Herbs & Supplements:
Vitamin C
Supplement Forms / Alternate Names
•
Ascorbic Acid, Ascorbate
Principal Proposed Uses
•
Colds (Treatment, Not Prevention)
, Post-exercise Colds
(Prevention)
Other Proposed Uses
•
Preeclampsia Prevention, RSD Prevention, Easy Bruising, Asthma, Hypertension, Maintaining Effectiveness of Nitrate Drugs, Muscle Soreness after Exercise, Bedsores, Low Sperm Count, Autism, Minor Injuries, Sunburn Prevention, Photosensitivity, Aging Skin, Acute Anterior Uveitis, Gallbladder Disease Prevention, Vascular Dementia Prevention, Bipolar Disorder, Bladder Infection, Diabetes, Hepatitis, Herpes, Insomnia, Menopausal Symptoms, Migraine Headaches, Nausea, Parkinson's Disease, Periodontal Disease, Restless Legs Syndrome, Rheumatoid Arthritis, Ulcers, Allergies, Cancer Treatment, HIV Support, Cancer Prevention, Heart Disease Prevention, Cataracts, Macular Degeneration, Osteoarthritis, General Antioxidant
Although most animals can make vitamin C from scratch, humans have lost the ability over the course of evolution. We must get it from food, chiefly fresh fruits and vegetables. One of this vitamin's main functions is helping the body manufacture collagen, a key protein in our connective tissues, cartilage, and tendons.
From ancient times through the early nineteenth century, sailors and others deprived of fresh fruits and vegetables developed a disease called scurvy. Scurvy involves so-called scorbutic symptoms, which include nonhealing wounds, bleeding gums, bruising, and overall weakness. Now we know that scurvy is nothing more than vitamin C deficiency.
Scurvy was successfully treated with citrus fruit during the mid-1700s. In 1928, when Albert Szent-Gyorgyi isolated the active ingredient, he called it the "anti-scorbutic principle," or ascorbic acid. This, of course, is vitamin C.
Vitamin C is a powerful antioxidant that protects against damaging natural substances called free radicals. It works in water, both inside and outside of cells. Vitamin C complements another antioxidant vitamin, vitamin E, which works in lipid (fatty) parts of the body.
Vitamin C is the single most popular vitamin supplement in the United States, and perhaps the most controversial as well. In the 1960s, two-time Nobel Prize winner Dr. Linus Pauling claimed that vitamin C could effectively treat both cancer and the common cold. Research has been mixed on both counts, but that hasn't dampened enthusiasm for this essential nutrient. The vitamin C movement has led to hundreds of clinical studies testing the vitamin on dozens of illnesses.
Requirements/Sources
Vitamin C is an essential nutrient that must be obtained from food or supplements; the body cannot manufacture it. The official US and Canadian recommendations for daily intake are as follows:
- Infants 0–6 months, 40 mg
7–12 months, 50 mg - Children 1–3 years, 15 mg
4–8 years, 25 mg
9–13 years, 45 mg - Males 14–18 years, 75 mg
19 years and older, 90 mg - Females 14–18 years, 65 mg
19 years and older, 75 mg - Pregnant women 85 mg (80 mg if 18 years old or younger)
- Nursing women 120 mg (115 mg if 18 years old or younger)
Note: Smoking significantly reduces levels of vitamin C in the body.1 The recommended daily intake for smokers is 35 mg higher across all age groups.
Scurvy, the classic vitamin C deficiency disease, is now a rarity in the developed world, although a more subtle deficiency of vitamin C is fairly common.2–6 According to one study, 40% of Americans do not get enough vitamin C.7 In fact, vitamin C deficiency sufficient to cause bleeding problems during surgery turns out to be more common than previously thought.141
Aspirin and possibly other anti-inflammatory drugs might lower body levels of vitamin C.8,9,10 Finally, oral contraceptives might decrease blood levels of vitamin C.11–15 Supplementation may be helpful if you are taking any of these medications.
Most of us think of orange juice as the quintessential source of vitamin C, but many vegetables are actually even richer sources. Red chili peppers, sweet peppers, kale, parsley, collard, and turnip greens are excellent sources, as are broccoli, Brussels sprouts, watercress, cauliflower, cabbage, and strawberries. (Oranges and other citrus fruits are good sources, too.)
One great advantage of getting vitamin C from foods rather than from supplements is that you will get many other healthy nutrients at the same time, such as bioflavonoids and carotenes. However, vitamin C in food is partially destroyed by cooking and exposure to air, so for maximum nutritional benefit you might want to try freshly made salads rather than dishes that require a lot of cooking.
Vitamin C supplements are available in two forms: ascorbic acid and ascorbate. The latter is less intensely sour.
Therapeutic Dosages
Ever since Linus Pauling, proponents have recommended taking vitamin C in enormous doses, as high as 20,000 to 30,000 mg daily. However, some evidence suggests that there might not be any reason to take more than 200 mg of vitamin C daily (10 to 100 times less than the amount recommended by vitamin C proponents).16 The reason is that if you consume more than 200 mg daily (researchers have tested up to 2,500 mg) your kidneys begin to excrete the excess at a steadily increasing rate, matching the increased dose. Your digestive tract also stops absorbing it well. The net effect is that no matter how much you take, your blood levels of vitamin C don't increase.
However, there are some flaws in this research. It is possible that vitamin C levels might rise in other tissues even if they remain constant in the blood. Furthermore, this study did not evaluate the possible effects of taking vitamin C several times daily rather than once daily.
Many nutritional experts recommend a total of 500 mg of vitamin C daily. This dose is almost undoubtedly safe. Others recommend that you take as much vitamin C as you can, up to 30,000 mg daily, cutting back only when you start to develop stomach cramps and diarrhea. This recommendation is not so much based on any evidence that such huge doses of vitamin C are good for you, but primarily on a semireligious enthusiasm.
Therapeutic Uses
According to numerous double-blind studies, regular use of vitamin C supplements can reduce symptoms of colds and shorten the length of the illness.17,18 However, regular use of vitamin C does not seem to help prevent colds.86 One exception is the “post-marathon sniffle”–colds that develop after endurance exercise.19,20 Vitamin C may be helpful for preventing this condition.
A sizable double-blind study suggests that the use of vitamin C and vitamin E supplements can reduce the risk of developing preeclampsia, a complication of pregnancy.21
One double-blind study suggests that vitamin C at a dose of 500 mg daily may help prevent reflex sympathetic dystrophy (RSD), a poorly understood condition that can follow injuries such as fractures.28
Another double-blind trial found that vitamin C supplements can decrease the tendency to bruise in individuals with marginal deficiency of the vitamin.29
Many studies have tried to evaluate whether vitamin C supplements can help asthma, with mixed results.30 The same may be said of using vitamin C to treat hypertension.31–35
Over time, the body develops tolerance to drugs in the nitrate family (such as nitroglycerin). Some evidence suggests that use of vitamin C can help maintain the effectiveness of these medications.142-145
One double-blind trial suggests that vitamin C may relieve the muscle soreness that typically develops after exercise.36
Small double-blind studies suggest that vitamin C may be able to speed recovery from bedsores.37
Vitamin C may improve sperm count and function;38 however, a recent double-blind study of 31 individuals found no benefit.39
A 10-week, double-blind, placebo-controlled study of 18 autistic children found some evidence that vitamin C might be helpful for improving behavior.40
Preliminary evidence from a somewhat poorly reported double-blind trial suggests that a combination of vitamin C and citrus bioflavonoids taken before practice can reduce the severity of athletic injuries.41
Studies on laboratory animals found that topical vitamin C and vitamin E, alone or together, helped prevent burning on exposure to ultraviolet light.42–45 One small, double-blind study found that 2 g of vitamin C and 1,000 IU of vitamin E taken for 8 days resulted in a modest decrease of "sunburn" induced by ultraviolet light.46 A 50-day, placebo-controlled study of 40 people found that higher doses of these vitamins provided a sun protection factor of about 2.47 (Compare this to the sun protection factor of 15 or higher in many sunscreens.) However, so far, research hasn't found that these vitamins, taken separately, are any more helpful than placebo.48,49 In addition, preliminary evidence suggests that cream containing vitamin C may improve the appearance of aging or sun-damaged skin.50,51
There is also evidence that vitamin C in combination with vitamin E (and the standard drug treatment) may be helpful for acute anterior uveitis. This condition involves painful inflammation of the iris of the eye, with acute sensitivity to light, increased ocular pressure, and a loss of visual clarity; it most often occurs in conjunction with autoimmune diseases. While acute anterior uveitis usually responds to conventional treatment after 10 to 14 days, vision problems may persist. In one study, the use of vitamin C at a dose of 500 mg twice daily and vitamin E (alpha-tocopherol) at 100 mg twice daily improved vision after 8 weeks as compared to placebo.52 Note: Acute anterior uveitis is a potentially dangerous condition. Physician supervision is mandatory.
Weak evidence suggests that vitamin C supplements may reduce the risk of gallbladder disease in women53 and help prevent vascular dementia (but not Alzheimer's disease).54
A small, double-blind, placebo-controlled study including 20 smokers suggests that vitamin C supplements may improve arterial function, but the effects aren't long-lasting.55
In addition, vitamin C supplements have been recommended for bipolar disorder, bladder infections, diabetes, hepatitis, herpes, insomnia, menopausal symptoms, migraine headaches, nausea, Parkinson's disease, periodontal disease, restless legs syndrome, rheumatoid arthritis, and ulcers, but there is no solid scientific basis for any of these uses.
Vitamin C is often suggested as a treatment for allergies, but the research results are very preliminary and somewhat contradictory.56,57,58
Observational studies tell us that regular use of vitamin C supplements is associated with a lower incidence of cataracts and macular degeneration.22–27 However, such findings do not indicate that if you take vitamin C supplements, you are less likely to develop these conditions. Observational trials are notoriously misleading; only intervention trials can demonstrate cause and effect.
Similarly, while a high intake of vitamin C from fruits and vegetables is associated with lower risk of cancer and heart disease, and slowed progression of osteoarthritis,59–66 there is no meaningful evidence that taking vitamin C supplements will provide any protection against these diseases.
Vitamin C has been proposed as a treatment for cancer, but this claim is very controversial, and there is as yet no scientifically meaningful evidence that it works.67–70
Massive doses of vitamin C have at times been popular among people with HIV infection based on highly preliminary evidence.71,72 An observational study linked high doses of vitamin C with slower progression to AIDS.73 However, a double-blind study of 49 people with HIV who took combined vitamins C and E or placebo for 3 months did not show any significant effects on the amount of HIV virus detected or the number of opportunistic infections.74
According to a double-blind, placebo-controlled study of 141 women with cervical dysplasia (early cervical cancer), vitamin C, taken at a dosage of 500 mg daily, does not help to reverse the dysplasia.75
Heated disagreement exists regarding whether it is safe or appropriate to combine vitamin C with standard chemotherapy drugs.76,77 The reasoning behind this concern is that many chemotherapy might drugs work in part by creating free radicals that destroy cancer cells. Antioxidants like vitamin C might interfere with this beneficial effect. Indeed, some cancer cells appear to accumulate vitamin C to protect themselves from injury! On the other hand, some evidence suggests that vitamin C may help reduce the side effects of certain chemotherapy drugs without decreasing their effectiveness.78,79 Nonetheless, in view of the high stakes involved, we strongly recommend that you do not take any supplements while undergoing cancer chemotherapy except on the advice of a physician.
What Is the Scientific Evidence for Vitamin C?
Colds (in General)
As the most famous of all natural treatments for colds, vitamin C has been subjected to irresponsible hype from both proponents and opponents. Enthusiasts claim that if you take vitamin C daily, you will never get sick, while critics of the treatment insist that vitamin C has no benefit at all.
However, a cool-headed evaluation of the research indicates something in between. Numerous studies have found that vitamin C supplements taken at a dose of 1,000 mg daily or more can modestly reduce symptoms of colds and help you get over a cold faster.
This evidence regards daily use of vitamin C throughout the cold season. Many people use vitamin C for colds in a different way: they only begin taking it when cold symptoms start. Relatively few studies have evaluated this approach.83
A large, double-blind, placebo-controlled trial was recently conducted to determine whether vitamin C helps when taken only at the onset of a cold.140 This double-blind trial enrolled 400 individuals with new-onset cold symptoms. Participants were divided into four different daily vitamin C dosage groups: 30mg daily (a dose lower than the minimum daily requirement, and used by the researchers as a placebo), 1000 mg, 3000 mg, or 3000 mg with bioflavanoids. Participants were instructed to take the vitamin at the onset of symptoms and for the following two days.
The results showed no difference in the duration or severity of cold symptoms among the groups. High dose vitamin C, in other words, didn't work.
There are two main possible explanations for these negative results. One possibility is that taking high dose vitamin C at the onset of a cold is not as effective as taking it continuously. Another possibility is that taking vitamin C at the onset of a cold could work, but that two days of treatment simply isn't long enough (It is actually a bit surprising that researchers used such a short treatment schedule.)
Preventing Colds
In general, vitamin C has failed to prove effective for preventing colds, except perhaps in individuals who are truly vitamin C deficient.86 (See Post-exercise Colds below for another situation in which vitamin C may help.)
Post-exercise Colds
Heavy endurance exercise temporarily weakens the immune system, leading to a high incidence of infection following marathons, triathlons, or similar forms of exercise. Vitamin C may be helpful.
According to a double-blind, placebo-controlled study involving 92 runners, taking 600 mg of vitamin C for 21 days prior to a race made a significant difference in the incidence of sickness afterwards.87 Within 2 weeks after the race, 68% of the runners taking placebo developed cold symptoms versus only 33% of those taking the vitamin C supplement. As part of the same study, non-runners of similar age and gender to those running were also given vitamin C or placebo. Interestingly, for this group, the supplement had no apparent effect on the incidence of upper respiratory infections. Vitamin C seemed to be effective in this capacity only for those who exercised intensively!
Two other studies found that vitamin C could reduce the number of colds experienced by groups of people involved in rigorous exercise in extremely cold environments.88 One study involved 139 children attending a skiing camp in the Swiss Alps, while the other enrolled 56 military men engaged in a training exercise in Northern Canada during the winter months. In both cases, the participants took either 1 g of vitamin C or placebo daily at the time their training program began. Cold symptoms were monitored for 1 to 2 weeks following training, and significant differences in favor of vitamin C were found.
However, one very large study of 674 marine recruits in basic training found no such benefit.89 The results showed no difference in the number of colds between the treatment and placebo groups.
What's the explanation for this discrepancy? There are many possibilities. Perhaps basic training in the marines is significantly different from the other forms of exercise studied. Another point to consider is that the marines didn't start taking vitamin C right at the beginning of training, but waited 3 weeks. The study also lasted a bit longer than the positive studies mentioned above, continuing for 2 months; maybe vitamin C is more effective at preventing colds in the short term. Of course, another possibility is that it doesn't really work. More research is needed to know for sure.
Preeclampsia Prevention
Preeclampsia is a dangerous complication of pregnancy that involves high blood pressure, swelling of the whole body, and improper kidney function. A double-blind placebo-controlled study of 283 women at increased risk for preeclampsia found that supplementation with vitamin C (1000 mg daily) and vitamin E (400 IU daily) significantly reduced the chances of developing this disease.96
While this research is promising, larger studies are necessary to confirm whether vitamins C and E will actually work. The authors of this study point out that similarly sized studies found benefits with other treatments, such as aspirin, that later proved to be ineffective when large-scale studies were performed. Furthermore, keep in mind that we don't know whether such high dosages of these vitamins are absolutely safe for pregnant women.
Cancer Treatment
Cancer treatment is one of the more controversial proposed uses of vitamin C. An early study tested vitamin C in 1100 terminally ill cancer patients. One hundred patients received 10,000 mg daily of vitamin C, while 1000 other patients (the control group) received no treatment. Those taking the vitamin survived more than 4 times longer on average (210 days) than those in the control group (50 days).102 A large (1826 subjects) follow-up study by the same researchers found a nearly doubled survival rate (343 days versus 180 days) in vitamin C–treated patients whose cancers were deemed "incurable," as compared to untreated controls.103 However, these studies were poorly designed, and other generally better-constructed studies have found no benefit of vitamin C in cancer.104,105 At the present time, vitamin C cannot be regarded as a proven treatment for cancer.
Reflex Sympathetic Dystrophy (RSD)
RSD is a set of symptoms that occasionally develops in the legs or arms after fractures and other injuries. The condition involves persistent pain, changes in skin temperature, redness, swelling, and difficulty in movement. Its cause is unknown, and it is very difficult, if not impossible, to treat, creating significant suffering and disability.
A double-blind study set out to find whether vitamin C could prevent RSD from developing in individuals who had sustained wrist fractures.110 A total of 123 adults with wrist fractures were enrolled and followed for 1 year. All were given 500 mg of vitamin C or placebo daily for 50 days. The results showed significantly fewer cases of RSD in the treated group.
If these results hold up in larger studies, vitamin C treatment could become part of the standard treatment of fractures.
Easy Bruising
A 2-month, double-blind study of 94 elderly people with marginal vitamin C deficiency found that vitamin C supplements decreased their tendency to bruise.111
Hypertension (High Blood Pressure)
According to a 30-day, double-blind study of 39 individuals taking medications for hypertension, treatment with 500 mg of vitamin C daily can reduce blood pressure by about 10%.112 Smaller benefits were seen in studies of individuals with normal blood pressure or borderline hypertension.113,114 However, other studies have failed to find evidence of benefit .115,116
Maintaining the Effectiveness of Nitrate Drugs
Nitroglycerin and related nitrate medications are used for the treatment of angina. However, the effectiveness of these medications tends to diminish over time.
According to a double-blind study of 48 individuals, use of vitamin C at a dose of 2000 mg three times daily helped maintain the effectiveness of nitroglycerin.142 These findings are supported by other studies as well.143-145
Note: Angina is too serious a disease for self-treatment. If you have angina, do not take vitamin C (or any other supplement) except on a physician’s advice.
Safety Issues
The US government has issued recommendations regarding "tolerable upper intake levels" (ULs) for vitamin C. The UL can be thought of as the highest daily intake over a prolonged time known to pose no risks to most members of a healthy population. The ULs for vitamin C are as follows:
- Children 1–3 years, 400 mg
4–8 years, 650 mg
9–13 years, 1200 mg - Males and females 14–18 years, 1800 mg
19 years and older, 2000 mg - Pregnant women 2000 mg (1800 mg if 18 years old or younger)
- Nursing women 2000 mg (1,800 mg if 18 years old or younger)
However, even within the safe intake range for vitamin C, some individuals may develop diarrhea. This side effect will likely go away with continued use of vitamin C, but you might have to cut down your dosage for a while and then gradually build up again.
Concerns have been raised that long-term vitamin C treatment can cause kidney stones.129,130 However, in large-scale observational studies, individuals who consume large amounts of vitamin C have shown either no change or a decreased risk of kidney stone formation.131,132,133 Still, there may be certain individuals who are particularly at risk for vitamin C–induced kidney stones.134 People with a history of kidney stones and those with kidney failure who have a defect in vitamin C or oxalate metabolism should probably restrict vitamin C intake to approximately 100 mg daily. You should also avoid high-dose vitamin C if you have glucose-6-phosphate dehydrogenase deficiency, iron overload, or a history of intestinal surgery.
Vitamin C supplements increase absorption of iron.121–128 Since it isn’t good to get more iron than you need, individuals using iron supplements shouldn’t take vitamin C at the same time except under a physician’s supervision.
One study from the 1970s suggests that very high doses of vitamin C (3 g daily) might increase the levels of acetaminophen (e.g., Tylenol) in the body.135 This could potentially put you at higher risk for acetaminophen toxicity. This interaction is probably relatively unimportant when acetaminophen is taken in single doses for pain and fever, or for a few days during a cold. However, if you use acetaminophen daily or have kidney or liver problems, simultaneous use of high-dose vitamin C is probably not advisable.
Finally, weak evidence suggests that vitamin C, when taken in high doses, might reduce the blood-thinning effects of Coumadin (warfarin) and heparin.136–139
The maximum safe dosages of vitamin C for people with severe liver or kidney disease have not been determined.
Interactions You Should Know About
If you are taking
- Aspirinother anti-inflammatory drugs, or oral contraceptives: You may need more vitamin C.
- Acetaminophen (Tylenol): The risk of liver damage from high doses of acetaminophen may be increased if you also take large doses of vitamin C.
- Coumadin (warfarin) or heparin: High-dose vitamin C might reduce their effectiveness.
- Iron supplements: High-dose vitamin C can cause you to absorb too much iron. This is especially a problem for people with diseases that cause them to store too much iron.
- Medications in the Nitrate Family: Vitamin C may help maintain their effectiveness. Note: Angina is too serious a disease for self-treatment. If you have angina, do not take vitamin C (or any other supplement) except on a physician’s advice.
View References
Last reviewed August 2002 by Medical Review Board
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