Vol.2, No.8, 819-823 (2010)
Copyright © 2010 SciRes. Openly accessible at http://www.scirp.org/journal/HEALTH/
Vitamin C twice a day enhances health
Alfred Roc Ordman
Biochemistry Program, Beloit College, Beloit, USA; Ordman@beloit.edu
Received 15 October 2009; revised 30 November 2009; accepted 3 December 2009.
This review provides information determining
how much vitamin C to take, including analysis
of the recent findings, which demonstrate ad-
vantages and problems with higher daily doses.
The Daily Value for vitamin C was raised to 90
mg for men in 2000, and Upper Limit of 2,000 mg
per day was established as being safe. This
followed a study of urinary excretion of vitamin
C (AA) that demonstrated 500 mg twice a day
provides levels sufficient to cause continuous
urinary excretion in humans. That is the lowest
oral dosage evaluated that significantly satu-
rates blood plasma. A later study endorsed 200
mg from dietary sources for the RDA. Even
though plasma concentration of AA is then sig-
nificantly lower, at 200 mg daily certain white
blood cells are saturated with AA. Recently
many studies have indicated the benefit of high
levels of AA to maintain the brain, bones, and
heart, reduce damage from stroke, brain trauma,
and cataracts, and lower the risk of cancer me-
tastasis and colds. Pro-oxidant activity has
been found only for i.v. adminstration, where
high concentrations appear effective destroying
tumor cells while not harming normal cells. This
review evaluates recent research and finds it
consistent with the hypothesis that people, es-
pecially older people and soldiers in combat
environments, are likely to benefit if they main-
tain the saturating level of AA by taking 500mg
supplements twice a day.
Keywords: Antioxidant; Vitamin C; Ascorbic
Acid; Stroke; Alzheimer’s; Oxidative Stress;
Cardiovascular Disease; Cataracts;
Osteoporosis; Mortality
In 1956, Denham Harman, founder of AGE, proposed
the free radical (ROS) theory of aging [1]. Linus Pauling
proposed megadoses of the water-soluble antioxidant
vitamin C (ascorbic acid, AA) to trap free radicals, re-
commending dosages up to 16 g per day [2]. Harman
and Pauling’s efforts moved the nutrition standards from
levels that merely prevent short-term deficiency diseases
to higher levels that reduce the risk for chronic disease
associated with aging. For nutrition studies to identify
risk for chronic disease, it is helpful to identify bio-
markers that reflect disease-generating processes. For
instance, oxidized DNA resulting from free radical
damage may be a biomarker for future cancer.
Pauling based his recommendation for megadoses on the
evolutionary development of an AA requirement in peo-
ple. The first biomarker for long-term optimal dosage
was published in 1994. King et al. [3] discovered that
500 mg of AA taken orally every 12 hr are sufficient to
provide continuous excretion of excess AA into the urine.
In 1996, a similar study by Levine M et al. [4] confirmed
those results, showing that that dosage provides the
highest statistically significant concentration in plasma
for protection from free radical damage. But they chose
a different biomarker, based on the plasma AA level
necessary to saturate certain white blood cells. The
choice of that biomarker led to their recommendation of
only 200 mg AA. That is also a level that does not re-
quire supplements for people whose food selections are
rich in AA. However, neither of these studies evaluated
tissue concentrations, particularly in the brain and eyes,
where high levels are likely to have substantial benefits,
as described below. If the higher level Pauling advocated
is useful, then 500 mg twice a day provides the highest
statistically significant level possible by oral dosing.
SAFETY: There are periodic reports in the media that
taking large amounts of AA over long periods of time
may be harmful. A potential AA hazard was widely re-
ported after a paper in Nature claimed that elevated AA
acted as a pro-oxidant, causing DNA mutations [5]. The
popular press gave much less publicity to articles re-
porting that that study was seriously flawed because it
A. R. Ordman / HEALTH 2 (2010) 819-823
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used obsolete methods [6], and that AA prevents DNA
damage [7]. It had already been reported that AA pre-
vents damage to DNA in sperm [8].
Even doses of 10 g/day in adults have not been re-
ported to cause harm in confirmed scientific studies. The
Food and Nutrition Board [9] of the Institute of Medi-
cine raised the RDA for AA for men and women to 90
and 75 mg resp., with an Upper Limit (UL) for safety
established at 2 g per day. The UL was based on osmotic
diarrhea and gastrointestinal disturbances. The Panel on
dietary antioxidants and related compounds stated that
the in vivo data do not clearly show a relationship be-
tween excess ascorbic acid intake and kidney stone for-
mation, pro-oxidant effects, or excess iron absorption
Another concern is that a high dose of AA distorts re-
sults of tests commonly used to measure the amount of
glucose in urine and blood. Combining oral anticoagu-
lant drugs and excessive amounts of AA can produce
abnormal results in blood-clotting tests (e.g., lowering
prothrombin time).
However, while media reports periodically raise con-
cerns about taking AA and vitamin E supplements, those
reports are based on misinterpretations of peer-reviewed
studies. In response, a major review by numerous au-
thorities, John N. Hathcock, Angelo Azzi, Jeffrey Blum-
berg, Tammy Bray, Annette Dickinson, Balz Frei, Ish-
warlal Jialal, Carol S. Johnston, Frank J. Kelly, Klaus
Kraemer, Lester Packer, Sampath Parthasarathy, Helmut
Sies and Maret G. Traber [11], was titled “Vitamins E
and C are safe across a broad range of intakes”. They
provided numerous human studies justifying the toler-
able upper limit (UL) established by the Food and Nutri-
tion Board, which is 1,000 mg for vitamin E and 2,000
mg for AA.
This review describes recent studies relevant to the
selection of a daily intake of AA for health maintenance.
In addition to serum and white blood cell concentrations
of vitamin C, there are many other tissues in which
maintaining high AA concentrations are important. Some
studies below reflect the metabolic and antioxidant func-
tions of AA in controlling cell differentiation through
nutrition signaling. In a process called nutrient signaling,
described only in the past decade, nutrients can trans-
form metabolic processes as hormones do [12,13]. AA
signals cell differentiation of brain and heart stem cells
described below.
BRAIN: Taking 500 mg of vitamin C twice daily may
lessen Alzheimer’s, stroke, and head trauma damage.
Stroke is the leading cause of disability worldwide [14],
and head trauma is common in war veterans. Several
studies reveal the value of elevated levels of AA for pro-
tecting brain function. First, in humans with Alzheimer’s
and mild cognitive impairment, tissues and biofluids
show evidence of oxidative stress [15]. Cognitive de-
cline in aging dogs is lessened simply by adding anti-
oxidants to the diet. The deposition of amyloid-beta is
decreased [16]. A review of 300 articles by Frank and
Gupta [17] concludes that the value of AA is often am-
biguous, but can be recommended based upon 1) epide-
miological evidence, 2) known benefits for prevention of
other maladies, and 3) benign nature of the substance.
AA is a water-soluble antioxidant that crosses the
blood-brain barrier. AA is produced in the liver of most
mammals. Hibernating animals naturally store high
concentrations of AA in the brain for protection from the
metabolic stress that accompanies arousal [18]. In hu-
mans, brain stroke damage continues when blood flow
resumes. AA substantially prevented this reperfusion
injury [19]. Polidori et al. [20] show that AA is much
lower in plasma for those with head trauma or intracran-
ial hemorrhage compared to healthy subjects, while
other antioxidants such as vitamin E are unaffected.
Even at a dose of 200 mg AA/day, ischemic stroke-
related lipid peroxidation decreased significantly in hu-
mans [21].
Via nutrient signaling, AA may exert effects on brain
maintenance and recovery. How neural progenitor cells
(NPCs) differentiate is determined by the redox state of
the brain [22]. In the reducing environment produced by
AA, NPCs become neurons. Under oxidizing conditions,
astrocytes are formed. Prozorovski et al. [23] conclude
that nontoxic manipulation of redox conditions in the
brain influences NPC fate to produce neurons. People
are able to generate new neurons throughout their entire
lives [24]. High AA concentrations in the brain maintain
the potential to generate new neurons.
HEART: The benefits of AA for the circulatory system
include improved circulation and heart health. AA re-
verses the endothelial dysfunction caused by oxidative
stress [25]. As with brain stem cells mentioned above,
sufficient AA causes embryonic stem cells to differenti-
ate into cardiac myocytes [26].
PLASMA: Heinz bodies reflect ROS damage to red
blood cells (RBCs). Johnston and Cox [27] measured
Heinz bodies in RBCs [28] in college students. They
concluded that the antioxidant protection afforded by
short-term vitamin C supplementation is maximal at the
500-1000 mg dosage. When taken in conjunction with
400 IU of vitamin E, a fat-soluble antioxidant, Moser
et al. [29] concluded that 500 mg of AA taken twice
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daily was superior to 200 mg in reducing Heinz bodies,
while vitamin E with minimal AA prevented protein
carbonyl and TBARS damage.
CATARACTS: A major study in Europe shows that
blood levels of AA above 49 µmol/L were associated
with a 64% reduced odds for cataract [30]. US daily
value for AA provides only about 20 µmol/L, while 500
mg twice daily provides about 75 µmol/L.
BONES: Sahni et al. [31] have shown a correlation
between AA intake and 4 yr change in bone mineral
density among 334 men of mean age 75 in the Framing-
ham Osteoporosis Study. They indicate results had only
borderline significance that may relate to other factors in
fruits and vegetables.
COLDS: Evidence for any benefit of AA for prevent-
ing or treating colds remains controversial. Hemilä [32]
completed a meta-analysis showing that the most influ-
ential reviews contain serious inaccuracies and short-
comings, and suggests this may be a remnant of hostility
to Linus Pauling’s opposition to nuclear weapons. He
found consistent evidence that studies using more than 1
g/d had physiologic effects on colds. However, a later
meta-analysis by Douglas and Hemilä [33] evaluated 55
comparative studies treating colds with oral doses of 200
mg of AA per day, and found at most limited evidence
for any benefit in prevention or treatment of colds. The
inconsistency may relate to the AA dosage. Although
twice-a-day dosing is necessary to maintain elevated
serum AA, I have not been able to locate any studies
conducted with this protocol. Recently Saszuki et al. [34]
reported that the risk of getting a cold decreased in re-
sponse to single doses of AA. In a study over the range
from 50 to 500 mg AA daily, the risk of contracting three
or more colds during a 5 yr period was decreased 66%
by daily intake of 500 mg.
CANCER: Research has demonstrated that mutations
of mitochondria accumulate with age [35]. Each mito-
chondrion generates about 4,000 free radicals per second
[36,37]. In human tumors, mitochondrial DNA muta-
tions occur at high frequency. Ishikawa et al. [38] dem-
onstrated that pretreatment of tumor cells in mice with
free radical scavengers prevents those cells from meta-
stasizing. By prevention of mitochondrial DNA muta-
tions, a mechanism to maintain a high concentration of
antioxidants in mitochondria could reduce the risk of
metastatic tumors.
But at higher plasma levels achieved by intravenous
(iv) doses, AA becomes a pro-oxidant that may be toxic
to cancer cells. In vitro AA killed cancer cells at ex-
tracellular concentrations above 1 mM [39]. Chen et al.
[40] found pharmacologic concentrations of AA caused
prooxidant cytotoxicity toward a variety of tumor cells
in vitro, without adversely affecting normal cells. Ovar-
ian, pancreatic, and glioblastoma tumors established in
mice significantly decreased growth rates in vivo in re-
sponse to parenteral AA administration.
Padayatty et al. [41] demonstrate using hospital vol-
unteers that this concentration can be achieved in vivo,
reaching 13 mM for a 50-g iv dose, 140-fold greater than
can be achieved by oral dosing. A review by Frei and
Lawson [6] also shows many recent studies demonstrat-
ing that millimolar AA by iv infusion will kill cancer
cells but not normal cells.
DEATH: Despite the evidence for mechanisms by
which AA may benefit, there remains doubt about the
benefits of antioxidants in general. A review in JAMA in
2007 by Bjelakovic et al. [42] reviewed all randomized
trials of adults taking antioxidants for the prevention of
several diseases, finally including 68 randomized trials
with 232,606 participants. They found beta-carotene,
vitamin A and vitamin E significantly increased mortal-
ity. Vitamin C had no significant effect on mortality.
DISCUSSION: One goal for a review is to elucidate
pros and cons. However, except for the flawed Podmore
study, I have found no reason for not taking 500 mg of
AA twice a day except inconvenience. Since the UL was
established at 2 g/d, studies have at most questioned the
benefit of AA supplements, and have often demonstrated
both theoretical and practical benefit.
Physicians accept the role of AA as a vitamin for im-
mediate health benefits such as prevention of scurvy.
Ever since Harman’s free radical theory of aging and
anti-nuclear activist Linus Pauling’s proposal to take
megadoses of AA as an ROS scavenger to maintain
long-term health, people have found higher doses con-
troversial. The American Aging Association held a con-
sensus conference in 1996 that concluded a daily dose
from 200 to 1,000 mg per day in divided doses was war-
ranted [43].
The dosages still advocated actively in the literature
are three. They are: 90 mg, found in most diets, provid-
ing the Daily Value, sufficient to prevent scurvy; 200 mg,
the highest intake likely through a conscientious diet,
sufficient to saturate certain WBCs, but substantially
lower than was found necessary in many studies cited in
Table 1, and 500 mg twice a day, which can be obtained
only through oral supplements, the highest dosage nec-
essary and sufficient for oral dosage saturation of plasma
in vivo.
Given the safety of AA supplements, greater education
might help people to alleviate the expense and suffering
that may be caused by inadequate AA intake. However,
the media report erroneous findings, like those of Pod-
more et al. [5] that AA may cause DNA mutations, while
they fail to report the UL for safe consumption of AA.
A. R. Ordman / HEALTH 2 (2010) 819-823
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Openly accessible at
Table 1. Summary of potential damage by oxidative stress and
benefits of aa intakes of 500 mg twice a day to humans.
ROS contribute to Alzheimer’s [15]
AA reduces cognitive decline [16]
AA reduces reperfusion injury [19]
AA reduces damage from head trauma [20]
AA generates new neurons [23]
AA reverses endothelial dysfunction [25]
AA generates new cardiac myocytes [26]
AA (500 mg twice daily) protects plasma [29]
EYES AA reduced cataract odds 64% [30]
COLDS AA (500 mg) reduced frequency of colds by
66% [34]
BONES Highest tertile had less bone loss [31]
AA reduced mutations causing metastases
iv AA reduced tumor growth in mice and did
not harm normal cells in vitro [40]
SAFETY Up to 2,000 mg AA daily is safe [11]
DEATH No significant effect on mortality [42]
As documented above, 500 mg of vitamin C taken every
12 hrs may reduce many major causes of chronic disease
and aging decline, not to mention colds. The safety and
benefit of vitamin C supplements is of critical impor-
tance, especially for those in war zones at risk for brain
trauma or of advancing years at risk for chronic disease
and stroke. People ought to be informed of the safety
and benefit of vitamin C supplements.
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