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.

Antioxidants may protect the skin from sunburn due to free radical–producing ultraviolet rays.⁸ 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.^9,10,11

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.^{12,13,14,15,16}

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.¹⁷ 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.¹⁸ 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.¹⁹

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.²⁰ 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.²¹ Similarly, in a controlled trial, a combination of selenium, copper, and vitamins was found to be ineffective.²²

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.^23,24

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,²⁵ and vitamin C deficiency causes delayed healing.²⁶ 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.^26,28 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.^29,30

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,³⁰ and vitamin C deficiency causes delayed healing.³¹ 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.^31,33 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.^34,35

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.³⁵Flavonoids are often recommended along with vitamin C. Flavonoids are vitamin-like substances that can help strengthen capillaries and therefore may also help with bruising.³⁶ 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.³⁷ 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.³⁸ Controlled research is needed to better establish whether vitamin C and flavonoids are effective for easy bruising.

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,^43,44 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.⁴⁵ 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.^46,47 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.⁴⁸

People who are deficient in vitamin C may be at increased risk for periodontal disease.⁴⁹ 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.⁵⁰ 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.

Animal studies suggest that supplementing with vitamin C can reduce blood levels of stress-related hormones and other measures of stress.^51,52,53,54 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.^55,56 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.⁵⁷ 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.⁵⁸ 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.

Several studies have shown that supplementing with vitamin C can significantly reduce elevated intraocular pressure in individuals with glaucoma.⁵⁹ 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.”⁶⁰ 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.

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.^61,62

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.^63,64,65 However, taking vitamin C only after such exercise was not effective in another double-blind study.⁶⁶ 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,^67,68,69 several studies have not found such benefits,^70,71,72,73 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,⁷⁴ 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.⁷⁵

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.^76,77 Similarly, vitamin E has not benefited exercise performance, ^78,79 except possibly at high altitudes. ^80,81

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.^82,83

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.^84,85,86 However, taking vitamin C only after such exercise was not effective in another double-blind study.⁸⁷ 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,^88,89,90 several studies have not found such benefits,^91,92,93,94 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,⁹⁵ 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.⁹⁶

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.^97,98 Similarly, vitamin E has not benefited exercise performance, ^99,100 except possibly at high altitudes. ^101,102

Vitamin C protects sperm from oxidative damage.¹⁰³ Supplementing vitamin C improves the quality of sperm in smokers.¹⁰⁴ When sperm stick together (a condition called agglutination), fertility is reduced. Vitamin C reduces sperm agglutination,¹⁰⁵ and supplementation with 200–1,000 mg per day increased the fertility of men with this condition in a controlled study.^106,107 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.¹⁰⁸