A review of 21 controlled trials using 1 to 8 grams of vitamin C per day found that “in each of the twenty-one studies, vitamin C reduced the duration of episodes and the severity of the symptoms of the common cold by an average of 23%.”¹ The optimum amount of vitamin C to take for cold treatment remains in debate but may be as high as 1 to 3 grams per day, considerably more than the 120 to 200 mg per day that has been suggested as optimal intake for healthy adults. A review of 23 controlled trials found that vitamin C supplementation produces a greater benefit for children than for adults.² The same review found that a daily amount of 2 grams or more was superior to a daily amount of 1 gram at reducing the duration of cold symptoms.

Vitamin C has antiviral activity, and may help prevent viral infections³ or, in the case of the common cold, reduce the severity and duration of an infection.⁴ Most studies on the common cold used 1 to 4 grams of vitamin C per day.

In a double-blind study of elderly patients hospitalized with acute bronchitis, those who were given 200 mg per day of vitamin C improved to a significantly greater extent than those who were given a placebo.⁵ The common cold may lead to bronchitis in susceptible people, and numerous controlled studies, some double-blind, have shown that vitamin C supplements can decrease the severity and duration of the common cold in otherwise healthy people.⁶

Vitamin C and vitamin E may prevent oxidative damage to the lung lipids by environmental pollution and cigarette smoke exposure. It has been suggested that amounts in excess of the RDA (recommended dietary allowance) are necessary to protect against the air pollution levels currently present in North America,⁷ although it is not known how much vitamin E is needed to produce that protective effect.

Most,^8,9 but not all,¹⁰ double-blind studies have shown that elderly people have better immune function and reduced infection rates when taking a multiple vitamin-mineral formula. In one double-blind trial, supplements of 100 mcg per day of selenium and 20 mg per day of zinc, with or without additional vitamin C, vitamin E, and beta-carotene, reduced infections in elderly people, though vitamins without minerals had no effect.¹¹ Burn victims have also experienced fewer infections after receiving trace mineral supplements in double-blind research.¹² These studies suggest that trace minerals may be the most important micronutrients for enhancing immunity and preventing infections in the elderly.

Vitamin C stimulates the immune system by both elevating interferon levels¹³ and enhancing the activity of certain immune cells.^14,15 Two studies came to opposite conclusions about the ability of vitamin C to improve immune function in the elderly,^16,17 and two other studies did not agree on whether vitamin C could protect people from hepatitis.^18,19 However, a review of 20 double-blind studies concluded that while several grams of vitamin C per day has only a small effect in preventing colds, when taken at the onset of a cold, it does significantly reduce the duration of a cold.²⁰ In controlled reports studying people doing heavy exercise, cold frequency was reduced an average of 50% with vitamin C supplements ranging from 600 to 1,000 mg per day.²¹ Thus, the overall effect of vitamin C on immune function is unclear, and its usefulness may vary according to the situation.

A combination of antioxidants vitamin A, vitamin C, and vitamin E significantly improved immune cell number and activity compared with placebo in a group of hospitalized elderly people.²² Daily intake of a 1,000 mg vitamin C plus 200 IU vitamin E for four months improved several measures of immune function in a preliminary study.²³ To what extent immune-boosting combinations of antioxidants actually reduce the risk of infection remains unknown.

Dockworkers given 100 mg of vitamin C each day for ten months caught influenza 28% less often than did their coworkers not taking vitamin C. Of those who did develop the flu, the average duration of illness was 10% less in those taking vitamin C than in those not taking the vitamin.²⁴ Other trials have reported that taking vitamin C in high amounts (2 grams every hour for 12 hours) can lead to rapid improvement of influenza infections.^25,26 Such high amounts, however, should only be used under the supervision of a healthcare professional.

Vitamin C has been shown to inactivate herpes viruses in the test tube.²⁷ In one study, people with herpes infections received either a placebo or 200 mg of vitamin C plus 200 mg of flavonoids, each taken three to five times per day. Compared with the placebo, vitamin C and flavonoids reduced the duration of symptoms by 57%.²⁸

Vitamin C deficiency can be detrimental to immune function in hospitalized patients,²⁹ and one study found that half of surgery patients recovering at home had low dietary intakes of vitamin C.³⁰ Vitamin C is also a critical nutrient for wound healing,^31,32 but studies of vitamin C supplementation have shown only minor effects on the healing of surgical wounds.^33,34 Vitamin C deficiency also can increase the risk of excessive bleeding in the surgical setting.³⁵

Antioxidants may protect the skin from sunburn due to free radical–producing ultraviolet rays.³⁶ Combinations of 1,000 to 2,000 IU per day of vitamin E and 2,000 to 3,000 mg per day of vitamin C, but neither given alone, have a significant protective effect against ultraviolet rays, according to double-blind studies.^37,38,39

Oral synthetic beta-carotene alone was not found to provide effective protection when given in amounts of 15 mg per day or for only a few weeks’ time in larger amounts of 60 to 90 mg per day, but it has been effective either in very large (180 mg per day) amounts or in smaller amounts (30 mg per day) in combination with topical sunscreen.^{40,41,42,43,44}

Natural sources of beta-carotene or other carotenoids have been more consistently shown to protect against sunburn. One controlled study found that taking a supplement of natural carotenoids (almost all of which was beta-carotene) in daily amounts of 30 mg, 60 mg, and 90 mg gave progressively more protection against ultraviolet rays.⁴⁵ In another controlled study, either 24 mg per day of natural beta-carotene or 24 mg per day of a carotenoid combination of equal amounts beta-carotene, lutein, and lycopene helped protect skin from ultraviolet rays.⁴⁶ A preliminary study compared synthetic lycopene (10.1 mg per day), a natural tomato extract containing 9.8 mg of lycopene per day plus additional amounts of other carotenoids, and a solubilized tomato drink (designed to increase lycopene absorption) containing 8.2 mg of lycopene plus additional amounts of other carotenoids. After 12 weeks, only the two tomato-based products were shown to give significant protection against burning by ultraviolet light.⁴⁷

Still other trials have tested combinations of several antioxidants. One preliminary study found that a daily combination of beta-carotene (6 mg), lycopene (6 mg), vitamin E (15 IU), and selenium for seven weeks protected against ultraviolet light.⁴⁸ However, a double-blind trial of a combination of smaller amounts of several carotenoids, vitamins C and E, selenium, and proanthocyanidins did not find significant UV protection compared with placebo.⁴⁹ Similarly, in a controlled trial, a combination of selenium, copper, and vitamins was found to be ineffective.⁵⁰

It should be noted that while oral protection from sunburn has been demonstrated with several types of antioxidants, the degree of protection (typically less than an SPF of 2) is much less than that provided by currently available topical sunscreens. On the other hand, these modest effects will provide some added protection to skin areas where sunscreen is also used and will give a small amount of protection to sun-exposed areas where sunscreen is not applied. However, oral protection from sunburn is not instantaneous; maximum effects are not reached until these antioxidants have been used for about eight to ten weeks.^51,52

Vitamin C is needed to make collagen (connective tissue) that strengthens skin, muscles, and blood vessels and to ensure proper wound healing. Severe injury appears to increase vitamin C requirements,⁵³ and vitamin C deficiency causes delayed healing.⁵⁴ Preliminary human studies suggest that vitamin C supplementation in non-deficient people can speed healing of various types of wounds and trauma, including surgery, minor injuries, herniated intervertebral discs, and skin ulcers.^54,56 A combination of 1–3 grams per day of vitamin C and 200–900 mg per day of pantothenic acid has produced minor improvements in the strength of healing skin tissue.^57,58

Antioxidants have been studied as topical agents for protection against sunburn. Animal studies have found sunscreen-like effects from topical application of a vitamin C and vitamin E combination, and a controlled human study reported ultraviolet protection from the use of a lotion containing 0.02% to 0.05% of the selenium-containing amino acid known as selenomethionine.^58,59 The topical use of the hormone melatonin has been shown to protect human skin against ultraviolet rays in double-blind research.^60,61 A double-blind human trial tested topical vitamins C and E and melatonin, alone and in combinations, and found the highest degrees of protection from combination formulations containing 2% vitamin E, 5% vitamin C, and 1% to 2.5% melatonin.⁶² Other studies in which topical antioxidants were applied after ultraviolet exposure have found no benefits.^63,64

Antioxidants have been studied as topical agents for protection against sunburn. Animal studies have found sunscreen-like effects from topical application of a vitamin C and vitamin E combination, and a controlled human study reported ultraviolet protection from the use of a lotion containing 0.02% to 0.05% of the selenium-containing amino acid known as selenomethionine.^65,66 The topical use of the hormone melatonin has been shown to protect human skin against ultraviolet rays in double-blind research.^67,68 A double-blind human trial tested topical vitamins C and E and melatonin, alone and in combinations, and found the highest degrees of protection from combination formulations containing 2% vitamin E, 5% vitamin C, and 1% to 2.5% melatonin.⁶⁹ Other studies in which topical antioxidants were applied after ultraviolet exposure have found no benefits.^70,71

Antioxidants such as vitamin C, vitamin E, and glutathione are depleted in healing skin tissue.⁷² One animal study found that vitamin E (alpha-tocopherol) applied to the skin shortened the healing time of skin ulcers.⁷³ Another animal study reported that administration of oral vitamin E before skin lesions were introduced into the skin prevented some of the tissue damage associated with the development of pressure ulcers.⁷⁴ A controlled human trial found that 400 IU of vitamin E daily improved the results of skin graft surgery for chronic venous ulcers.⁷⁵ No further research has investigated the potential benefit of vitamin E for skin ulcers.

Animal research has suggested that vitamin C may help prevent skin ulcers,⁷⁶ and in a preliminary study,⁷⁷ elderly patients with pressure ulcers had lower blood levels of vitamin C than did ulcer-free patients. Supplementation with vitamin C (3 grams per day) increased the speed of healing of leg ulcers in patients with a blood disorder called thalassemia, according to a double-blind study.⁷⁸ And while a double-blind trial of surgical patients with pressure ulcers found that supplementation with 500 mg of vitamin C twice a day accelerated ulcer healing,⁷⁹ a similar double-blind trial found no difference in the effectiveness of either 20 mg per day or 1,000 mg per day of vitamin C.⁸⁰

In 1989, Medical World News reported that researchers from the University of Texas found that vitamin C, at 50–75 mg per 2.2 pounds of body weight, reduced symptoms of eczema in a double-blind trial.⁸¹ In theory, vitamin C might be beneficial in treating eczema by affecting the immune system, but further research has yet to investigate any role for this vitamin in people with eczema.

A clinical report describes the use of vitamin supplements in the treatment of vitiligo.⁸²Folic acid and/or vitamin B12 and vitamin C levels were abnormally low in most of the 15 people studied. Supplementation with large amounts of folic acid (1–10 mg per day), along with vitamin C (1 gram per day) and intramuscular vitamin B12 injections (1,000 mcg every two weeks), produced marked repigmentation in eight people. These improvements became apparent after three months, but complete repigmentation required one to two years of continuous supplementation. In another study of people with vitiligo, oral supplementation with folic acid (10 mg per day) and vitamin B12 (2,000 mcg per day), combined with sun exposure, resulted in some repigmentation after three to six months in about half of the participants.⁸³ This combined regimen was more effective than either vitamin supplementation or sun exposure alone.

Vitamin C is needed to make collagen (connective tissue) that strengthens skin, muscles, and blood vessels and to ensure proper wound healing. Severe injury appears to increase vitamin C requirements,⁸⁴ and vitamin C deficiency causes delayed healing.⁸⁵ Preliminary human studies suggest that vitamin C supplementation in non-deficient people can speed healing of various types of wounds and trauma, including surgery, minor injuries, herniated intervertebral discs, and skin ulcers.^85,87 A combination of 1–3 grams per day of vitamin C and 200–900 mg per day of pantothenic acid has produced minor improvements in the strength of healing skin tissue.^88,89

Doctors often suggest that people who experience easy bruising supplement with 100 mg to 3 grams of vitamin C per day for several months. Controlled research is limited, but vitamin C supplements have been shown to reduce bruising in people with low vitamin C intake.⁸⁹Flavonoids are often recommended along with vitamin C. Flavonoids are vitamin-like substances that can help strengthen capillaries and therefore may also help with bruising.⁹⁰ Flavonoids may also increase the effectiveness of vitamin C; citrus flavonoids, in particular, improve the absorption of vitamin C. Older preliminary research suggested that vitamin C, 400–800 mg per day, in combination with 400–800 mg per day of the flavonoid, hesperidin, reduced bruising in menopausal women.⁹¹ A small, preliminary trial in Germany gave three people with progressive pigmented purpura (a chronic bruising disorder) 1,000 mg per day of vitamin C and 100 mg per day of the flavonoid rutoside. After four weeks, noticeable bruising was no longer apparent and did not recur in the three month period after treatment was stopped.⁹² Controlled research is needed to better establish whether vitamin C and flavonoids are effective for easy bruising.

Antioxidant supplements, including vitamin C and vitamin E, may help prevent exercise-related muscle injuries by neutralizing free radicals produced during strenuous activities.⁹³ Controlled research, some of it double-blind, has shown that 400–3,000 mg per day of vitamin C may reduce pain and speed up muscle strength recovery after intense exercise.^94,95 Reductions in blood indicators of muscle damage and free radical activity have also been reported for supplementation with 400–1,200 IU per day of vitamin E in most studies,^96,97,98 but no measurable benefits in exercise recovery have been reported.⁹⁹ A combination of 90 mg per day of coenzyme Q10 and a very small amount of vitamin E did not produce any protective effects in one double-blind trial.¹⁰⁰

Vitamin C is needed to make collagen, the “glue” that strengthens connective tissue. Injury, at least when severe, appears to increase vitamin C requirements,¹⁰¹ and vitamin C deficiency causes delayed healing from injury.¹⁰² Preliminary human studies have suggested that vitamin C supplementation in non-deficient people can speed healing of various types of trauma, including musculoskeletal injuries,^102,104 but double-blind research has not confirmed these effects for athletic injuries, which included sprains and strains.¹⁰⁵

A preliminary report in 1964 suggested that 500–1,000 mg per day of vitamin C helped many people avoid surgery for their disc-related low back pain.¹⁰⁵ No controlled research has been done to examine this claim further.

Severe vitamin C deficiency (scurvy) is a well-recognized but uncommon cause of increased capillary fragility. Whether vitamin C supplementation can help capillary fragility in people who do not have scurvy is less clear. Patients undergoing dialysis may develop low levels of vitamin C,^110,111 which can lead to capillary fragility, but giving dialysis patients 50 mg of vitamin C per day had no effect on capillary fragility in one study.¹¹² People with kidney failure and those undergoing dialysis should not supplement with more than 100 mg per day, unless supervised by a doctor.

According to preliminary studies, vitamin C may reduce capillary weakness in diabetics, who often have low blood levels of vitamin C compared to non-diabetics.^113,114 In a double-blind trial, elderly people with low vitamin C levels and capillary fragility were helped with supplementation of one gram per day of vitamin C.¹¹⁵

Experimentally increasing homocysteine levels in humans has led to temporary dysfunction of the cells lining blood vessels. Researchers are concerned this dysfunction may be linked to atherosclerosis and heart disease. Vitamin C has been reported in one controlled study to reverse the dysfunction caused by increases in homocysteine.¹¹⁶ Vitamin C also protects LDL.¹¹⁷

Despite the protective mechanisms attributed to vitamin C, some research has been unable to link vitamin C intake to protection against heart disease. These negative trials have mostly been conducted using people who consume 90 mg of vitamin C per day or more—a level beyond which further protection of LDL may not occur. Studies of people who eat foods containing lower amounts of vitamin C have been able to show a link between dietary vitamin C and protection from heart disease. Therefore, leading vitamin C researchers have begun to suggest that vitamin C may be important in preventing heart disease, but only up to 100–200 mg of intake per day.¹¹⁸ In a double-blind trial,¹¹⁹ supplementation with 250 mg of timed-release vitamin C twice daily for three years resulted in a 15% reduction in the progression of atherosclerosis, compared with placebo. Many doctors suggest that people take vitamin C—often 1 gram per day—despite the fact that research does not yet support levels higher than 500 mg per day.

A review of nutrition and lung health reported that people with a higher dietary intake of vitamin C were less likely to be diagnosed with bronchitis.¹³⁶ As yet, the effects of supplementing with vitamin C in people with COPD have not been studied.

Vitamin C has been reported to protect blood vessels from problems associated with heart attack risk in a variety of ways.^137,138,139 However, research attempting to link vitamin C directly to protection from heart attacks has been inconsistent.^140,141 The reason for this discrepancy appears related to the amount of vitamin C intake investigated in these studies. True or marginal vitamin C deficiencies do appear to increase the risk of suffering heart attacks.^142,143 However, in trials comparing acceptable (i.e., non-deficient) vitamin C levels to even higher levels, additional vitamin C has not been protective.¹⁴⁴ Therefore, though many doctors recommend that people at high risk for heart attack take vitamin C—often 1 gram per day—most evidence currently suggests that consuming as little as 100–200 mg of vitamin C per day from food or supplements may well be sufficient.

People who are deficient in vitamin C may be at increased risk for periodontal disease.¹⁴⁵ When a group of people with periodontitis who normally consumed only 20–35 mg of vitamin C per day were given an additional 70 mg per day, objective improvement of periodontal tissue occurred in only six weeks.¹⁴⁶ It makes sense for people who have a low vitamin C intake (e.g., people who eat few fruits and vegetables) to supplement with vitamin C in order to improve gingival health.

Vitamin C has been shown to inactivate herpes viruses in the test tube.¹⁴⁷ In one study, people with herpes infections received either a placebo or 200 mg of vitamin C plus 200 mg of flavonoids, each taken three to five times per day. Compared with the placebo, vitamin C and flavonoids reduced the duration of symptoms by 57%.¹⁴⁸

People who are deficient in vitamin C may be at increased risk for periodontal disease.¹⁴⁹ When a group of people with periodontitis who normally consumed only 20–35 mg of vitamin C per day were given an additional 70 mg per day, objective improvement of periodontal tissue occurred in only six weeks.¹⁵⁰ It makes sense for people who have a low vitamin C intake (e.g., people who eat few fruits and vegetables) to supplement with vitamin C in order to improve gingival health.

For people who consume adequate amounts of vitamin C in their diet, several studies have found that supplemental vitamin C has no additional therapeutic effect. Research,¹⁵¹ including double-blind evidence,¹⁵² shows that vitamin C fails to significantly reduce gingival inflammation in people who are not vitamin C deficient. In one study, administration of vitamin C plus flavonoids (300 mg per day of each) did improve gingival health in a group of people with gingivitis;¹⁵³ there was less improvement, however, when vitamin C was given without flavonoids. Preliminary evidence has suggested that flavonoids by themselves may reduce inflammation of the gums.¹⁵⁴

Animal studies suggest that supplementing with vitamin C can reduce blood levels of stress-related hormones and other measures of stress.^{155,156,157,158} Controlled studies of athletes have shown that vitamin C supplementation (1,000 to 1,500 mg per day) can reduce stress hormone levels after intense exercise.^159,160 Surgery patients given 2,000 mg per day of vitamin C during the week before and after surgery had a more rapid return to normal of several stress-related hormones compared with patients not given vitamin C.¹⁶¹ In a double-blind trial, young adults took 3,000 mg per day of vitamin C for two weeks, then were given a psychological stress test involving public speaking and mental arithmetic.¹⁶² Compared with a placebo group, those taking vitamin C rated themselves less stressed, scored better on an anxiety questionnaire, had smaller elevations of blood pressure, and returned sooner to lower levels of an adrenal stress hormone following the stress test.

Premature rupture of membranes (PROM) affects 10 to 20% of all pregnancies. It is an important cause of preterm delivery and is associated with increased rates of complications in both the mother and child. In a double-blind study, supplementing with 100 mg of vitamin C per day, beginning in the twentieth week of pregnancy, reduced the incidence of PROM by 74%.¹⁶⁶ The women in this study were consuming only about 65 mg of vitamin C per day in their diet, which is less than the RDA of 80 to 85 mg per day for pregnant women. In a double-blind study of pregnant smokers, supplementation with 500 mg per day of vitamin C, beginning at 23 weeks of pregnancy or earlier and continuing until delivery, improved lung function and decreased the incidence of wheezing in the offspring.¹⁶⁷

In some women, infertility is due to a hormonal abnormality known as luteal phase defect. In this condition, the uterine lining does not develop and mature properly, presumably because of a deficiency of the hormone progesterone. In a study of infertile women with luteal phase defect, supplementation with 750 mg of vitamin C per day for up to six months resulted in a pregnancy rate of 25%, compared with a rate of 11% in an untreated control group, a statistically significant difference.¹⁶⁹

In 1964, a preliminary trial reported that 1,200 mg each of vitamin C and the flavonoid hesperidin taken over the course of the day helped relieve hot flashes.¹⁷⁰ Although placebo effects are strong in women with hot flashes, other treatments used in that trial failed to act as effectively as the flavonoid/vitamin C combination. Since then, researchers have not explored the effects of flavonoids or vitamin C in women with menopausal symptoms.

Both vitamin C and flavonoids protect capillaries (small blood vessels) from damage. In so doing, they might protect against the blood loss of menorrhagia. In one small study, 88% of women with menorrhagia improved when given 200 mg vitamin C and 200 mg flavonoids three times per day.¹⁷¹ In another study, 70% of women with excessive menstrual bleeding experienced at least a 50% reduction in bleeding after taking a flavonoid product.¹⁷² The preparation used in this study contained 90% diosmin and 10% hesperidin and was given in the amount of 1,000 mg per day, beginning five days prior to the expected start of menstruation and continuing until the end of bleeding for three cycles.

Several studies have shown that supplementing with vitamin C can significantly reduce elevated intraocular pressure in individuals with glaucoma.¹⁷³ These studies used at least 2 grams per day of vitamin C; much larger amounts were sometimes given. Higher quantities of vitamin C appeared to be more effective than smaller amounts.

Doctors often suggest that people with glaucoma take vitamin C to “bowel tolerance.”¹⁷⁴ The bowel-tolerance level is determined by progressively increasing vitamin C intake until loose stools or abdominal pain occurs, and then reducing the amount slightly, to a level that does not cause these symptoms. The bowel tolerance level varies considerably from person to person, usually ranging from about 5 to 20 or more grams per day. Vitamin C does not cure glaucoma and must be used continually to maintain a reduction in intraocular pressure.

Sunlight triggers oxidative damage in the eye, which in turn can cause macular degeneration.¹⁷⁵ Because vitamin C functions as an antioxidant, it has the potential to protect against macular degeneration. However, in a double-blind trial, supplementing with 500 mg of vitamin C daily for eight years did not decrease the incidence of macular degeneration in healthy male physicians.¹⁷⁶

People with low blood levels of antioxidants and those who eat few antioxidant-rich fruits and vegetables have been reported to be at high risk for cataracts.^177,178

The major antioxidants in the lens of the eye are vitamin C¹⁷⁹ and glutathione (a molecule composed of three amino acids).¹⁸⁰ Vitamin C is needed to activate vitamin E,¹⁸¹ which in turn activates glutathione. Both nutrients are important for healthy vision. People who take multivitamins or any supplements containing vitamins C or E for more than 10 years have been reported to have a 60% lower risk of forming a cataract.¹⁸²

Vitamin C levels in the eye decrease with age.¹⁸³ However, supplementing with vitamin C prevents this decrease¹⁷⁹ and has been linked to a lower risk of developing cataracts.^177,186 Healthy people are more likely to take vitamin C and vitamin E supplements than those with cataracts according to some,¹⁸⁷ but not all,¹⁸⁸ studies. Dietary vitamin C intake has not been consistently associated with protection from cataracts.^189,190 Nonetheless, because people who supplement with vitamin C have developed far fewer cataracts in some research,^191,192 doctors often recommend 500 to 1,000 mg of vitamin C supplementation as part of a cataract prevention program. The difference between successful and unsuccessful trials may be tied to the length of time people actually supplement with vitamin C. In one preliminary study, people taking vitamin C for at least ten years showed a dramatic reduction in cataract risk, but those taking vitamin C for less than ten years showed no evidence of protection at all.¹⁹³

Vitamin C, at a dose of 500 mg twice per day, was found to reduce fasting blood glucose levels, although not HbA1c, after four months in a placebo-controlled trial.¹⁹² A meta-analysis of other controlled trials found vitamin C supplementation can reduce blood glucose levels (but not HbA1c) in people with type 2 diabetes, and is especially effective in those who are older and those who have taken vitamin C for 30 days or more.¹⁹³

Through its anti-inflammatory and antioxidant effects, vitamin C has been shown to lower blood pressure and improve cardiovascular health in people with type 2 diabetes.¹⁹⁴ In addition, like other antioxidants, vitamin C reduces formation of advanced glycation end-products (AGEs), which cause widespread damage to proteins and DNA.¹⁹⁵ Antioxidants like vitamins C and E protect against damage to micro-vessels and therefore have the potential to help prevent type 2 diabetes complications such as retinopathy, nephropathy, and neuropathy.^196,197 Furthermore, low vitamin C levels in blood and in the eye have been correlated with increased risk of retinopathy.^198,199 In a pilot study, a combination supplement with vitamin C and other antioxidant vitamins, minerals, and plant extracts improved vision in subjects with type 2 diabetes and early stage retinopathy.²⁰⁰

High-dose vitamin C supplementation has been reported to interfere with some devices for self-monitoring of blood glucose levels. This can raise the risk of overtreatment leading to hypoglycemia.²⁰¹ Therefore it is important that people with type 2 diabetes who rely on self-monitoring glucose devices talk with their healthcare provider before starting a vitamin C supplement.

Research has shown that supplementing with chromium (200 mcg per day)²¹⁶ or magnesium (340 mg per day)²¹⁷ can prevent blood sugar levels from falling excessively in people with hypoglycemia. Niacinamide (vitamin B3) has also been found to be helpful for hypoglycemic people.²¹⁸ Other nutrients, including vitamin C, vitamin E, zinc, copper, manganese, and vitamin B6, may help control blood sugar levels in diabetics.²¹⁹ Since there are similarities in the way the body regulates high and low blood sugar levels, these nutrients might be helpful for hypoglycemia as well, although the amounts needed for that purpose are not known.

Both vitamin C and flavonoids protect capillaries (small blood vessels) from damage. In so doing, they might protect against the blood loss of menorrhagia. In one small study, 88% of women with menorrhagia improved when given 200 mg vitamin C and 200 mg flavonoids three times per day.²²⁴ In another study, 70% of women with excessive menstrual bleeding experienced at least a 50% reduction in bleeding after taking a flavonoid product.²²⁵ The preparation used in this study contained 90% diosmin and 10% hesperidin and was given in the amount of 1,000 mg per day, beginning five days prior to the expected start of menstruation and continuing until the end of bleeding for three cycles.

Supplementation with 1 gram of vitamin C per day reduces the tendency of the bronchial passages to go into spasm,²²⁶ an action that has been confirmed in double-blind research.²²⁷ Beneficial effects of short-term vitamin C supplementation (i.e., less than three days) have been observed. In double-blind trials, supplementation with 1,000 to 1,500 mg of vitamin C per day for 2 to 14 days prevented attacks of exercise-induced asthma.^228,229 Two other preliminary trials found that vitamin C supplementation reduced bronchial reactivity to metacholine, a drug that causes bronchial constriction.^230,231 However, other studies,²³² including two double-blind trials,^233,234 have failed to corroborate these findings. The only double-blind trial of a long duration found that vitamin C supplementation (1 gram per day for 14 weeks) reduced the severity and frequency of attacks among Nigerian adults with asthma.²³⁵ A buffered form of vitamin C (such as sodium ascorbate or calcium ascorbate) may work better for some asthmatics than regular vitamin C (ascorbic acid).²³⁶

Although vitamin C has antihistamine activity, and supplementation, in preliminary research,^237,238 has been reported to help people with hay fever, 2,000 mg of vitamin C per day did not reduce hay fever symptoms in a placebo controlled trial.²³⁹ Thus, while some doctors recommend that hay fever sufferers take 1,000–3,000 mg of vitamin C per day, supportive evidence remains weak.

In theory, high amounts of vitamin C might help people with hives by lowering histamine levels.²⁴⁰ Amounts of at least 2,000 mg daily appear necessary to produce these effects.²⁴⁰ No research trials have yet explored the clinical effects of vitamin C supplementation in people with hives.

There is some evidence that combinations of antioxidants such as vitamin E, vitamin C, and selenium may help improve symptoms of asthma throught to be caused by air pollution.²⁴¹ In one double-blind study, 46 Dutch bicyclists were randomly assigned to receive a placebo or 100 mg of vitamin E and 500 mg of vitamin C daily for 15 weeks.²⁴² Lung function was measured before and after each training session on 380 different occasions, and ambient ozone concentrations were measured during each training session. After analysis, researchers concluded that bicyclists with the vitamins C and E blunted the adverse effects of ozone on measures of lung function. In another double-blind study, 17 adults (18 to 39 years old) were randomly assigned to receive either 400 IU per day of vitamin E and 500 mg per day of vitamin C or a placebo for five weeks.²⁴³ Tests showing improved measures of lung function led researchers to conclude that supplementation with vitamins C and E inhibited the decline in pulmonary function induced in asthmatics by exposure to air pollutants. Also using a double-blind design, another study of 158 children with asthma living in Mexico City were randomly assigned to receive, a daily supplement containing 50 mg of vitamin E and 250 mg of vitamin C or a placebo.²⁴⁴ Tests results suggested that supplementing with vitamins C and E may reduce the adverse effect of ozone exposure on lung function of children with moderate to severe asthma.

When H. pylori causes gastritis, free radical levels rise in the stomach lining.²⁴⁵ These unstable molecules contribute to inflammation and damage to the stomach lining. Vitamin C, an antioxidant that helps quench free radical molecules, is low in the stomach juice of people with chronic gastritis. This deficiency may be the link between chronic gastritis and the increased risk of stomach cancer. When people with gastritis took 500 mg of vitamin C twice a day, vitamin C levels in their gastric juice rose, though not to normal levels.²⁴⁶ In another trial, vitamin C supplementation (5 grams per day divided into several doses for four weeks) appeared to eliminate H. pylori infection.²⁴⁷ While no direct evidence proves that taking vitamin C reduces gastritis symptoms, scientists widely believe that any agent capable of knocking out H. pylori should help people with this condition.

Little is known about the effects of vitamin C supplementation for people with peptic ulcer. People with gastritis, a related condition, have been found to have low levels of vitamin C in their stomach juice. Vitamin C may also help eradicate H. pylori in people with gastritis. Vitamin C may one day prove to have a therapeutic effect for people with peptic ulcer; however, further research in this area is needed.²⁴⁸

Vitamin C is needed to convert cholesterol to bile acids. In theory, such a conversion should reduce gallstone risks. Women who have higher blood levels of vitamin C have a reduced risk of gallstones.²⁴⁹ Although this does not prove that vitamin C supplements can prevent or treat gallstones, some researchers believe this is plausible.²⁵⁰ One study reported that people who drink alcohol and take vitamin C supplements have only half the risk of gallstones compared with other drinkers, though the apparent protective effect of vitamin C did not appear in non-drinkers.²⁵¹ In another trial, supplementation with vitamin C (500 mg taken four times per day for two weeks before gallbladder surgery) led to improvement in one parameter of gallstone risk (“nucleation time”), though there was no change in the relative level of cholesterol found in bile.²⁵² While many doctors recommend vitamin C supplementation to people with a history of gallstones, supportive evidence remains preliminary.

Vitamin K and vitamin C, taken together, may provide relief of symptoms for some women. In one study, 91% of women who took 5 mg of vitamin K and 25 mg of vitamin C per day reported the complete disappearance of morning sickness within three days.²⁵³ Menadione was removed from the market a number of years ago because of concerns about potential toxicity. Although some doctors still use a combination of vitamin K1 (the most prevalent form of vitamin K in food) and vitamin C for morning sickness, no studies on this treatment have been done.

In one small study, people who took 4 grams of vitamin C (but not lower amounts) had an increase in urinary excretion of uric acid within a few hours, and those who took 8 grams of vitamin C per day for several days had a reduction in serum uric acid levels.²⁵⁴ Thus, supplemental vitamin C could, in theory, reduce the risk of gout attacks. However, the authors of this study warned that taking large amounts of vitamin C could also trigger an acute attack of gout by abruptly changing uric acid levels in the body. Another study showed that taking lower amounts of vitamin C (500 mg per day) for two months significantly reduced blood levels of uric acid, especially in people whose initial uric acid levels were elevated.²⁵⁵ For people with a history of gout attacks, it seems reasonable to begin vitamin C supplementation at 500 mg per day, and to increase the amount gradually if uric acid levels do not decrease.

A preliminary report in 1964 suggested that 500–1,000 mg per day of vitamin C helped many people avoid surgery for their disc-related low back pain.²⁵⁶ No controlled research has been done to examine this claim further.

Use of vitamin C or vitamin E supplements, or both, has been associated with better cognitive function and a reduced risk of certain forms of dementia (not including Alzheimer’s disease).²⁵⁷ Clinical trials of these antioxidants are needed to confirm the possible benefits suggested by this study.

Sunlight triggers oxidative damage in the eye, which in turn can cause macular degeneration.²⁵⁸ Because vitamin C functions as an antioxidant, it has the potential to protect against macular degeneration. However, in a double-blind trial, supplementing with 500 mg of vitamin C daily for eight years did not decrease the incidence of macular degeneration in healthy male physicians.²⁵⁹

Many doctors recommend 5,000 mg or more of vitamin C per day for an acute UTI, as well as long-term supplementation for people who are prone to recurrent UTIs. Vitamin C has been shown to inhibit the growth of E. coli, the most common bacterial cause of UTIs.²⁶⁰ In addition, supplementation with 4,000 mg or more of vitamin C per day, results in a slight increase in the acidity of the urine,²⁶¹ creating an “unfriendly” environment for some infection-causing bacteria. In one controlled trial, pregnant women who supplemented with 100 mg of vitamin C per day experienced 56% less UTI frequency, compared with a placebo.²⁶²

Use of vitamin C or vitamin E supplements, or both, has been associated with better cognitive function and a reduced risk of certain forms of dementia (not including Alzheimer’s disease).²⁶³ Clinical trials of these antioxidants are needed to confirm the possible benefits suggested by this study.

Up to 6 grams daily of vitamin C has been reported to be beneficial for people with schizophrenia;^264,265 in one case the addition of 400 IU daily of vitamin E enhanced this benefit.²⁶⁵ A small preliminary trial using 8 grams daily of vitamin C showed decreases in hallucinations, suspiciousness, and unusual and disorganized thoughts in 77% of schizophrenic patients.²⁶⁷ In all reported cases, patients were also being treated with sychiatric medications. Some early studies found no difference between blood and urine vitamin C levels in schizophrenics and non-schizophrenics, either before or after supplementation.^268,269,270 However, later studies found that blood and urine levels of vitamin C were lower in schizophrenics than in non-schizophrenics before and after a single 1,000 mg “load” of vitamin C was taken. After four weeks of daily supplementation with 1,000 mg of vitamin C, blood levels became the same, but urinary levels remained lower in the schizophrenic group, leading the researchers to conclude that the amount of vitamin C required by people with schizophrenia may be greater than that of the general population.^271,272

Vitamin C helps the body to reduce its load of vanadium and this has been studied for its possible role in treatment of bipolar disorder.²⁷² A double-blind trial found that both manic and depressed bipolar patients were significantly improved after one-time administration of 3 grams of vitamin C, compared with a placebo.²⁷³ The same study found that both manic and depressed patients did better on a reduced-vanadium diet compared to a normal diet. Another double-blind study reported that 4 grams per day of vitamin C in combination with a drug known as EDTA (which also helps remove elements such as vanadium from the body) was helpful to depressed bipolar patients but not to those experiencing mania.²⁷⁴ Until more is known, people with bipolar illness should avoid supplements containing vanadium and consider supplementing with vitamin C.

Premature rupture of membranes (PROM) affects 10 to 20% of all pregnancies. It is an important cause of preterm delivery and is associated with increased rates of complications in both the mother and child. In a double-blind study, supplementing with 100 mg of vitamin C per day, beginning in the twentieth week of pregnancy, reduced the incidence of PROM by 74%.²⁷⁵ The women in this study were consuming only about 65 mg of vitamin C per day in their diet, which is less than the RDA of 80 to 85 mg per day for pregnant women. In a double-blind study of pregnant smokers, supplementation with 500 mg per day of vitamin C, beginning at 23 weeks of pregnancy or earlier and continuing until delivery, improved lung function and decreased the incidence of wheezing in the offspring.²⁷⁶

In one double-blind trial lasting ten weeks, autistic children given 1 gram vitamin C per day for each 20 pounds of body weight showed a reduction in symptom severity compared with placebo.²⁷⁷ The authors speculate that vitamin C may play a positive role because of its known effects on a hormone pathway typically disturbed in children with autism.

Vitamin C has been demonstrated in test tube, animal, and human studies to have immune-enhancing and direct antiviral properties.²⁷⁸ Preliminary observations made on the effect of vitamin C on viral infections have involved both measles and chicken pox.²⁷⁹ An active immune system uses vitamin C rapidly, and blood levels fall in children with bacterial or viral infections.²⁸⁰ Reduced immune cell activity has been observed in people with measles, but in one preliminary study, supplementation with 250 mg daily of vitamin C in children 18 months to 3 years old had no impact on the course of the illness.²⁸¹ The authors of this study admit that this amount of vitamin C may have been too low to bring about an observable increase in immune cell activity and thus an increase in speed of recovery.

Vitamin C supplementation has been reported to stimulate immune function.^282,283 As a result, some doctors recommend between 500 mg and 1,000 mg of vitamin C per day for people with ear infections. Nonetheless, vitamin C supplementation has not been studied by itself in people with ear infections.

In 1964, a preliminary trial reported that 1,200 mg each of vitamin C and the flavonoid hesperidin taken over the course of the day helped relieve hot flashes.²⁸⁴ Although placebo effects are strong in women with hot flashes, other treatments used in that trial failed to act as effectively as the flavonoid/vitamin C combination. Since then, researchers have not explored the effects of flavonoids or vitamin C in women with menopausal symptoms.

Vitamin C supplementation has been reported to stimulate immune function.^285,286 As a result, some doctors recommend between 500 mg and 1,000 mg of vitamin C per day for people with ear infections. Nonetheless, vitamin C supplementation has not been studied by itself in people with ear infections.

Most research has demonstrated that strenuous exercise increases production of harmful substances called free radicals, which can damage muscle tissue and result in inflammation and muscle soreness. Exercising in cities or smoggy areas also increases exposure to free radicals. Antioxidants, including vitamin C and vitamin E, neutralize free radicals before they can damage the body, so antioxidants may aid in exercise recovery. Regular exercise increases the efficiency of the antioxidant defense system, potentially reducing the amount of supplemental antioxidants that might otherwise be needed for protection. However, at least theoretically, supplements of antioxidant vitamins may be beneficial for older or untrained people or athletes who are undertaking an especially vigorous training protocol or athletic event.^287,288

Placebo-controlled research, some of it double-blind, has shown that taking 400 to 3,000 mg of vitamin C per day for several days before and after intense exercise may reduce pain and speed up muscle strength recovery.^289,290,291 However, taking vitamin C only after such exercise was not effective in another double-blind study.²⁹² While some research has reported that vitamin E supplementation in the amount of 800 to 1,200 IU per day reduces biochemical measures of free radical activity and muscle damage caused by strenuous exercise,^293,294,295 several studies have not found such benefits,^{296,297,298,299} and no research has investigated the effect of vitamin E on performance-related measures of strenuous exercise recovery. A combination of 90 mg per day of coenzyme Q10 and a very small amount of vitamin E did not produce any protective effects for marathon runners in one double-blind trial,³⁰⁰ while in another double-blind trial a combination of 50 mg per day of zinc and 3 mg per day of copper significantly reduced evidence of post-exercise free radical activity.³⁰¹

In most well-controlled studies, exercise performance has not been shown to improve following supplementation with vitamin C, unless a deficiency exists, as might occur in athletes with unhealthy or irrational eating patterns.^302,303 Similarly, vitamin E has not benefited exercise performance, ^304,305 except possibly at high altitudes. ^306,307

Most research has demonstrated that strenuous exercise increases production of harmful substances called free radicals, which can damage muscle tissue and result in inflammation and muscle soreness. Exercising in cities or smoggy areas also increases exposure to free radicals. Antioxidants, including vitamin C and vitamin E, neutralize free radicals before they can damage the body, so antioxidants may aid in exercise recovery. Regular exercise increases the efficiency of the antioxidant defense system, potentially reducing the amount of supplemental antioxidants that might otherwise be needed for protection. However, at least theoretically, supplements of antioxidant vitamins may be beneficial for older or untrained people or athletes who are undertaking an especially vigorous training protocol or athletic event.^308,309

Placebo-controlled research, some of it double-blind, has shown that taking 400 to 3,000 mg of vitamin C per day for several days before and after intense exercise may reduce pain and speed up muscle strength recovery.^310,311,312 However, taking vitamin C only after such exercise was not effective in another double-blind study.³¹³ While some research has reported that vitamin E supplementation in the amount of 800 to 1,200 IU per day reduces biochemical measures of free radical activity and muscle damage caused by strenuous exercise,^314,315,316 several studies have not found such benefits,^{317,318,319,320} and no research has investigated the effect of vitamin E on performance-related measures of strenuous exercise recovery. A combination of 90 mg per day of coenzyme Q10 and a very small amount of vitamin E did not produce any protective effects for marathon runners in one double-blind trial,³²¹ while in another double-blind trial a combination of 50 mg per day of zinc and 3 mg per day of copper significantly reduced evidence of post-exercise free radical activity.³²²

In most well-controlled studies, exercise performance has not been shown to improve following supplementation with vitamin C, unless a deficiency exists, as might occur in athletes with unhealthy or irrational eating patterns.^323,324,325 Similarly, vitamin E has not benefited exercise performance, ^326,327 except possibly at high altitudes. ^328,329

Most research has demonstrated that strenuous exercise increases production of harmful substances called free radicals, which can damage muscle tissue and result in inflammation and muscle soreness. Exercising in cities or smoggy areas also increases exposure to free radicals. Antioxidants, including vitamin C and vitamin E, neutralize free radicals before they can damage the body, so antioxidants may aid in exercise recovery. Regular exercise increases the efficiency of the antioxidant defense system, potentially reducing the amount of supplemental antioxidants that might otherwise be needed for protection. However, at least theoretically, supplements of antioxidant vitamins may be beneficial for older or untrained people or athletes who are undertaking an especially vigorous training protocol or athletic event.^330,331

Placebo-controlled research, some of it double-blind, has shown that taking 400 to 3,000 mg of vitamin C per day for several days before and after intense exercise may reduce pain and speed up muscle strength recovery.^332,333,334 However, taking vitamin C only after such exercise was not effective in another double-blind study.³³⁵ While some research has reported that vitamin E supplementation in the amount of 800 to 1,200 IU per day reduces biochemical measures of free radical activity and muscle damage caused by strenuous exercise,^336,337,338 several studies have not found such benefits,^{339,340,341,342} and no research has investigated the effect of vitamin E on performance-related measures of strenuous exercise recovery. A combination of 90 mg per day of coenzyme Q10 and a very small amount of vitamin E did not produce any protective effects for marathon runners in one double-blind trial,³⁴³ while in another double-blind trial a combination of 50 mg per day of zinc and 3 mg per day of copper significantly reduced evidence of post-exercise free radical activity.³⁴⁴

In most well-controlled studies, exercise performance has not been shown to improve following supplementation with vitamin C, unless a deficiency exists, as might occur in athletes with unhealthy or irrational eating patterns.^345,346 Similarly, vitamin E has not benefited exercise performance, ^347,348 except possibly at high altitudes. ^349,350

Vitamin C protects sperm from oxidative damage.³⁵¹ Supplementing vitamin C improves the quality of sperm in smokers.³⁵² When sperm stick together (a condition called agglutination), fertility is reduced. Vitamin C reduces sperm agglutination,³⁵³ and supplementation with 200–1,000 mg per day increased the fertility of men with this condition in a controlled study.^354,355 Many doctors recommend 1 gram of vitamin C per day for infertile men, particularly those diagnosed with sperm agglutination. However, a double-blind trial studying the effects of combined vitamin C and vitamin E supplementation found no improvements in semen quality among men with low sperm motility.³⁵⁶