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African Journal of Food, Agriculture, Nutrition and Development
Rural Outreach Program
ISSN: 1684-5358 EISSN: 1684-5374
Vol. 5, Num. 1, 2005
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African Journal of Food Agriculture Nutrition and Development, Vol. 5, No. 1 , 2005
PEER REVIEWED ARTICLE
ROLE OF VITAMIN C (ASCORBIC ACID) ON HUMAN HEALTH-
A REVIEW
Walingo KM*
*Corresponding Author *Dr Mary Khakoni Walingo, School Of Consumer
Sciences & Tech, Maseno University, Box 333, Maseno, Kenya. e-Mail: marywalingo@yahoo.com, maelo@maseno.ac.ke
Code Number: nd05011
ABSTRACT
Vitamin C, also known as ascorbic
acid, abounds in nature and is highly labile. It is a water-soluble vitamin
that is lost in large amounts during food processing. It is a vitamin whose
prescribed requirement across cultures is not uniform. For example , the
prescribed requirement of vitamin C in Great Britain is 30mg/day, while in
the U.S.A., it is 60mg/day and 100mg/day in Japan. These variations are
unusual and point to the need for further research to establish the
acceptable RDAs for diverse populations. The RDA for vitamin C should be more
than the amount needed to prevent the occurrence of disease. Vitamin C plays
significant functions in the body that enhance its role in the health status
of the human body. The biochemical functions of vitamin C include:
stimulation of certain enzymes, collagen biosynthesis, hormonal activation,
antioxidant, detoxification of histamine, phagocytic functions of leukocytes,
formation of nitrosamine, and proline hydroxylation amongst others. These
functions are related to the health effects of vitamin C status in an
individual. In human health, vitamin C has been associated with reduction of
incidence of cancer, blood pressure, immunity, and drug metabolism and
urinary hydroxyproline excretion, tissue regeneration. This vitamin is needed
for the proper metabolism of drugs in the body through adequate hepatic mixed
function oxidase system. Epidemiological data have revealed the preventive
and curative role of vitamin C on certain disease conditions in the body
though controversies still persist. Vitamin C is effective in protecting
against oxidative damage in tissues and also suppresses formation of
carcinogens like nitrosamines. There is an inverse relationship with blood
pressure and both plasma vitamin C and Vitamin C. Vitamin C has a lowering
effect on blood pressure, especially on systolic pressure more than a
diastolic pressure. Low levels of plasma vitamin C are associated with stroke
and with an increased risk of all cause mortality. Increased consumption of
ascorbic acid raises serum ascorbic levels and could decrease the risk of
death.
KEY WORDS: Vitamin C, cancer, blood pressure, drug
metabolism, immunity
RÉSUMÉ
La vitamine C, appelée également acide ascorbique, abonde
en nature et elle est fortement labile. C’est une vitamine soluble dans
l’eau qui est perdue en grandes quantités pendant le traitement des
produits alimentaires. Il s’agit d’une vitamine dont la prise
prescrite dans les cultures n’est pas uniforme. Par exemple, la prise
de la vitamine C prescrite en Grande-Bretagne est de 30mg/jour, tandis
qu’aux Etats-Unis elle est de 60mg/jour et de 100mg/jour au Japon. Ces
variations sont inhabituelles et elles indiquent la nécessité d’une
recherche plus approfondie en vue d’établir les AQR (Apports
Quotidiennement Recommandés) acceptables pour diverses populations. Les AQR
en vitamine C devraient être supérieurs à la quantité requise pour prévenir
l’apparition de maladies. La vitamine C joue dans le corps des
fonctions considérables qui augmentent son rôle dans l’état de santé du
corps humain. Les fonctions biochimiques de la vitamine C sont notamment la
stimulation de certains enzymes, la biosynthèse des collagènes,
l’activation des hormones, des antioxydants, la détoxification de
l’histamine, des fonctions des phagocytes, des leucocytes, la formation
de la nitrosamine, et l’hydroxylation proline, entre autres. Ces
fonctions ont trait aux effets sanitaires de la teneur en vitamine C dans un
individu. En matière de santé humaine, la vitamine C a été associée à la
réduction de l’incidence du cancer et de la tension artérielle, à
l’immunité, au métabolisme des médicaments et à l’excrétion
hydroxyproline urinaire, ainsi qu’à la régénération des tissus. Cette
vitamine est nécessaire pour un bon métabolisme des médicaments dans le corps
par un système d’oxydase de fonctions mixtes hépatiques adéquates. Des
données épidémiologiques ont révélé le rôle préventif et curatif de la
vitamine C sur certaines conditions de maladies dans le corps bien que des
controverses persistent encore. La vitamine C est efficace dans la protection
contre des lésions oxydatives dans les tissus; elle supprime également la
formation de cancérigènes tels que les nitrosamines. Il existe un rapport
inverse entre la tension artérielle et la vitamine C du plasma et la vitamine
C. La vitamine C a un effet de réduction sur la tension artérielle, plus
particulièrement sur la pression systolique que sur la pression diastolique.
Les niveaux peu élevés de la vitamine C du plasma sont associés à la
congestion cérébrale et à un risque élevé de toutes les causes de la
mortalité. Une consommation élevée d’acide ascorbique augmente les
niveaux ascorbiques du sérum et pourrait réduire les dangers de mort.
Mots-clés: Vitamine C, cancer, tension artérielle,
métabolisme des médicaments, immunité.
INTRODUCTION
Ascorbic acid, commonly known as vitamin C plays
significant functions in the human body, though its function at the cellular
level is not very clear. Vitamin C is needed for collagen synthesis, the
protein that serves so many connective functions in the body. Among the
body’s collagen-containing materials and structures are the framework
of bone, gums and binding materials in skin muscle or scar tissue. Production
of certain hormones and of neurotransmitters and the metabolism of some amino
acids and vitamins require vitamin C. This vitamin also helps the liver in
the detoxification of toxic substances in the system, and the blood in
fighting infections. Ascorbic acid is important in the proper function of the
immune system. As an antioxidant, it reacts with compounds like histamines
and peroxides to reduce inflammatory symptoms. Its antioxidant property is
associated with the reduction of cancer incidences [1, 2].
The requirement for vitamin C for adults does not seem to
be uniform across cultures. This may be a pointer to the need for
cultural-specific requirements for the nutrient. It has now been appreciated
that ascorbic acid functions in cellular reactions and processes.
Epidemiological data now point to reduction of colds with increased
consumption of foods rich in vitamin C [3]. People are tempted to
over-consume vitamin C because of its health benefits. Miniscule increases in
blood vitamin C levels decrease the risk of death from all conditions [4].
This paper presents a review of epidemiological data on the role of ascorbic
acid in certain human health conditions. Though there are many functions of
vitamin C, its role in health is discussed in relation to cancer, blood
pressure, immunity, drug metabolism and urinary hydroxyproline excretion.
Vitamin C and Cancer
Cancer is a global public health problem with increased
mortality levels. Though there are many types of cancers, the review presents
a general picture of the role of ascorbic acid in cancer. Vitamin C is
effective in protecting against oxidative damage in tissues, and also
suppresses formation of carcinogens like nitrosamines [4]. The WHO estimated
an increase of over 10 million new cases of cancer. Despite progress against
some rare forms of cancer, the overall death rate has increased [5]. Though
Vitamin C is cytotoxic to tumor cells but non-toxic to normal cells,
conventional medicine favors more powerful and toxic chemotherapeutic agents.
Many studies have shown that vitamin C intake is inversely related to cancer,
with protective effects shown for cancer of the lung, breast, pancreas,
stomach, cervix, rectum and oral cavity [6]. The authors found that the
oxidation of guanine, a purine in DNA was significantly reduced after vitamin
C supplementation, but the oxidation of adenine also a purine was
significantly elevated. The reduced level in the oxidation of guanine
suggests that vitamin C acts as an antioxidant [5]. Extensive in vitro
and in vivo vitamin C tests to determine its ability to prevent the
adverse effects of, decrease the resistance to and increase the effects of
chemotherapeutic have been conducted [7]. Combination of vitamin C and
vitamin K given prior to chemotherapy increased survival and the effects of
several chemotherapeutic agents in a murine ascitic living tumor model [8].
Vitamin C has also been shown to be safe to be used concurrently with
radiation [9]. Administration of Vitamin A, β-carotene, E and C can
reduce the incidence, and delay the progression of various cancers such as
skin, those of the colon, stomach, esophagus, mammary glands and bladder [1,
4, 10, 11]. Epidemiological studies have revealed an inverse relationship
between the intake of vitamins A, β-carotene, C and E, and the incidence
of different cancers in humans. There is a decrease in cancer incidence and
risk, in population with high content of these vitamins in the plasma.
Carcinogenesis is related to cell differentiation, progression and
metabolism, and collagen synthesis.
The basic mechanism for carcinogenesis is cell
differentiation, because cancer develops when there is loss of cell
differentiation. Vitamins A, β-carotene, E and C have a profound
influence on cell growth and differentiation. Vitamin C is a strong
antioxidant that acts synergistically with Vitamin E as scavengers for free
radicals in the body system, which are carcinogenic. Ascorbic acid, as sodium
ascorbate exerts marked cytotoxic effects on several mammalian cells in
culture. These effects are dose dependent. For example, Lupulescus [2]
reported that vitamin C (upto 200 ug/ml) did not cause any morphological
changes in mouse melanoma, mouse neuroblastoma, rat glioma and mouse
fibroblasts in culture. At high concentrations (500-1000 ug/ml), vitamin C
was lethal for neuroblastoma cells. Cytotoxic effects are also cell
dependent, being stronger in human melanoma as compared to mouse melanoma
cells. It has been suggested that vitamin C induced cytotoxicity is mediated
primarily by hydrogen peroxide formation at cell surface, or thin cells.
Cytotoxic activity can also be mediated by presence of cupric ions
(Cu2+) in malignant melanoma cells that react with vitamin C and
generate free radicals in solution. Vitamin C also reverses the chemically
transformed cells to thin normal phenotype.
Ultra structure and cell surface studies of cancer cells
following vitamin C administration revealed observed cytolysis, cell membrane
disruption, mitochondrial alterations, nuclear and nucleolar reduction, and
increased phagolysosome formation in cancer cells following vitamin C
administration [10]. Cell surface changes such as cytolysis, increased
collagen synthesis, and cell membrane disorganization were predominant, and
increased apoptotic and phagocytic activity were also seen. Quantitative
estimation of cell organelles revealed that vitamin C affects the
intracellular organelle distribution, playing an important role in
cyto-differentiation of cell cancer [1, 10]. Hence, vitamins A, E and C
markedly affect the cancer cells differentiation by exerting direct cytotoxic
effects, modifying membrane biogenesis, light and gap junction formation,
Golgi’s complex, autophagic and apoptotic activity, cell surface
changes and sometimes reversing transformed cells to their normal cell type.
This reversal is necessary in the reduction of possible cancer incidences.
Changes of DNA, RNA and protein synthesis have been
associated with cell differentiation and proliferation. The mechanism of
action, however, is not fully understood. It has been suggested that most of
the metabolic effects are mediated by transcription and translation processes
at the genome level. Further, vitamins A, E and C modulate DNA synthesis and
gene expression in a similar manner to that of steroid hormones. These
vitamins may affect chemical mutagenicity and cellular status [2]. By
affecting DNA, RNA and proteins at specific sites that are targets of
electrophiles, the vitamins can control cell replication rate, leading to an
altered cell rearranged codons and translocation of specific genes, or
oncogenes. The vitamins A, E and C affect differently DNA, RNA and protein
synthesis in cancer cells. Administration of vitamin C decreases DNA
synthesis in the nuclei, RNA synthesis in the nucleoli, and proteins in
cytoplasm of cancer cells. This inhibition of DNA, RNA and protein synthesis
is accompanied by advanced ultra structural and cell surface changes, and
will decrease progression of cancer.
Vitamin C plays an important role in collagen synthesis.
Collagen is an extracellular protein synthesized intracellularly as
tropocollagen by the fibroblasts, and contains amino acids prolines, glycine
and lysine. The major reaction in collagen synthesis is hydroxylation of
proline to 4-hydroxyproline. This provides proper structural configuration
and stability of collagen. The enzyme 4-hydroxylase, which is a tetramer,
catalyzes hydroxylation of proline and its genes are differently located in
human chromosomes. In some experiments ascorbic acid and its analogues
specifically enhanced the collagen synthesis almost four-fold in cultured
human skin fibroblasts, by increasing the incorporation of (3H-) proline into
colagenase sensitive proteins. L-ascorbate was found to be the most active
compound, [2]. It is also possible that ascorbic acid can stimulate collagen
synthesis by other mechanisms, independent of hydroxylation. Ascorbic acid
induces lipid peroxidation and reactive aldehydes, a step required for
collagen expression, collagen m-RNA levels, and collagen production in
cultured human fibroblasts. Collagen gene expression is probably influenced
by lipid perodixation, or through acetaldehyde formation, which consequently
increases collagen gene transcription in cultured human fibroblasts. The
mechanism by which vitamin C stimulates collagen gene expression is not very
clear. Ascorbic acid can, therefore, stimulate collagen synthesis independent
of proline hydroxylation by inducing lipid peroxidation. Ascorbic acid is
thus the major vitamin regulating collagen synthesis, and provides stability
to the procollagen single helix [1, 2]
Blood Pressure
Elevated blood pressure is a powerful determination of
cerebro-vascular and coronary disease. Great efforts have been made towards
the detection and treatment of hypertension since the 1960s. Some of the
observed decrease in cardiovascular disease over the period has been
attributed to better control of blood pressure among diagnosed hypertensives.
The importance of nutrition in control of blood pressure is well documented.
Obesity, dietary sodium and alcohol are associated with lower or higher blood
pressure. Higher intake of polyunsaturated fatty acids and magnesium have
also been linked with lower blood pressure [12].
Inverse associations with blood pressure and both plasma
vitamin C and vitamin C have been reported. A study of the relationship
between fasting vitamin C and cerebrovascular disease risk factors revealed
reduction in hypertension in men. Men with serum vitamin C of 0.5 mg/dl had a
mean systolic pressure of 122 mmHg compared with a mean of 113 mmHg for men
with serum vitamin C of 0.9 mg/dl, a relative difference of -7%. The same men
had a similar difference in diastolic pressure of 78 mmHg vs. 73 mmHg –
6% relative difference. Prevalence of hypertension was 7.5% in the low serum
vitamin C group versus 1.0% among those with high levels of vitamin C [13].
An investigation to identify the association between BP plasma antioxidant
vitamin revealed a significant trend for both systolic and diastolic pressure
over qualities of plasma vitamin C with a – 5% difference in systolic
pressure and a –4% difference in diastolic BP between highest and
lowest quintiles of plasma vitamin C [14].
Similar significant negative trends between plasma vitamin
C and both systolic and diastolic BP were observed in Chinese-American males
and females aged 60-96 years who were not taking anti-hypertensive
medication. The difference between mean systolic and diastolic BP levels in
the highest and lowest quartiles were –21 mmHg (-14%) and – 8
mmHg (-9%), respectively [15]. Vitamin C appears to have a lowering effect on
systolic pressure, more than it has on diastolic pressure. Other negative
correlation between dietary vitamin C intake and both systolic and diastolic
BP [16] and negative association between BP, and vitamin C intake and use of
vitamin C supplements in Honolulu Heart Study [17] have also been reported.
Diastolic pressure was not associated with vitamin intake. Individuals
consuming vitamin C in adequate quantities are likely to have lower BP. It is
a cheaper form of supplementation for developing communities.
In a randomized placebo-controlled, cross-over trial on a
mixed sample of 12 borderline hypertensives and 8 normotensives, there was
reported a 4.4% drop in systolic BP between the placebo treatment and vitamin
C treatment after four weeks of supplementation with 1 gram of vitamin C
[18]. Vitamin C supplementation did not influence diastolic pressure. A
cross-sectional study of the relationship of vitamin C intake and blood
pressure in the elderly revealed relative differences in systolic and
diastolic BP between subjects consuming 240 mg/day compared to those
consuming 60 mg/day were –6.9% and –6.6%, respectively. The
prevalence of elevated BP (systolic 160 mmHg or diastolic 100 mmHg) was
approximately 50% lower across this range vitamin C intake (60, 60-119,
120-179, 180-239 240mg/day) [19]. Vitamin C thus appears to have a lowering
effect on blood pressure of the elderly.
A prospective population-based cohort study of 2419
randomly selected middle aged men (42-60yrs) with no history of stroke was
conducted to examine whether plasma vitamin C modifies the association
between overweight and hypertension and stroke [20]. Men who had the lowest
levels of plasma vitamin C were at a higher risk of developing any stroke
compared to men who had the highest levels of plasma vitamin C. Hypertensive
men who had the lowest vitamin C levels were at an increased risk for any
stroke and so were overweight men with low plasma vitamin C. Low plasma
vitamin C was associated with increased stroke risk.
An increase in the intake of vitamin C during the dietary
fat reduction period could have accounted for a part of the blood pressure
reduction [14, 15]. Decreased plasma ascorbic acid levels were associated
with reduced plasma concentrations of 6-ketoprostaglandin-F, the metabolite
prostacylin. Thus dietary antioxidants enhance production of prostacyclin by
scavenging free radicals and peroxides, which inhibit prostacylin synthetase
at concentrations above a certain threshold. Thus vitamin C and blood
pressure appear to be related, in that adequate dietary vitamin C could exert
a reducing effect on blood pressure, and especially if dietary fat intake is
reduced.
Vitamin C and Immunity
In stressful situations adrenal glands react by releasing
hormones that trigger the “fight or flight” reaction. It has been
indicated that 200mg of vitamin C a day may reduce the levels of stress
hormones. Stress suppresses the immune system. Mega doses of vitamin C
increase the levels of antibody that fights against germs and viruses in both
stressed and unstressed rats, with greater antibody increase in the
unstressed rats [21]. Stressed animals may need larger vitamin C doses for
proper function of the immune system.
Healing of wounds is characterized by synthesis of connective tissue, the
major component of which is collagen. Ascorbic acid has important biological
and metabolic functions, particularly with respect to its role in the
biosynthesis of connective tissue [22]. Ascorbic acid is required for the
hydroxylation of prolyl and lysl residues during collagen biosynthesis [23,
24, 25]. The supplementation of ascorbic acid is necessary to heal wounds
since ascorbic acid may be oxidized during collagen synthesis [26].
Considerable evidence is available to document an interaction between
vitamin C and phagocytic cells. Cells collected from blood, peritoneal or
alveolan fluids, normally contain high (1-2 ug/mg protein) concentrations of
vitamin C and in other in vitro conditions leukocytes can take up
vitamin C. Neutrophils have been studied in vitro under conditions of
vitamin C deprivation and excess [27, 28, 29]. Neutrophils from guinea pigs
produce H2O2 and kill staphylococci as well as control cells. Ascorbate and
dehydroascorbate are both utilized during phagocytic process [29].
Neutropholis may avoid poisoning themselves by absorbing extra ascorbic acid,
which can neutralize antioxidants.
However, the glycolytic activity did not increase maximally in neutrophils
from ascorbic guinea pigs and the stimulation of NADPH oxidase activity was
depressed [27, 28]. The addition of ascorbate to cultures of normal
macrophages increases the cellular concentrations of cyclic GMP (cGMP) as
well as HMP shunt [30]. Although the in vitro addition of ascorbate in
very large amounts may inhibit H2O2 –
myeloperoxidase-halide activity, it does not alter bacteriocidal capacity of
cells. An increase in the in vitro concentrations of ascorbate
increased phagocytic activity of cultural mouse peritoneal macrophages [31].
The addition of vitamin C cultures of neutrophil or macrophages increased
their motility and chemotactic activity.
A defect in which a failure of the umbilicus to separate in newborn
infants led to severe local and disseminated bacterial infection [32]. Two
surviving had decreased neutrophil mobility that improved with the oral
administration of ascorbic acid in doses of 200-800 mg/day, as well as in
vitro addition of vitamin C to defective neutrophils. The ability of human
neutrophils to kill Candida albicans occurs best at physiological in vitro
concentrations of L-ascorbic acid. The locomotor functions, for example.
random motion and chemotactic migration of neutrophils and macrophages are
impaired in the absence of vitamin C.
Vitamin C may influence the ability of certain cell lines to produce
interferon when approximately stimulated. The addition of ascorbic acid to
cultures of human embryo skin or to fibroblasts led to induction of
interferon. Vitamin C appears to be required by thymus for the maintenance of
certain cells, possibly certain reticular cells that are concerned with the
production of thymic humoral factor. Thymic content of dehydroascorbate
decreased in direct proportion to vitamin C. The hormone activity of thymic
extracts correlated with thymic ascorbate and inversely with dehydroascorbate
[33]. A prospective controlled study of students (18-32 years) in a technical
training facility was conducted to assess the effect megadose vitamin C in
preventing and relieving cold and flu symptoms [3]. The numbers of reports of
cold and flu symptoms were recorded for both the test and control groups.
Students in the test group were treated with hourly doses of 100 mg of
vitamin C for the first six hours and then three times thereafter while those
in the control group were treated with pain relievers and decongestants. The
reported cold and flu symptoms in the test group decreased by 85% more than
the control group after the administration of the megadose vitamin C.
Drug Metabolism
Vitamin C functions as a cofactor in hydroxylation
reactions of collagen and catecholamine synthesis. It appears to be involved
in a wide range of processes including macrophage function, carnitine
synthesis and drug metabolism. Studies in guinea pigs have demonstrated that
vitamin C deficiency alters the concentration and activity of components of
the hepatic mixed function oxidase system and impairs the metabolism of a
wide range of drugs. Much less is known about the effects of different
vitamin C intakes on drug metabolism in humans. The few studies that have
been conducted have not always been properly controlled and have yielded
conflicting results [34, 35]. Discrepancies between studies may be due to the
different drugs used and the extent of depletion achieved. This calls for the
need to conduct research of vitamin C on particular drugs which could be
financially and resourcefully expensive, though may clarify the role of
vitamin C in drug metabolism.
Nutritional deficiencies have been shown to affect the
kinetics of many drugs directly by altering their metabolism. Specifically
vitamin C has been shown to influence the oxidative metabolism of a variety
of drugs, including some oxidative demethylation processes in guinea pigs. A
study of the influence of vitamin C on pharmacokinetics of caffeine in
healthy elderly men reported that short-term alterations in vitamin C intake
do not affect caffeine metabolism in elderly men [36]. In another study of
the effects of vitamin C on the elimination of antipyrine in 14 healthy
adults (24–25 years), added vitamin C had no effect on the
pharmacokinetics of antipyrine [37]. In contrast, there was an increased
metabolic clearance of antipyrine after administration of vitamin C to non
deficient young subjects and vitamin C deficiency elderly subjects [35, 37].
Assessment of the influence of subclinical ascorbic acid deficiency on drug
metabolism revealed that pronounced ascorbic acid deficiency of relatively
short duration does not alter antipyrine metabolism in man [38]. In contrast,
vitamin C has repeatedly been shown to depress drug-metabolizing enzyme
activity in guinea pigs [39, 40]. There is need to understand these effects
in human populations.
Hepatic mixed function oxidase activity was measured in
Gambian men during a period of low seasonal vitamin D intake after
supplementation. Demythylation of methodyphelacetamide (methacetin) was
followed using breath test, in which exhalation of 13C-enriched
CO2 was measured following an oral dose of 13C-methacetin. Vitamin
supplementation, sufficient to increase plasma levels significantly, did not
influence methacetin metabolism. However, methacetin metabolism in
unsupplemented men appeared normal in majority of cases. Hepatic tissue may
not have been sufficiently depleted of vitamin C to impair the activity of
the mixed function oxidase system. There is need to understand the effect of
vitamin C on the mixed function oxidase system in hepatic tissues that are
sufficiently depleted of the vitamin [41].
Urinary Hydroxyproline Excretion
Many clinical problems associated with ascorbic acid
deficiency such as abnormalities of skeleton seen in infantile scurvy,
lesions of gingiva, as well as impairment of wound healing of bony structures
are related to collagen metabolism. Urinary excretion of hydroxyproline is
considered to be a useful indicator of changes in metabolism of collagen
because with the exception of the small amounts of hydroxyproline in elastin
and component Uq (a hydroxy-proline rich in protein) in plasma no other
protein contains a significant amount of this amino acid [42]. Collagen is
much more abundant than these other hydroxyproline-containing proteins and
comprises about one-third of the total body protein, which justifies the use
of hydroxyproline in following changes in collagen turnover. There was
reported an increase in urinary hydroxyproline by an average of 16% and 30%
after first and second depletion periods, and decreased to baseline after
supplementation with normal or high doses of ascorbic acid [43]. They
concluded that urinary hydroxyproline excretion increases during human
vitamin C deficiency but that this effect is not strong to provide a reliable
marker of vitamin C deficiency.
CONCLUSION
Ascorbic acid is needed for various functions in the body,
though its function at the cellular level remains unclear. Ascorbic acid is
needed for collagen synthesis, a production of certain hormones and of
neurotransmitters, metabolism of some amino acids and vitamins,
detoxification of toxic substances in the body, and proper function of the
immune system. The vitamin C requirements are not uniform across cultures,
and hence need for further research in setting appropriate RDAs for vitamin
C.
Epidemiological data is increasing on the effect of
vitamin C in cancer, blood pressure, immunity drug metabolism and urinary
hydroxyproline excretion. Vitamin C is effective in protecting against
oxidative damage in tissues and also suppresses formation of carcinogens like
nitrosamines. High levels of serum ascorbic acid reduce blood pressure,
especially systolic blood pressure. Low plasma vitamin C is associated with
increased risk of stroke. Mega doses of vitamin C increases the levels of an
antibody that fights against infections in both stressed and unstressed rats,
with greater increase in the unstressed rats. Larger doses of vitamin C may
be necessary for proper function of the immune system. Mega doses of vitamin
C decreases the symptoms of cold and flu in humans. Though reports show that
vitamin C influences the oxidative metabolism of a variety of drugs in guinea
pigs, there is need to understand the effects in human populations. The role
of vitamin C in the prevention and control of disease states is encouraging,
since it is a cheaper way of enhancing vitamin C status through diets,
instead of use of supplements that may be quite expensive. Vitamin C abounds
in nature and thus in diets. However, it is highly labile and is destroyed
through the methods of processing employed in food handling from harvesting
through conservation and preparation stages. Principles of nutrient
preservation through all these stages should be carefully applied.
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