Function of vitamin C in the body

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.

Immune system
Vitamin C is critical to immune function as it is involved in antibody production and white blood cell function and activity. Other functions include the production of interferon, an antiviral and anticancer substance. Vitamin C requirements are raised when the immune system is under stress.

Antioxidant properties
Vitamin C is a powerful water soluble antioxidant and plays a vital role in protecting against oxidative damage. It neutralizes potentially harmful reactions in the watery parts of the body, such as the blood and the fluid inside and surrounding cells. It 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 (See page 417 for more information.)

Hormones
Vitamin C is important in the synthesis of adrenal hormones and is depleted from the adrenal glands in times of stress.

Nervous system
Vitamin C plays a role in the manufacture of neurotransmitters. It is necessary for the conversion of tryptophan to serotonin, and of tyrosine to dopamine and adrenaline.

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 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.

Deficiency
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 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.

In a 1998 study, researchers at Arizona State University assessed the vitamin C status of nonsmoking college students during fall and winter. From one to two per cent of students were vitamin C-deficient and 12 per cent of those tested in the fall were marginally deficient while 16 per cent of those tested in winter were. If smokers had been included in the sample, it is likely that the number of students with deficiency would have been greater.

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.

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 was begun 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 following up the subjects from nine to 12 years later 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. 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.

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 intakes had around half the risk of death from stroke when compared with those with the lowest intakes. However in this study, no link was found between vitamin C status and risk of death from coronary heart disease.

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.

However, not all studies have shown protective effects of vitamin C. These include the large Nurses and Health Professionals Studies.

Researchers from Cambridge University in the UK examined the relationship between blood levels of vitamin C status and angina in women aged from 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. 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.

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. 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.

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.

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 lung. 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. 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 beta carotene intakes were 40 per cent less likely to die of cancer than those with the lowest intakes.

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 beta carotene 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 beta carotene were linked to improved survival.

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.

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.

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.

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. However, results from the Nurses Health Study did not show a protective effect against the disease.

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.

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.

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.

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.

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.

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 brussels sprouts. Vitamin C from natural sources such as these is associated with bioflavonoids which enhance the beneficial effects of vitamin C (See page 364 for more information.)

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.
 

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.

Supplements
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.

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.

Doctors who practise 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.

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.

Therapeutic uses of supplements
Vitamin C supplements are used to treat and prevent many diseases and conditions. For some of these there is research evidence while for others the evidence is mainly anecdotal. Vitamin C supplement use appears to be associated with a lower risk of death in elderly people and vitamin C seems to enhance the beneficial effects of vitamin E. Researchers involved in a study published in 1996 found that those elderly people who took vitamin C and vitamin E supplements had a lower risk of death from any cause and also from both cancer and heart disease. Those taking vitamin E supplements had a 34 per cent lower risk, and those taking both vitamin C and vitamin E had a 42 per cent reduced risk.

Cardiovascular disease
The evidence from epidemiological studies, animal experiments and some clinical trials suggests that vitamin C supplements may protect against the development of cardiovascular disease. In the same study mentioned in the previous paragraph, those taking vitamin E supplements had a 47 per cent lower risk of death from heart disease and those taking both vitamin C and vitamin E had a 53 per cent reduced risk.

Vitamin C may exert its protective effects by lowering total blood cholesterol and harmful LDL cholesterol and raising beneficial HDL cholesterol. Vitamin C also increases the production of prostacyclin, a prostaglandin which decreases the clumping of blood platelets and dilates blood vessels, therefore reducing the risk of heart disease, atherosclerosis and stroke.

High fat meals cause damage to artery linings, which may contribute to the development of atherosclerosis. Research published in 1997 suggests that taking the antioxidant vitamins C and E before a meal may help to prevent this damage. The study which was carried out at the University of Maryland School of Medicine involved 13 women and seven men with normal blood cholesterol levels. Once a week for three weeks, the subjects ate either a high fat meal, a low fat meal, a high fat meal after taking 1000 mg of vitamin C and 800 IU of vitamin E, or a low fat meal after taking the antioxidants. Before and after the meals, the researchers measured blood fat and cholesterol levels, blood pressure and heart rate in the subjects. They also used ultrasound to measure the dilation of an artery in the arm after release of a tourniquet which had been applied for five minutes. If the artery lining is functioning normally, it releases nitric oxide which causes dilation. The results showed that the high fat meal decreased artery lining function for up to four hours afterwards, whereas the low fat meal did not. This is probably due to oxidative stress caused by an accumulation of triglyceride-rich lipoproteins (blood fats). Vitamins C and E prevented this decrease in artery lining function.

A recent US study looked at the effect of either 2 g of vitamin C or a placebo on patients with coronary artery disease. In such patients the arteries leading to the heart are unable to open when the heart requires increased blood flow. Those patients given the vitamin C experienced expansion in their arteries while those given the placebo experienced no effect. Vitamin C has similar effects in those with high cholesterol and in those with chronic heart failure.

When blood is re-supplied to an organ from which it was previously cut off, oxidative damage can occur. This has been found in many types of surgery, for example in heart bypass operations. Vitamin C has also been shown to protect against this reperfusion injury.

Smoking
Vitamin C needs are higher in smokers and several studies suggest that vitamin C may protect against smoking-related damage. It may help to decrease the smoking-related build-up of atherosclerotic plaque by limiting the amount of white blood cells that stick to artery walls.40 Vitamin C supplements may be helpful in restoring reduced plasma vitamin C concentrations in smokers.41 Like those with high cholesterol levels and coronary heart disease, the arteries of smokers have a reduced ability to dilate. Vitamin C supplements may counteract this impairment.

High blood pressure
Vitamin C may also be of benefit in the treatment of mild high blood pressure, another risk factor for heart disease and stroke. Some research suggests that vitamin C may have beneficial effects in lowering high blood pressure.43 Vitamin C supplements improve abnormal artery lining function in hypertensive people.

Cancer prevention
Vitamin C supplements may have a part to play in cancer prevention. Some research suggests that the risk of cancer is lower in those taking supplements. Vitamin C may exert its anticancer effects by acting as an antioxidant and shielding the genetic mechanism of the cell from damage that can lead to cancerous changes. Vitamin C may also strengthen the ability of the immune system to track down and destroy pre-cancerous cells. Vitamin C may exert its protective effects against some cancers by inhibiting the formation of toxic compounds known as nitrosamines from nitrite food additives. These compounds are also found in cigarette smoke and are linked to an increased risk of stomach and lung cancers.

Stomach cancer
Supplements may be useful in helping to prevent stomach cancer. In a 1996 study, researchers gave 32 patients 500 mg of vitamin C twice daily for two weeks. Levels in gastric juices and gut tissues were increased, raising the possibility of increased protection against free radicals.

A 1997 report in the journal, Cancer, suggests that vitamin C may inhibit the growth of Helicobacter pylori, a stomach bacterium that increases the risk of ulcers and stomach cancer. High concentrations of vitamin C inhibited the growth of bacteria in culture dishes and also in the stomachs of Mongolian gerbils, according to researchers at the International Medical Center of Japan in Tokyo. Vitamin C-rich diets have been found to decrease the risk of stomach cancer. This has been attributed to the antioxidant ability of vitamin C. However, vitamin E, which is also an antioxidant, does not inhibit the growth of Helicobacter pylori. This suggests that vitamin C may exert its protective effects through a biochemical mechanism. This research suggests the possibility of a safe, side effect-free alternative to antibiotics for the treatment of ulcers.

Colon cancer
Vitamin C supplements have also been shown to have beneficial effects against the pre-cancerous changes which occur in colon cancer. In a 1992 study, 20 patients with colorectal cancer were given vitamins A, C, and E for six months and 21 patients with adenomas received placebo. The results showed that supplementation with vitamins A, C, and E was effective in reducing pre-cancerous abnormalities.47 Vitamin C supplements may also be beneficial in the treatment of prostate cancer.

Cancer treatment
Controversy surrounds the use of vitamin C in the treatment of cancer. The Nobel Prize winner, Linus Pauling and his colleagues have used vitamin C to improve survival times in cancer patients, but these results have not been repeated in other studies. Vitamin C may also benefit cancer patients who are undergoing radiation treatment by enabling them to withstand greater doses of radiation with fewer side effects.

Asthma and allergy
There is some evidence that vitamin C is of benefit in reducing the bronchial constriction and impaired breathing seen in asthma and allergic responses. This effect may be due to the antioxidant effect of vitamin C as oxidizing agents promote inflammation and can increase allergic responses. Vitamin C may also improve lung and white blood cell function and decrease respiratory infections and hypersensitivity reactions by reducing histamine levels. However, some studies do not support a beneficial role in vitamin C in asthma. Most studies have been short term and have assessed immediate effects of vitamin C supplementation. The effect of long-term supplementation with vitamin C is unclear.

According to researchers from the University of Washington, antioxidant vitamin supplements may help relieve the symptoms of asthma. The researchers measured the amount of breath expelled by the lungs in 17 asthma sufferers. The subjects took peak flow lung function tests while running on a treadmill and breathing in high levels of polluted air. In those asthmatics whose diets were supplemented with daily doses of 400 IU of vitamin E and 500 mg of vitamin C, an 18 per cent increase in peak flow capacity was seen.

In a 1997 study, 20 asthma patients underwent lung function tests at rest, before and one hour after receiving 2 g of oral ascorbic acid. They were then randomly assigned in a double-blind manner to receive 2 g of ascorbic acid or a placebo one hour before a 7-minute exercise session on a treadmill. Lung function tests were performed after an 8-minute rest. This procedure was repeated one week later, with each patient receiving the alternative medication. In nine patients, a protective effect on exercise-induced hyperreactive airways was seen.

Immunity
Vitamin C boosts immunity by increasing the production of B and T cells and other white blood cells, including those that destroy foreign micro-organisms. It also increases interferon levels and antibody responses and has antiviral and antibacterial effects. These effects lead to improved resistance against infections.

Vitamin C has been shown to help the immune system recover from exposure to toxic chemicals. In a 1997 study, researchers studied the effect of vitamin C on the function of several immune cells (natural killer, T and B cells) in patients who had been exposed to toxic chemicals. Fifty-five patients were given buffered vitamin C in water at a dosage of 60 mg per kg body weight (around 4g for the average man). Twenty-four hours later, the researchers tested immune cell function. The results showed that natural killer cell activity was enhanced up to ten-fold in 78 per cent of patients. B and T cell function was restored to normal.

HIV/AIDS
Vitamin C supplements are likely to be useful in HIV-positive individuals as they have been shown to boost the immune system and prevent damage to nerves. However, caution should be used with very high doses as they can cause diarrhea. Vitamin C has been shown to inhibit HIV in the laboratory and may also kill HIV-infected cells.

Common cold
Vitamin C may reduce the duration of the common cold and also the severity of symptoms such as sneezing, coughing and sniffling. Its use as a cold treatment is controversial but it seems to have several effects, including reducing blood levels of histamine which can trigger tissue inflammation and a runny nose. It may also protect the immune cells and surrounding tissue from damaging oxidative reactions that occur when cells fight bacteria.

It is possible that the effects of supplementation are greater in those with low dietary vitamin C intake. In general, men have lower vitamin C levels than women. In four studies with British girls and women, vitamin C supplementation had no marked effect on common cold. However, in four studies involving British male schoolchildren and students, a reduction in common cold occurrence was found in groups supplemented with vitamin C.

Research suggests that vitamin C supplementation may be beneficial for people who do heavy exercise and who have problems with frequent upper respiratory tract infections. Three placebo-controlled studies have examined the effect of vitamin C supplementation on common cold occurrence in people under acute physical stress. In one study the subjects were school children at a skiing camp in the Swiss Alps; in another they were military troops training in Northern Canada; and in the third they were participants in a 90 km running race. In each of the three studies, a considerable reduction in common cold incidence in the group supplemented with vitamin C at levels of 600 mg to 1000 mg per day was seen.

Cataracts
Many studies show that vitamin C can protect against cataracts, possibly by reducing oxidative damage caused by ultraviolet light. Vitamin C may act to protect the lens of the eye from this damage and protect enzymes within the lens that remove oxidation damaged proteins.

In a study published in 1992, researchers at Harvard Medical School examined the link between dietary intake of vitamins C and E, carotene, and riboflavin and cataract extraction in over 50 000 women taking part in the Nurses Health Study. The results showed that the risk of cataract was 45 per cent lower among women who used vitamin C supplements for ten or more years.

Further results from this study reported in 1997 in the American Journal of Clinical Nutrition also suggests that vitamin C supplements taken for long periods can reduce the development of cataracts. Researchers from the US Department of Agriculture and Harvard School of Public Health examined the link between cataract development and vitamin C supplement use over a ten to 12 year period. The subjects were 247 Boston area nurses aged from 56 to 71. The researchers performed detailed eye examinations to determine the degree of opacity (clouding) of the lenses of the eyes of the subjects. Results showed that use of vitamin C supplements for over ten years was associated with a 77 per cent lower prevalence of early lens opacities and an 83 per cent lower prevalence of moderate lens opacities.

Diabetes
Increasing vitamin C intake may improve blood sugar regulation in diabetics. Vitamin C administration in pharmacological doses for four months in Type II diabetes has been shown to have beneficial effects on glucose and lipid metabolism, blood circulation and capillary fragility.

In a 1995 study the effect of magnesium and vitamin C supplements on metabolic control was assessed in 56 diabetics. The study involved a 90 day run-in period followed by two 90 day treatment periods, during which patients received 600 mg of magnesium and 2 g of vitamin C per day. The results showed that vitamin C supplementation improved glycemic control, fasting blood glucose, cholesterol and triglyceride levels.

High blood sugar levels in diabetes cause a compound known as sorbitol to be manufactured from glucose. This contributes to the progression of diabetic complications. Vitamin C has been shown to reduce levels of sorbitol in diabetics. In a 58 day study carried out in 1994, researchers investigated the effect of two different doses of vitamin C supplements (100 or 600 mg) on young adults with Type I diabetes. The results showed that vitamin C supplementation at either dose normalized sorbitol levels in those with diabetes in 30 days. Vitamin C may also help to reduce capillary fragility, which also contributes to complications. The ability of the arteries to dilate is impaired in diabetics. Vitamin C supplements improve the response.

Skin protection
Vitamins C and E taken together may protect against sunburn. In a study published in 1998, German dermatologists found that people who took these vitamins had a higher threshold for sunburn reaction. The researchers tested ultraviolet sensitivity in two groups of ten Caucasian people by exposing a section of skin to UV light. Subjects in one of the groups then took 2 g of vitamin C and 1000 IU of vitamin E for eight days. The UV test was then re-done. Those taking the vitamins showed increased tolerance, particularly at higher UV doses. However, in comparison with the protection afforded by topical sunscreens, this level of protection is small.

Vitamin C-containing cosmetic skin creams such as Cellex-C have also become extremely popular in the last few years. They are designed to protect against pollutants and to promote healing.

Gallstones
Vitamin C is involved in cholesterol metabolism and deficiency may increase the risk of gallstones. In a 1998 study published in the Journal of Clinical Epidemiology, researchers in San Francisco found that vitamin C supplements reduced the prevalence of gall bladder disease by half in 2744 postmenopausal women who regularly drank alcohol. Supplement use was also associated with a 62 per cent decrease in gallstone removal. The supplements had no effect on those who did not drink.

Researchers involved in a 1997 study to test the effect of vitamin C supplements on gallstones analyzed blood fat levels, cholesterol metabolism, bile fat composition and cholesterol saturation in 16 gallstone patients. They then treated the patients with 500 mg of vitamin C four times a day for two weeks before surgery. Their findings indicated that vitamin C supplementation may also influence the conditions for cholesterol gallstone formation.

Exercise
Strenuous exercise appears to increase the levels of free radicals in the body, increasing the risk of disorders in which oxidative damage play a part. As an antioxidant, vitamin C may help to prevent this damage. In a 1997 study researchers examined the effects of supplements on oxidative stress in athletes. They found that exercise-induced oxidative stress was highest when those involved in the study did not supplement with vitamin C.

Interactions with other nutrients
Vitamin C acts together with the other antioxidants, vitamin E and beta carotene in many body processes. High levels of vitamin C appear to increase blood levels of the other antioxidants and therapeutic effects appear to be greater when combinations of antioxidants are used. Vitamin C improves the stability and use of vitamin E. However, it may interfere with selenium absorption and supplements should be taken at different times.

Vitamin C may protect against the harmful effects of beta carotene supplements in smokers. (See page 55 for more information.) Smokers tend to have low levels of vitamin C and this may allow a build-up of a harmful form of beta carotene called the carotene free radical which is formed when beta carotene acts to regenerate vitamin E. Smokers who take beta carotene supplements should also take vitamin C.

Vitamin C aids in the body’s absorption of iron by helping convert dietary iron to a soluble form. It helps to reduce the ability of food components such as phytates to form insoluble complexes with, and reduce the absorption of, iron. Vitamin C decreases the absorption of copper. Calcium and manganese supplements may decrease vitamin C excretion and vitamin C supplements may increase manganese absorption. Vitamin C also helps to reduce folic acid excretion and deficiency may lead to increased excretion of vitamin B6. Vitamin C helps to protect against the toxic effects of cadmium, copper, vanadium, cobalt, mercury and selenium.

Interactions with drugs
Large doses of vitamin C may increase estrogen levels when taken at the same time as the contraceptive pill. Oral contraceptives may increase requirements but supplements should be taken at a different time.

Aspirin, alcohol, antibiotics and steroids may increase vitamin C requirements. Vitamin C may be useful for preventing the development of tolerance to nitrate drugs which are often used to treat angina.

Cautions
There is some concern that large doses of vitamin C may cause kidney stones because part of the oxalate in calcium oxalate kidney stones comes from metabolized vitamin C. However, this is unlikely to happen in healthy people. It may be advisable for anyone suffering from recurrent kidney stones, kidney disease or who has a defect in vitamin C metabolism to keep their daily intake of vitamin C to around 100 mg.

Large doses of vitamin C may cause a false positive test result for diabetes and affect hemoglobin tests. Large doses may also affect the action of anticoagulant drugs.

A state of dependency can result from prolonged high dose consumption of vitamin C. If you have been taking large doses and decide to stop, a gradual reduction in dose is advisable.