]. Moreover, a recent
report demonstrated that large doses of exogenous iron
(200 mg) and ascorbic acid (75 mg) promote the release
of iron from iron binding proteins and also enhance in
vitro lipid peroxidation in serum of guinea pigs. This find-
ing supports the hypothesis that high intake of iron along
with ascorbic acid could increase in vivo lipid peroxidation
of LDL and therefore could increase risk of atherosclerosis
[52]. However, another study demonstrated that in iron-
overloaded plasma, ascorbic acid acts as an antioxidant
and prevent oxidative damage to lipids in vivo [97].
Is ascorbic acid harmful to cancer patients?
Agus et al [98] have reported that the tumor cells contain
large amounts of ascorbic acid, although the role of ascor-
bic acid in tumors is not yet known. They have established
that vitamin C enters through the facilitative glucose
transporters (GLUTs) in the form of dehydroascorbic acid,
which is then reduced intracellularly and retained as
ascorbic acid. It is speculated that high levels of ascorbic
acid in cancer cells may interfere with chemotherapy or
radiation therapy since these therapies induce cell death
by oxidative mechanism. Thus, ascorbic acid supplemen-
tation might make cancer treatment less effective because,
ascorbic acid being a strong antioxidant may scavenge or
neutralize the oxidative stress induced by chemotherapy
in cancer patients. However, more studies are needed to
understand the role of ascorbic acid in tumors cells and
the speculative contraindication of ascorbic acid for can-
cer chemotherapy.
Does ascorbic acid cause cancer ?
Recently, it has been reported that lipid hydroperoxide
can react with ascorbic acid to form products that could
potentially damage DNA, suggesting that it may form gen-
otoxic metabolites from lipid hydroperoxides implicating
that ascorbic acid may enhance mutagenesis and risk of
cancer. Lee et al [99], demonstrated that ascorbic acid
induces decomposition of lipid hydroperoxide (13-(S)-
hydroperoxy-(Z,E)-9,11-octadecadienoic acid;(13-
HPODE) in presence of transition metals to DNA-reactive
bifunctional electro-philes namely 4-oxo-2-nonenal, 4,5-
epoxy-2(E)-decenal and 4-hydroxy-2-nonenal. 4-oxo-2-
nonenal being a genotoxin can react with DNA bases to
form mutations [100] or apoptosis [101].
Thus, the above process can give rise to substantial
amounts of DNA damage in vivo. However there are many
questions, which need to be considered before we accept
the hypothesis that ascorbic acid can cause cancer by pro-
ducing genotoxic metabolites from lipids. The hydroper-
oxides formed through lipid peroxidation reaction are
rapidly reduced to aldehydes by a number of enzymes.
Further, ascorbic acid being a strong antioxidant effec-
tively inhibits the formation of lipid peroxides as ascorbic
acid forms the first line of antioxidant defense mechanism
in human plasma. The formation of lipid hydroperoxides
occur only after ascorbic acid has been exhausted. Hence,
interaction of ascorbic acid and hydroperoxide may not
arise in human plasma. Recently, high intracellular vita-
min C was reported to prevent oxidation-induced muta-
tions in human cells [102]. Thus, the physiological
relevance of these results is yet to be established in in vivo
experiments.
Ascorbic acid -HPODE DNA reactive electrophiles
-oxo-2-n
+→13
4oonenal
4,5-epoxy-2-decenal
4-hydroxy-nonenal
DNA damage






Nutrition Journal 2003, 2 http://www.nutritionj.com/content/2/1/7
Page 8 of 10
(page number not for citation purposes)
Conclusion
Ascorbic acid is one of the important and essential vita-
mins for human health. It is needed for many physiologi-
cal functions in human biology. Fresh fruits, vegetables
and also synthetic tablets supplement the ascorbic acid
requirement of the body. However, stress, smoking, infec-
tions and burns deplete the ascorbic acid reserves in the
body and demands higher doses of ascorbic acid supple-
mentation. Based on available biochemical, clinical and
epidemiological studies, the current RDA for ascorbic acid
is suggested to be 100–120 mg/day to achieve cellular sat-
uration and optimum risk reduction of heart diseases,
stroke and cancer in healthy individuals. In view of its
antioxidant property, ascorbic acid and its derivatives are
widely used as preservatives in food industry. Many health
benefits have been attributed to ascorbic acid namely anti-
oxidant, anti-atherogenic and anti-carcinogenic activity.
Lately some of these beneficial effects of ascorbic acid are
contradicted. The relation between ascorbic acid and can-
cer is still a debatable as the molecular mechanism under-
lying anti-carcinogenic activity of ascorbic acid is not
clearly elucidated. Regarding the pro-oxidant activity of
vitamin C in presence of iron, there is compelling evi-
dence for antioxidant protection of lipids by ascorbic acid
both with and without iron co-supplementation in ani-
mals and humans. Current evidences also suggest that
ascorbic acid protects against atherogenesis by inhibiting
LDL oxidation. The data on vitamin C and DNA damage
are conflicting and inconsistent. However, more mecha-
nistic and human in vivo studies are warranted to establish
the beneficial claims on ascorbic acid. Thus, though ascor-
bic acid was discovered in 17
th
century, the role of this
important vitamin in human health and disease still
remains a mystery in view of many beneficial claims and
contradictions.
Acknowledgements
Author gratefully acknowledges Dr.S.G.Bhat, Head, Department of Bio-
chemistry and Nutrition and Dr.V.Prakash, Director, CFTRI, Mysore for
their encouragement in preparing this review. The author also acknowl-
edges Dr.Santo V.Nicosia and Dr. D.Coppola, Moffitt Cancer Research
Center, University of South Florida, Tampa, FL, USA for supporting the
work on ascorbyl stearate in his laboratory.
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Fruits mg/100 g edible portion
Banana 8–16
Apple 3–30
Mango 10–15
Pineapple 15–25
Cherry 15–30
Papaya 39
Orange 30–50
Grape fruit 30–70
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-1
s
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Reactive oxygen species
Hydroxyl radical 1.1 × 10
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Alkoxyl radical 1.6 × 10
9
Peroxy radical 1.2 × 10
6
Superoxide anion/ hydroperoxy radical 1.0 × 10
5
Reactive nitrogen species
Dinitrogen trioxide/dinitrogen tetroxide 1.2 × 10
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Peroxynitrite/peroxynitrous acid 235
Antioxidant derived radicals
Alpha-tocopherol radical 2 × 10
5
Urate radical 1 × 10
6
Thiyl/sulphenyl radical 6 × 10
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