Vitamin C, also
referred to as ascorbic acid or ascorbate, belongs to the water-soluble
class of vitamins. Humans are one of the few species who lack
the enzyme to convert glucose to vitamin C (13). Ascorbic acid
(AA) is an odorless, white solid having the chemical formula
C6H8O6. The vitamin is easily oxidized to form dehydroascorbic
acid (DHAA), and thus oxidation is readily reversible. Vitamin
C is a generic name for all compounds that exhibit the same biologic
activity as AA. Consequently, the term includes both AA and DHAA.
The importance of vitamin C was first was discovered in 1747.
During the 16th century, numerous sea voyagers died due to the
disease known as scurvy. James Lind found that men suffering
from scurvy were cured when given oranges and lemons and he published
his findings in the Treatise of the Scurvy in 1753. He developed
a hypothesis based upon the results he observed; although his
ideas were incorrect, he was the first person to understand the
importance of what would later be called vitamin C. These findings
were not widely accepted by the rest of the world and scurvy
continued to lead to wide spread death throughout the 19th century
(17). Finally, in 1907 scurvy was induced in lab animals and
this opened a new opportunity to understand the disease. Around
1930, two scientists working independently isolated and published
their findings on vitamin C. The men found that vitamin C prevented
and treated scurvy. The term ascorbic acid was adopted to describe
its ability to prevent scurvy. The vitamin was then synthesized
in the laboratory during 1933 (5).
A wide variety
of food that exists contains vitamin C. A well-balanced diet
easily obtains the DRI for vitamin C. It is widely known by the
general public today that the best sources of vitamin C are citrus
fruits and their juices. Fruits with a high vitamin C content
include, but are not limited to oranges, lemons, peaches, strawberries,
bananas and grapefruit. A wide variety of other foods also contain
sufficient quantities of vitamin C. Cabbage, broccoli, cauliflower,
leaf lettuce, tomatoes, potatoes, and beans also have relatively
high (7 mg/100 g to 163 mg/100g) vitamin C content (17).
Transport of vitamin
C is a saturable and dose dependent process that occurs by active
transport. At the intestine and cells AA is oxidized to DHAA,
which is more quickly transported across the cell membrane. Once
inside the tissue or intestinal epithelium, the vitamin is reduced
back to AA. | The degree of intestinal absorption decreases as
intake of AA increases. Intakes of 1 to 1.5 grams results in
50% absorption, but at intakes over 12 grams, only 16% of the
vitamin is absorbed. In contrast, an intake of less than 20 mg,
has a 98% absorption rate (13). Absorption of vitamin C is greater
when several individual doses of vitamin C, in quantities less
than one gram, are taken throughout the day rather than one megadose
(17). Eighty to ninety-five percent of the vitamin C found in
foods is absorbed (13). Furthermore, the bioavailability of synthetic
and "natural" forms of the vitamin differ very little
despite the claims made by manufacturers (13,17). Vitamin C absorption
can be impaired by a number of factors. A single large dose saturates
the enzyme kinetics for vitamin C, leading to excess AA in the
intestinal lumen, which causes numerous gastrointestinal problems.
Pectin and zinc also inhibit AA absorption, but this mechanism
is not well understood. A high iron concentration in the gastrointestinal
tract may cause oxidative destruction and in turn impair uptake
is the main mechanism of vitamin C distribution within the body.
Simple diffusion may occur in the mouth and stomach but accounts
for only a very small percentage of uptake (13). Sodium-independent
transport systems shuttle vitamin C across the basolateral membrane
of the intestinal cells. In the plasma absorbed ascorbic and
dehydroascorbate (DHAA) can either be transported freely or be
bound to albumin. Ascorbate can also move into body cells and
tissues (13). As previously mentioned DHAA is the primary form
of vitamin C that crosses cellular membranes. The adrenal and
pituitary glands, red blood cells, lymphocytes, and neutrophils
all receive vitamin C in the form of DHAA (13,17).
Vitamin C is stored
throughout body tissues and blood. Ascorbic acid content of blood
components, fluid, and tissue varies widely on an individual
basis. Tissue concentrations exceed those found in the plasma
by three to ten times. Energy-driven transport pumps are responsible
for the higher tissue concentrations of vitamin C versus the
plasma. Both tissue and plasma levels of vitamin C are correlated
to intake up to 90 mg / day (13). The total body pool of vitamin
C has been estimated using radiolabeled isotopes, to a maximum
of 20 mg/kg body weight. This corresponds to a plasma AA concentration
of 57 umol/L. Alternate techniques of measurement have estimated
maximum total body AA stores to be 22 mg/kg (17). The pituitary
glands, adrenal glands, and lens of the eye contain the highest
vitamin C content (at least140umol/ 100 g wet weight) within
the body (13,17). In contrast, the saliva and plasma have the
lowest AA content (17). Vitamin C content of cardiac tissue is
between 28 and 85 ml/100g wet weight, while that in skeletal
muscle is approximately 17 ml/100g wet weight (16). Other tissues
with intermediate levels of vitamin C include the kidneys, brain,
liver, lungs, and thyroid. The water-soluble properties of vitamin
C prevent it from being stored in the adipose tissue of the body.
The average half-life
of AA is believed to be between 16 and 20 days (17). Its half-life
is inversely related to intake. The water-soluble properties
of vitamin C lead to urinary excretion of the vitamin. Metabolites
of vitamin C including dehydroascorbate (DHAA), oxalic acid,
2-O-methyl ascorbate, and 2-ketoascorbitol are also excreted
from the body via the urinary system (13,17). The kidneys play
a major role in vitamin C excretion and retention. DHAA and AA
can be reabsorbed by the kidney tubules as long as body pool
levels are equal to or less than 1500 mg. Levels within the body
that are 1500 mg or less will result in no urinary excretion
of vitamin C (13). As levels increase above 1500 mg, the reabsorption
efficiency of the kidneys decreases. Thus, body pool levels from
1500 to 3000 mg relate to tissue saturation of the vitamin (13).
Plasma ascorbate levels between 0.8 and 1.4 mg/dl are considered
the renal threshold. Above these levels, vitamin C will be excreted
rather than reabsorbed by the kidneys (13).
Vitamin C has
been studied for many years. It participates in numerous biochemical
reactions, suggesting that vitamin C is important for every body
process from bone formation to scar tissue repair (13). The only
established role of the vitamin appears to be in curing or preventing
scurvy. Vitamin C is the major water-soluble antioxidant within
the body. The vitamin readily donates electrons to break the
chain reaction of lipid peroxidation. The water-soluble properties
of vitamin C allow for the quenching of free radicals before
they reach the cellular membrane. Tocopherol and glutathione
also rely on AA for regeneration back to their active isoforms.
The relationship between AA and glutathione is unique. Vitamin
C reduces glutathione back to the active form. Once reduced glutathione
will regenerate vitamin C from its DHAA or oxidized state. The
prophylactic effects of vitamin C as an antioxidant during exercise,
when free radical formation is high, will be discussed in future
sections of this literature review. A well-known function of
AA is the role it plays in hydroxylation reactions that are essential
for the formation of collagen. Vitamin C is important in collagen
formation as it allows for a tight cross-linking of the triple
helix, thereby resulting in stabilization of the peptide. Evidence
also suggests that AA may be involved in collagen gene expression.
However, this mechanism is not well understood. Carnitine synthesis
prefer to use vitamin C as the reducing agent (13). Carnitine
facilitates the beta-oxidation of fat, through its role transporting
long chain fatty acids from the cytoplasm into the mitochondrial
matrix of cardiac and skeletal muscle. High concentrations of
AA are found in adrenal and brain tissue where they are fairly
resistant to AA depletion. Vitamin C is directly involved in
the enzyme activity of two copper dependent mono-oxygenases,
which are important in the formation of norepinephrine and serotonin
(13,17). Furthermore, AA regulates the activity of some neurons
within the brain. Some of these functions include neurotransmitter
membrane receptor synthesis, and neurotransmitter dynamics. Indirectly,
AA plays important regulatory roles throughout the entire body
due to its involvement in the synthesis of hormones, hormone-releasing
factors, and neurotransmitters (13). Animal models have also
shown that AA is an important factor in development of the nervous
system, specifically in the maturation of glial cells and myelin
(17) Vitamin C is important to a host of numerous other functions
within the body. The vitamin is an important aid in the absorption
and conversion of iron to its storage form. Bile acid formation,
and hence cholesterol degradation are highly dependent on AA.
Some hypothesize that vitamin C may even have a hypocholesterolemic
effect. This has been suggested because the enzyme needed for
the first step in bile acid synthesis, cholesterol 7-alpha hydroxylase,
is dependent upon the presence of vitamin C. Ascorbic acid may
also has vasodilatory and anticlotting effects within the body
by stimulating nitric oxide release. Physiological effects such
as an antihistamine modified bronchial tone, and insulin responses
have been linked to AA. The protection of neural and endothelial
tissue, along with effects on cellular tone can also be attribute
to vitamin C. Multiple other mechanisms of function for vitamin
C have been proposed, but experimental results addressing these
topics are variable. Other possible functions for vitamin C include
regulation of cellular nucleotide concentrations, immune function,
and the endocrine system. Vitamin C has been proposed by some
to have pharmacological benefits in preventing cancer, infections,
and the common cold. However, these benefits have yet to be reported
in the scientific literature. The role of vitamin C in preventing
cancer is controversial, but has been studied for cancers of
the oral cavity, uterus, esophagus, bladder, and pancreas. The
research is at best equivocal and more studies are needed to
further address the role of vitamin C in preventing cancer.
Dietary Reference Intake (DRI)
Daily Allowance (RDA) has been replaced by a DRI for vitamin
C in the year 2000. In 1989, the RDA was established at 60 mg
for adults. This level was believed to be sufficient enough to
maintain body pool levels at 1500 mg and does not differ from
that established in 1980. A RDA for smokers was established in
1989 at 100 mg/day. This greater RDA was established because
smokers have a higher turnover rate of vitamin C versus non-smokers
(13). The DRI does not differentiate the need between smokers
and non-smokers. Dietary reference intakes for vitamin C have
been established at 90 mg for men and 75 mg for women (28).
Between the 16th
and 18th centuries, numerous sea voyagers died mysterious deaths,
but symptoms could be reversed with the consumption of citrus
fruits. Dr. James Lind made this discovery in 1747 after the
British Admiralty demanded that a cure for the disease be found.
The disease was later termed scurvy and the cure ascorbic acid
because of its antiscurvy properties (16). Today the disease
still exists, but is rare in the United States. However, the
symptoms and cure are well known. The disease is most commonly
seen in people who have poor diets, cancers, are alcoholics,
or have been institutionalized. The disease is more common in
those who have cancer and those who are alcoholics due to the
increased turnover rate of the vitamin.
kinetics of vitamin C make toxicity more likely when multiple
large doses (~1gram) are consumed throughout a day versus one
single dose. A common symptom of unabsorbed vitamin C left in
the gastrointestinal tract is osmotic diarrhea (13). Vitamin
C can be transformed in the body to oxalate, which is a common
constituent of kidney stones. Doses up to 10 grams have shown
to be associated with a higher prevalence of oxalate excretion,
but the level does not fall outside of the normal range. As a
precaution, people who are prone to kidney stones may want to
avoid large doses (10 times the DRI or greater) of the vitamin
(13). People who lack the control to regulate iron uptake should
also avoid large doses of the vitamin. As stated earlier, vitamin
C enhances iron absorption, which can lead to toxicity of iron
in some people. Furthermore, excess ascorbate in the urine and
feces can falsify lab tests such as glucose in the urine and
fecal occult blood test.
Vitamin C And Exercise
Effects of Exercise
on Vitamin C Requirements
The evidence addressing
the vitamin C requirement of athletes is abundant and contradictory.
Multiple studies have found blood and plasma levels of vitamin
C to be diminished in those who exercise. Keith (24) summarizes
the findings of Namyslowski who published two papers addressing
the requirement of ascorbate in exercising individuals. The second
study concluded that blood vitamin C levels decreased in athletes
ingesting 100 mg per day. A dietary intake of 300 mg/day maintained
blood levels of the vitamin. This was some of the first evidence
to suggest vitamin C needs are increased in those who exercise.
Athletes receiving a one-gram vitamin C supplement showed increased
work capacity at a heart rate of 170 beats per minute. Subjects
served as self-controls and were given a placebo for two weeks
and then vitamin C for two weeks. During vitamin C supplementation,
subjects repeatedly demonstrated decreased heart rates at all
levels of work when compared with the placebo trial (16). However,
treatments (placebo or vitamin) were given in succession. An
alternate study by Telford et al. (35) provided evidence that
supplementation for 7 to 8 months did not have any significant
effect on blood levels of the vitamin. However, females were
shown to have significantly higher levels of vitamin C in their
blood versus the male population. Also, this study showed that
plasma vitamin C levels could remain elevated for 24 hours after
strenuous exercise (11,39). Vitamin C has shown favorable effects
when used during heat acclimatization in humans. Two separate
studies (26, 33) from the 1970s have both shown similar results.
Plasma levels of ascorbic acid in thirteen male volunteers rose
to a level fourfold higher in supplemented (250 mg or 500 mg)
than unsupplemented subjects. The higher plasma levels of ascorbate
were associated with reduced body temperature and sweat loss.
These results support the hypothesis that vitamin C is beneficial
to those trying to acclimatize themselves to heat (27). In a
study done one year earlier, the same results were found. However,
it was determined that doses of 500 mg versus 250 mg result in
no enhanced benefits in subjects acclimatizing to heat (33).
The effects of vitamin C supplementation on anaerobic and aerobic
work capacity were investigated by Keren and Epstein (26). Thirty-three
healthy males partook in a 21-day training session that primarily
involved aerobic work. Measures of aerobic and anaerobic work
capacity were then measured. Vitamin C supplementation provided
no enhancement in either aerobic or anaerobic work. Many ultra
long-distance runners experience upper respiratory tract infections.
Peters et al. (30) addressed the possible role of vitamin C in
preventing these infections. Symptoms of upper respiratory infection
were monitored for 14 days after an ultramarathon (> 26.2
miles) in subjects receiving either placebo or vitamin C supplementation.
Sixty-eight percent of the runners on the placebo reported the
development of respiratory tract infections. In contrast only
one-third of the vitamin C group reported the infections. The
scientists concluded that vitamin C supplementation may actually
be beneficial in helping prevent upper respiratory infections
in ultramarathoners. Blood vitamin C levels greater than 0.6mg/100ml
have been established as being adequate (41). Multiple studies
have shown blood vitamin C levels of various athletes including
runners (11,40) to be adequate. Concentrations of plasma AA have
been reported to be significantly higher five minutes after a
21 km (13.1 mile) run than baseline values. The levels then fell
20% below pre-exercise values within 24 hours and remained depressed
for 2 days (11). This may be due to a loss in plasma volume following
exercise. Contradictory results were found by Rokitzki et al.
(31) and Glesson et al. (11) who found that vitamin C was higher
immediately after exercise. However, Rokitzki et al. (31) also
noted that the levels remained high for 24 hours following a
marathon. The fluctuating levels of vitamin C are likely controlled
by the adrenal gland (11). In conclusion, a high probability
exists that a large majority of athletes consume sufficient vitamin
C in their diet. At the same time it is know that a diet lacking
vitamin C will in turn inhibit performance (5). A daily intake
between 100-300 mg of vitamin C may be warranted to meet the
needs of all people who exercise (5). However, until an alternative
RDI for athletes is established, the recommended intake for vitamin
C shall remain at 75 mg for women and 90 mg for men. Doses of
1500 milligrams or greater oversaturate the body pool and are
excreted in the urine. Therefore, multiple large doses are not
warranted. One must also be aware of the enzyme kinetics of vitamin
C transport. Multiple small doses help prevent side effects and
are advised for those choosing to consume large quantities or
supplement with the vitamin. The previously mentioned levels
are easily obtainable by substituting a well-balanced diet for
a supplement. Clarkson summarizes (7) that athletes taking a
multivitamin do not appear to obtain ergogenic benefits and likely
do not need the supplementation (40). However, a small portion
of those who exercise will lack an adequate vitamin C intake.
Bazzarre et al. (3) have shown some bodybuilders have vitamin
C intakes below 40 mg. Similar conclusions were found in basketball
players (15), cyclists (25), and even Navy Seals (8). Currently
it appears that the variety and adequacy of individual diets
will determine if supplementation with vitamin C is needed
Of Oxidative Stress / Lipid Peroxidation
Vitamin C has
the ability to sequester the singlet oxygen radical, stabilize
the hydroxyl radical, and regenerate reduced vitamin E back to
the active state. These functions work to halt peroxidation of
cellular lipid memebranes (21). Despite these functions, the
studies involving AA and lipid peroxidation are disappointing
at best. Vitamin C has been shown to induce a lower-frequency
fatigue (indicates less muscle damage) when compared with those
deficient in the vitamin (18). A second study found that vitamin
C does control reactive oxidant species formed during exercise
(32). If not controlled, these species have the ability to react
with cell membranes and damage them, initiating lipid peroxidation.
In 1992, Kaminski et al. (21) examined the relationship between
AA given to 19 subjects for three days before exercise (and seven
after) and the muscle damage induced by two bouts of eccentric
exercise that were 3 weeks apart. The authors concluded that
AA reduced muscle damage, however they did not measure any indices
of oxidative stress other than muscle soreness (21) Supplementation
with two 500-gram dosages of vitamin C for one day is associated
with a decreased shift from pre-exercise to post exercise prooxidant
activity when compared to a placebo. The same dosage given over
a two-week period did not elicit as great a change in peroxidation
activity as a one-day supplementation provided (1). This difference
may be related to the fact that a one-day dose of vitamin C,
such as that used in this study, helped to regenerate other antioxidants
in the body (i.e. Vitamin E). The authors hypothesize that a
two-week period of vitamin C supplementation may replenish other
antioxidants and then lead to prooxidant properties within the
body (1), likely via the Fenton reaction. As shown here there
is an obvious lack of research addressing antioxidants and exercise,
particularly vitamin C. The existing preliminary data addressing
vitamin C to-date seems promising, but also demonstrates the
need for further research in this area.
C And Exercise Recovery
concerning vitamin C and exercise recovery is limited at best.
A previously mentioned study found that vitamin C supplementation
prevented muscle soreness (21). However, the aim of the study
was not that of exercise recovery, but rather peroxidation of
membranes. Vasankari et al. (39) performed one of the few studies
addressing the role of vitamin C in exercise recovery. Conjugated
dienes decreased by 11% after exercise in those individuals who
ingested vitamin C versus those receiving a placebo. The design
of this study should be noted as the subjects received one gram
of vitamin C in supplement form immediately after a bout of exercise.
However, these methods and results may be the basis for future
research addressing vitamin C (and antioxidant) supplementation
immediately after exercise. This author believes that vitamin
C likely only aids in recovery if a person is deficient in the
vitamin. The adrenal gland has been shown to regulate vitamin
C release into the plasma (11). The significance of this has
yet to be determined. However, it is likely that vitamin C does
not directly function in muscle recovery because post-exercise
values in the previous study (11) fell to values 20% below baseline
within the first 24 hours of recovery. The possibility does exist
that vitamin C may play an indirect role in exercise recovery.
The vitamin has the ability to regenerate vitamin E. This means
that any function vitamin E has within the body can also be linked
back to vitamin C. The literature suggests that the role vitamin
E plays in muscle recovery is limited and contradictory at this
time. The relationship between vitamin E and muscle recovery
is further addressed in a separate section. In summary, the role
of vitamin C in exercise recovery is not known. The literature
to date seems to imply that vitamin C probably has no direct
significant role in muscle recovery from exercise, but may possibly
play a significant indirect role in the process.
Summary / Current Recommendations
C levels can ultimately cause decreased performance. Vitamin
C can regenerate other antioxidants and act as an antioxidant
itself. Therefore the need for vitamin C is likely increased
in those who exercise regularly. An intake of 100 to 500 mg seems
to be sufficient to meet the needs of the exercising individual.
This level can easily be obtained through a fruit and vegetable
inclusive diet. As stated earlier these dosages would best be
absorbed if taken in small quantities at multiple intervals.
Some athletes will experience inadequate vitamin C intakes that
are below the RDA. For this group of people increased vitamin
C may be beneficial to performance. The abundance of vitamin
C rich foods establishes that inadequate vitamin C levels should
be increased via food sources, not by supplementation, if possible.
The paucity of research investigating vitamin C and the prevention
of oxidative damage makes interpretation difficult. This confusion
has led to difficulty in formation of a clear-cut recommendation,
so one will not be presented. Many studies have looked at the
effects of combined vitamin C and E supplementation. These were
not addressed in this literature review. Future research needs
to investigate the role that vitamin C, alone, has in preventing
or lessening oxidative stress. This too may be difficult as AA
has the ability to regenerate vitamin E. Furthermore, research
addressing the validity of acclimatization studies warrants further
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