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Vitamin C 1 of 3 : next >>


[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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