|
[Antioxidant
properties | Immune system]
[Collagen and connective tissue | Hormones
| Nervous system]
[Other Functions | Absorption
& metabolism | Deficiency | Sources]
[RDA | Supplements | Therapeutic
uses]
[Interactions w/ other nutrients |
References]
Vitamin C is involved
in hundreds of vital biological processes in the body. Vitamin C is an
antioxidant. Vitamin C has beneficial effects on
- The immune system
- Collagen and connective
tissue
- Hormones
- Nervous system
Low vitamin C intake
has been linked to
- Cardiovascular
disease
- High blood pressure
- Cancer
- Cataracts
- Diabetes
- Asthma
- Immunity problems
Antioxidant
properties
Vitamin C is a powerful water-soluble antioxidant
and plays a vital role in protecting against oxidative damage. Vitamin
C neutralizes potentially harmful reactions in the watery parts of the
body, such as the blood and the fluid inside and surrounding cells. Vitamin
C also helps protect LDL cholesterol against free radical damage. This
antioxidant action helps to protect against cancer, the effects of aging,
heart disease, and an array of other health problems.
Immune
system
Vitamin C is critical to immune function as it is
involved in antibody production and white blood cell function and activity.
Other Vitamin C functions include the production of interferon, an antiviral
and anticancer substance. Vitamin C requirements are raised when the immune
system is under stress.
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Collagen
and connective tissue
The main role of vitamin C is in the manufacture of
collagen. This protein forms the basis of connective tissue, the most
abundant tissue in the body, and acts as a cementing substance between
cells. It helps support and protect blood vessels, bones, joints, organs
and muscles, and forms a sizable proportion of skin, tendons, the cornea
of the eye, ligaments, cartilage, teeth and bone. Collagen forms a protective
barrier against infection and disease, and promotes healing of wounds,
fractures and bruises.
Hormones
Vitamin C is important in the synthesis of adrenal
hormones and Vitamin C is depleted from the adrenal glands in times of
stress.
Nervous
system
Vitamin C plays a role in the manufacture of neurotransmitters.
Vitamin C is necessary for the conversion of tryptophan to serotonin,
and of tyrosine to dopamine and adrenaline.
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Other
functions
Vitamin C is involved in the manufacture of carnitine,
a substance necessary for the production of energy from fatty acids in
cells, especially cardiac and skeletal muscle cells. Vitamin C is necessary
for the activity of the enzyme system which metabolizes drugs in the body.
Vitamin C is also necessary for iron absorption and plays a role in the
conversion of cholesterol to bile acids for excretion. Vitamin C may also
affect prostaglandin metabolism.
Absorption
and metabolism
Absorption of vitamin C occurs in the intestine. The amount absorbed depends
on the dose, as the absorption mechanism is saturable and any excess is
excreted in the urine in two to three hours. As vitamin C is water soluble,
only a small amount (about 4 to 5 g) is stored in the body. Vitamin C
levels in the body are regulated by absorption and kidney excretion mechanisms.
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Deficiency
[Cardiovascular
disease | High blood pressure | Cancer]
[Prostate cancer | Stomach
cancer | Lung cancer | Colon
cancer]
[Breast cancer | Cataracts
| Diabetes | Lung
function and asthma]
[Immunity | Other
disorders]
A lack of vitamin
C leads eventually to scurvy. The symptoms are mainly due to poorly formed
collagen and include the breaking open of small blood vessels, the reddening
and bleeding of gums, loose teeth, joint pains, dry scaly skin and blood
vessel damage. Other Vitamin C deficiency symptoms include general weakness,
fluid retention, depression and anemia. Vitamin C deficiency can also
cause slower wound-healing, increased susceptibility to infections, male
infertility and increased genetic damage to sperm cells, which may lead
to birth defects.
Scurvy and severe
vitamin C deficiency are rare in developing countries but marginal deficiencies
may be relatively common and may play a role in the development of diseases
such as cancer and heart disease. The first National Health and Nutrition
Examination Survey (NHANES I) looked at the vitamin C intake of over 11
000 people during a five-year period. Results showed that men whose intakes
of vitamin C were greater than 50 mg daily had a 34 per cent lower chance
of death from all causes than those whose intakes were lower than 50 mg
daily.1
Men, the elderly,
smokers, diabetics, those with high blood pressure and perhaps oral estrogen-containing
contraceptive users have lowered plasma vitamin C levels and are at greatest
risk of deficiency-related diseases.2
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Cardiovascular
disease
Many population studies have linked low vitamin
C intakes to an increased risk of cardiovascular disease. These include
a study reported in 1996 in the American Journal of Epidemiology. During
the study, which began in 1981, USDA researchers assessed the health
and nutrition status of 747 people aged 60 years and over. Particular
attention was paid to the foods the participants usually ate and the
levels in their blood of the antioxidant vitamins C, E and beta carotene.
The researchers followed up the subjects from nine to 12 years later
and found that among people who ate lots of dark green and orange vegetables,
there were fewer deaths from heart disease and other causes. The results
showed that a daily intake of more than 400 mg and higher blood levels
of vitamin C were linked to reduced risk of death from heart disease.3
In a study published in 1993, Swiss researchers found an increased risk
of death from ischemic heart disease in people with low vitamin C levels.4
In a study published
in the British Medical Journal in 1995, UK researchers assessed the
links between dietary intake and blood levels of vitamin C, and death
from stroke and coronary heart disease in people aged 65 and over. The
study involved 730 men and women who were followed up for a 20-year
period. The results showed that those with the highest vitamin C intakes
had around half the risk of death from stroke when compared with those
with the lowest vitamin C intakes. However in this study, no link was
found between vitamin C status and risk of death from coronary heart
disease.5
Low vitamin C levels
are also associated with an increased risk of heart attack. In a 1997
study, Finnish researchers examined this link in 1605 men aged between
42 and 60 who were free from heart disease when they entered the study.
During the follow-up period there were 70 heart attacks. The results
showed that men with vitamin C deficiency were three-and-a-half times
more likely to have a heart attack than those who were not deficient.6
However, not all
studies have shown protective effects of vitamin C. These include the
large Nurses and Health Professionals Studies.7,8
Researchers from
Cambridge University in the UK examined the relationship between blood
levels of vitamin C and angina in women aged 45 to 74. Forty-two women
with previously undiagnosed angina were compared with 877 women with
no disease. Those with higher vitamin C levels had a 66 per cent reduced
risk of angina.9 The same
researchers examined the link between blood levels of vitamin C and
blood fat levels. Their results showed that a high intake of vitamin
C from food raises beneficial HDL cholesterol and lowers serum triglyceride.10
Other studies suggest
that people with low vitamin C levels have higher total and harmful
LDL cholesterol levels and lower beneficial HDL cholesterol levels.
In a study reported in the American Journal of Clinical Nutrition, USDA
researchers found that high blood levels of vitamin C were associated
with high levels of HDL cholesterol in 316 women and 511 men aged from
19 to 95.11 Vitamin C also
helps to protect blood fats and artery walls against oxidative damage
by free radicals, and seems to have beneficial effects on clotting.
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High
blood pressure
Vitamin C deficiency also appears to be linked to an
increased risk of high blood pressure. In a study done in Cambridge,
UK researchers examined the relationship between blood pressure and
vitamin C levels in the blood in 835 men and 1025 women aged from 45
to75. The results showed that low vitamin C levels were associated with
higher systolic and diastolic blood pressures.12
Cancer
Low intake of vitamin C appears to be a risk factor
for many forms of cancer. Diets high in fruit and vegetables, and therefore
high in vitamin C, have been found to be associated with lower risk
for cancers of the oral cavity, esophagus, stomach, colon, and lungs.
Many studies have found a reduced risk of cancer in people who have
high vitamin C intakes. The protective effect seems to be strongest
for cancers of the esophagus, larynx, mouth and pancreas. Vitamin C
also seems to provide some protection against cancers of the cervix,
liver, stomach, rectum, breast and lungs.13
However, in many of these studies it is not possible to tell whether
the protective effect is due to vitamin C, vitamin E, or carotene, to
a combined effect of these nutrients, or even due to additional substances
found in food.
Results from the
Western Electric Study published in 1995 suggest a link between low
vitamin C levels and death from cancer. The researchers obtained information
on diet and other factors from 1556 employed, middle-aged men. During
the follow-up period 231 men died from cancer. The results showed that
those with the highest vitamin C and betacarotene intakes were 40 per
cent less likely to die of cancer than those with the lowest intakes.14
Prostate
cancer
Further results from the Western Electric Study
reported in 1996, suggest that vitamin C improves survival in those
with prostate cancer. Researchers examined the links between dietary
betacarotene and vitamin C and the risk of prostate cancer in 1899 middle-aged
men over a 30-year period. During this time, prostate cancer developed
in 132 men. The results showed that associations between vitamin C intake
and risk of prostate cancer differed depending on whether the cancer
was diagnosed during the first 19 years of follow-up or the next 11
years of follow-up. Overall, higher intakes of vitamin C and betacarotene
were linked to improved survival.15
Stomach
cancer
Results from the Seven Countries Study published in 1995
suggest that low vitamin C intake is linked to an increased risk of
stomach cancer. In the 1960s, researchers collected detailed dietary
information and in 1987, they assessed average food intakes. They then
examined the links between this information and death from stomach cancer.
The results showed that the average intake of vitamin C was strongly
related to the risk of stomach cancer. However, vitamin C intake was
not related to the risk of lung and colorectal cancer in this study.16
Other studies have shown similar results.17
Lung
Cancer
Results from a Dutch study published in 1997 suggest
a weak protective effect of vitamin C against lung cancer. Researchers
obtained dietary information from 561 men from the town of Zutphen,
in 1960, 1965, and 1970. During the period from 1971 to 1990, 54 new
cases of lung cancer were identified and analysis of the diets of the
men showed an increased risk of lung cancer in those with lower fruit
and vegetable and vitamin C intakes.18
Colon
cancer
Researchers at the University of Southern California
assessed the links between fruits and vegetables and vitamin C intake
in 11,580 residents of a retirement community who entered the study
free from cancer. During the period from 1981 to 1989 a total of 1335
cases of cancer were diagnosed. The results showed a decreased risk
of colon cancer in women with higher vitamin C intakes. Supplemental
use of vitamins A and C also showed a protective effect on colon cancer
risk in women.19
Italian researchers
investigated the relationship between estimated intake of selected micronutrients,
including vitamin C, and the risk of disease in 828 patients with colon
cancer, 498 with rectal cancer and 2024 people without cancer. Those
in the highest intake group for vitamin C had a 60 per cent lower risk
of cancer than those in the low intake group.20
Breast
cancer
In a study published in 1994, researchers examined
the effect of diet before diagnosis on the risk of dying of breast cancer
in 678 women who were diagnosed with the disease from January 1982 through
June 1992. The results showed that those women with the highest vitamin
C intakes had a 57 per cent lower chance of dying of breast cancer than
those with the lowest intakes.21
However, results from the Nurses Health Study did not show a protective
effect against the disease.22
Cataracts
The vitamin C content of the eye is 20 times greater
than that in the blood. Results from some studies including the Beaver
Dam Eye Study, suggest that people with high levels of vitamin C are
at less risk of cataracts than those with low levels of vitamin C.23
Diabetes
Diabetics often have lower levels of antioxidants,
which can increase the risk of diabetic complications such as cardiovascular
disease. The cellular uptake of vitamin C is promoted by insulin and
inhibited by high blood sugar; and as diabetics have low insulin levels,
they are at greater risk of vitamin C deficiency. Most studies have
found people with diabetes to have at least 30 per cent lower vitamin
C concentrations than people without the disease. Levels seem to be
lower in diabetic people as a result of the disease rather than as a
result of poor dietary intake.24
Elevated fasting
insulin concentrations and insulin resistance have been associated with
non-insulin-dependent diabetes mellitus (NIDDM), obesity, atherosclerosis,
and hypertension; and some research suggests that antioxidant vitamins
may help to reduce insulin resistance. However, a study reported in
the American Journal of Clinical Nutrition in 1997 suggests that vitamin
E and vitamin C intakes are not linked to improved insulin sensitivity.
Researchers working on the Insulin Resistance and Atherosclerosis Study
(IRAS) assessed insulin concentrations and insulin sensitivity in 1151
African American, Hispanic, and non-Hispanic white men and women with
a wide spectrum of glucose tolerance. They also assessed the intake
of vitamins E and C in the subjects. They did not find a link between
vitamin intake and insulin resistance.25
Lung
function and asthma
Low vitamin C levels seem to impair lung function.
Researchers in Cambridge, UK examined the links between vitamin C levels
in the blood and respiratory function in 835 men and 1025 women aged
45 to 75. The results showed that vitamin C was protective for lung
function.26 Vitamin C intake
in the general population appears to be linked to the incidence of asthma,
suggesting that a diet low in vitamin C is a risk factor for asthma.
Symptoms of ongoing asthma in adults may be decreased by vitamin C supplementation,
although not all studies show positive results. Vitamin C is the major
antioxidant substance present in the airway surface liquid of the lung,
where it could be important in protecting against both damage from toxic
chemicals and free radicals, which may worsen the symptoms of asthma.27
Low vitamin C levels are associated with increased bronchial reactivity.28
Immunity
Vitamin C is important for the functioning of the
immune system, and deficiency can increase susceptibility to infection.
In a study published in 1997, French researchers assessed vitamin C
levels in 18 elderly patients in hospital. The patients were divided
into three groups: those with acute infection, those who were malnourished,
and a control group. Those with acute infection had considerably lower
vitamin C levels than those in the other groups.29
Other
disorders
Vitamin C deficiency may also play a role in macular
degeneration of the eye, arthritis, Parkinson's disease, pre-eclampsia
of pregnancy, the common cold, low sperm counts and skin ulcers.
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Sources
Good sources of vitamin C include citrus fruits such
as oranges and grapefruits. Other sources include strawberries, kiwifruit,
blackcurrants, papaya; and vegetables such as red peppers, broccoli and
brussel sprouts. Vitamin C from natural sources such as these is associated
with bioflavonoids which enhance the beneficial effects of vitamin C.
Vitamin C is easily lost during storage and cooking. Aging, bruising,
overcooking and re-heating all destroy vitamin C. Slicing vegetables exposes
a higher surface area to heat and light, leading to loss of vitamin C.
- Blackcurrants
1 cup 202
- Red pepper, raw
1 cup, sliced 174
- Guavas 1 fruit
165
- Orange juice,
commercial 1 cup 124
- Grapefruit juice
1 cup 94
- Kohlrabi, boiled
1 cup 89
- Papaya 1 cup,
cubes 86
- Lemons 1 fruit
83
- Strawberries 1
cup 82
- Green pepper 1
cup. sliced 82
- Kiwi fruit, peeled
1 medium 74
- Oranges 1 fruit
68
- Cantaloupe melon
1 cup, diced 66
- Broccoli, boiled
˝ cup 58
- Mangoes 1 fruit
57
- Kale 1 cup 53
- Brussels sprouts,
boiled ˝ cup 48
- Grapefruit ˝ fruit
47
- Honeydew melon
1 cup, diced 42
- Raspberries 1
cup 37
- Cauliflower, boiled
˝ cup 27
- Tangerines 1medium
26
- Pineapples, raw
1 cup, diced 24
- Cabbage, boiled
˝ cup, shredded 15
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Recommended
dietary allowances
USA
Men 60 mg
Women 60 mg
Pregnancy 70 mg
Lactation 95 mg
In a paper published
in 1996, researchers at the National Institutes of Health recommended
that the RDA for vitamin C be raised to 200 mg per day.30
Supplements
[Dosage | Toxic
effects of excess intake]
Vitamin C is the most
widely taken supplement in developing countries. It is available in pills,
powders, effervescent tablets, syrups and pastilles. Ascorbic acid is
the most widely used and least expensive form, but it causes stomach upsets
in some people and can damage tooth enamel. Calcium ascorbate and sodium
ascorbate are also readily available and are less likely to have these
effects. Some supplements provide vitamin C in the form of C complex which
contains bioflavonoids. These compounds occur naturally with vitamin C
and, in high enough doses, increase its activity.
Supplements are particularly
beneficial for anyone who smokes, eats an unhealthy diet, is under physical
or emotional stress, drinks alcohol, lives in a polluted environment,
is exposed to toxic chemicals, suffers from recurrent infections or has
an increased risk of cancer. Women who take the contraceptive pill, elderly
people, pregnant women and those with absorption difficulties are also
likely to benefit.
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Dosage
Opinions vary widely as to the optimal dose of vitamin
C. Linus Pauling, the Nobel Prize winner who studied the effects of
large doses of vitamin C on the common cold, flu and cancer, recommended
an optimum intake of between 2 g and 9 g per day. Many experts believe
that 500 mg is ideal to meet body needs while others feel that 200 mg
is adequate. Vitamin C needs vary with age, weight, activity, energy
levels, general metabolism and state of health.
In order to maintain
blood levels of vitamin C, it is best to take it in divided doses throughout
the day. Taking vitamin C with food minimizes adverse effects on the
digestive system.
A study reported
in 1997 in the American Journal of Clinical Nutrition suggests that
doses of vitamin C above 200 mg do not increase blood levels of the
vitamin significantly and may be excreted. Researchers at the University
of Tucson in Arizona, measured blood levels of vitamin C when the dose
given was 200 mg and then again when 2500 mg was administered. They
found negligible absorption increases between the lower and higher doses.31
Doctors who practice
orthomolecular medicine use megadose vitamin C therapy in times of specific
illness, especially viral infections. They typically use 20 to 40 g
daily, often intravenously. With oral doses, some doctors believe that
the amount of vitamin C needed is related to the severity of the disease
and increase the dose until 'bowel tolerance' is exceeded and diarrhea
results.32
Toxic
effects of excess intake
Vitamin C is safe in relatively large doses but
excessive intakes may cause diarrhea, nausea, stomach cramping, excess
urination and skin rashes. There is the possibility of kidney stones
in those with kidney disease. These effects may occur when doses above
1 g are taken regularly. Chewable vitamin C may lead to tooth decay.
Large doses of vitamin
C taken by pregnant women have caused 'rebound scurvy' in newborn babies
whose intake returns to normal. It may be advisable to reduce vitamin
C intake slowly after taking large amounts.
Results of a study
reported in 1998 in Nature Medicine suggest that vitamin C may cause
cell damage in doses above 500 mg. The researchers gave daily doses
of 500 mg of vitamin C to 30 healthy volunteers and then assessed two
indicators of oxidative damage in DNA from their blood cells. One of
these indicators showed less oxidation in the volunteers, and the other
indicator showed more oxidation than before they began taking the supplements.
However, this study directly contradicts other studies and focuses only
on a single biological marker that is not necessarily known to be a
good indicator of oxidative stress.33
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