Vitamin C: What does it do?
is a water-soluble vitamin that has a number of biological functions.
Acting as an antioxidant, one of vitamin
C’s important functions is to protect LDL
cholesterol from oxidative damage. (Only when LDL is damaged does cholesterol appear to
lead to heart disease, and vitamin C may be
one of the most important antioxidant protectors of LDL.)1 may also
protect against heart disease by reducing the stiffness of arteries and the tendency of
platelets to clump together.2
The antioxidant properties of vitamin C are thought to protect smokers, as well as people
exposed to secondhand smoke, from the harmful effects of free radicals. A controlled trial
demonstrated the ability of 3 grams of vitamin C, taken by nonsmokers two hours prior to being
exposed to cigarette smoke, to reduce the free radical damage and LDL cholesterol oxidation
associated with exposure to cigarette smoke.3 The smoke-induced decline in total
antioxidant defense was also diminished. These beneficial effects were not observed in
nonsmokers under normal conditions (no free radical exposure).
is needed to make collagen, the "glue" that strengthens many parts of the body,
such as muscles and blood vessels. also plays important roles in wound healing and as a natural antihistamine. This
vitamin also aids in the formation of liver bile and helps to fight viruses and to detoxify
alcohol and other substances.
Recently, researchers have shown that vitamin C improves nitric oxide activity.4
Nitric oxide is needed for the dilation of blood vessels, potentially important in lowering blood pressure and preventing spasms of arteries
in the heart that might otherwise lead to heart
attacks. has reversed dysfunction of cells lining blood vessels.5 The
normalization of the functioning of these cells may be linked to prevention of heart
disease.
Evidence indicates that vitamin C levels in the eye decrease with age6 and that
supplementing with vitamin C prevents this decrease,7 possibly leading to a lower
risk of developing cataracts.8
9 Healthy people have been reported in some, but not all, studies10 to
be more likely to take vitamin C and vitamin E
supplements than are people with cataracts.11
has been reported to reduce activity of the enzyme, aldose reductase, in people.12
Aldose reductase is the enzyme responsible for accumulation of sorbitol in eyes, nerves, and
kidneys of people with diabetes. This
accumulation is believed to be responsible for deterioration of these parts of the body
associated with diabetes. Therefore, interference with the activity of aldose reductase
theoretically helps protect people with diabetes.
may help protect the body against accumulation or retention of the toxic mineral,
lead. In one preliminary study, people with higher blood levels of vitamin C had much lower
risk of having excessive blood levels of lead.13 In a controlled trial, male
smokers with moderate to high levels of lead received supplements of 1,000 mg per day of
vitamin C, 200 mg per day of vitamin C, or a placebo.14 Only those people takin
Who is likely to be deficient of Vitamin C?
Although scurvy (severe vitamin C deficiency) is uncommon in Western societies, many
doctors believe that most people consume less than optimal amounts. Fatigue, easy bruising, and bleeding gums are early signs of vitamin
C deficiency that occur long before frank scurvy develops. Smokers have low levels of vitamin
C and require a higher daily intake to maintain normal vitamin C levels. Women with preeclampsia have been found to have lower blood
levels of vitamin C than women without the condition.18 Women who have lower blood
levels of vitamin C have an increased risk of
gallstones.19
People with kidney failure have an increased risk of vitamin C deficiency.20
However, people with kidney failure should take vitamin C only under the supervision of a
doctor.
How much Vitamin C is usually taken?
The recommended dietary allowance (RDA) for vitamin C in nonsmoking adults is 75 mg per day
for women and 90 mg per day for men. For smokers, the RDAs are 110 mg per day for women and
125 mg per day for men. Most clinical vitamin C studies have investigated the effects of a
broad range of higher vitamin C intakes (100–1,000 mg per day or more), often not
looking for (or finding) the “optimal” intake within that range. In terms of heart disease prevention, as little as
100–200 mg of vitamin C appears to be adequate.21 Although some doctors
recommend 500–1,000 mg per day or more, additional research is needed to determine
whether these larger amounts are necessary. Some vitamin C experts propose that adequate
intake be considered 200 mg per day because of evidence that the cells of the human body do
not take up any more vitamin C when larger daily amounts are used.22
Some scientists have recommended that healthy people take multi-gram amounts of vitamin C
for the prevention of illness. However, little or no research supports this point of view and
it remains controversial. Supplementing more results in an excretion level virtually identical
to intake, meaning that consuming more vitamin C does not increase the amount that remains in
the body.23 On the basis of extensive analysis of published vitamin C studies,
researchers at the Linus Pauling Institute at Oregon State University have called for the RDA
to be increased, but only to 120 mg.24 This same report reveals that “. . .
90–100 mg vitamin C per day is required for optimum reduction of chronic disease risk in
nonsmoking men and women.” Thus, the multiple gram amounts of vitamin C taken by many
healthy people may be superfluous.
The studies that ascertained approximately 120–200 mg daily of vitamin C is correct
for prevention purposes in healthy people have typically not investigated whether people
suffering from various diseases can benefit from larger amounts. In the case of the common cold, a review of published trials found that
amounts of 2 grams per day in children appear to be more effective than 1 gram per day in
adults, suggesting that large intakes of vitamin C may be more effective than smaller amounts,
at least for this condition.25
Are there any side effects or interactions with Vitamin C?
Some people develop diarrhea after as
little as a few grams of vitamin C per day, while others are not bothered by ten times this
amount. Strong scientific evidence to define and defend an upper tolerable limit for vitamin C
is not available. A review of the available research concluded that high intakes (2–4
grams per day) are well-tolerated by healthy people.26 However, intake of large
amounts of vitamin C can deplete the body of
copper27 28 —an essential nutrient. People should be sure to
maintain adequate copper intake at higher intakes of vitamin C. Copper is found in many multivitamin-mineral supplements.
increases the absorption of iron and should be
avoided by people with iron overload diseases (e.g., hemochromatosis, hemosiderosis). Vitamin
C helps recycle the antioxidant, vitamin E.
It is widely (and mistakenly) believed that mothers who consume large amounts of vitamin C
during pregnancy are at risk of giving birth
to an infant with a higher-than-normal requirement for the vitamin. The concern is that the
infant could suffer “rebound scurvy,” a vitamin C deficiency caused by not having
this increased need met. Even some medical textbooks have subscribed to this
theory.29 In fact, however, the concept of “rebound scurvy” in infants
is supported by extremely weak evidence.30 Since the publication in 1965 of the
report upon which this mistaken notion is based, millions of women have consumed high amounts
of vitamin C during pregnancy and not a single new case of rebound scurvy has been
reported.31
A preliminary study found that people who took 500 mg per day of vitamin C supplements for
one year had a greater increase in wall thickness of the carotid arteries (vessels in the neck
that supply blood to the brain) than those who did not take vitamin C.32 Thickness
of carotid artery walls is an indicator of progression of atherosclerosis. Currently, no evidence supports a
cause-and-effect relationship for the outcome reported in this study. The vast preponderance
of research suggests either a protective or therapeutic effect of vitamin C for heart disease, or no effect at all.
People with the following conditions should consult their doctor before
supplementing with vitamin C: glucose-6-phosphate dehydrogenase deficiency, iron overload
(hemosiderosis or hemochromatosis), history of kidney stones, or kidney failure.
It has been suggested that people who form calcium oxalate kidney stones should avoid vitamin C supplements,
because vitamin C can be converted into oxalate and increase urinary oxalate.33
34 Initially, these concerns were questioned because of potential errors in the
laboratory measurement of oxalate.35 36 However, using newer methodology
that rules out this problem, recent evidence shows that as little as 1 gram of vitamin C per
day can increase the urinary oxalate levels in some people, even those without a history of
kidney stones.37 38 In one case, 8 grams per day of vitamin C led to
dramatic increases in urinary oxalate excretion and kidney stone crystal formation causing
bloody urine.39
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The information presented in Healthnotes is for informational
purposes only. It is based on scientific studies (human, animal, or in
vitro), clinical experience, or traditional usage as cited in each article.
The results reported may not necessarily occur in all individuals. For many of
the conditions discussed, treatment with prescription or over the counter
medication is also available. Consult your doctor, practitioner, and/or
pharmacist for any health problem and before using any supplements or before
making any changes in prescribed medications.
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