Nutrients useful for maintaining healthy immune function are also applicable for preventing infections. Vitamin A plays an important role in immune system function and helps mucous membranes, including those in the lungs, resist invasion by microorganisms.¹ However, most research shows that while vitamin A supplementation helps people prevent or treat infections in developing countries where deficiencies are common,² little to no positive effect, and even slight adverse effects, have resulted from giving vitamin A supplements to people in countries where most people consume adequate amounts of vitamin A.^{3,4,5,6,7,8,9} Moreover, vitamin A supplementation during infections appears beneficial only in certain diseases. An analysis of trials revealed that vitamin A reduces mortality from measles and diarrhea, but not from pneumonia, in children living in developing countries.¹⁰ A double-blind trial for vitamin A supplementation in Tanzanian children with pneumonia confirmed its lack of effectiveness for this condition.¹¹ In general, parents in the developed world should not give vitamin A supplements to children unless there is a reason to believe vitamin A deficiency is likely, such as the presence of a condition causing malabsorption (e.g., celiac disease). However, the American Academy of Pediatrics recommends that all children with measles should be given high-dose vitamin A for several days.

Vitamin A plays an important role in immune system function and helps mucous membranes, including those in the lungs, resist invasion by microorganisms.¹² However, most research shows that while vitamin A supplementation helps people prevent or treat infections in developing countries where deficiencies are common,¹³ little to no positive effect, and even slight adverse effects, have resulted from giving vitamin A supplements to people in countries where most people consume adequate amounts of vitamin A.^{14,15,16,17,18,19,20} Moreover, vitamin A supplementation during infections appears beneficial only in certain diseases. An analysis of trials revealed that vitamin A reduces mortality from measles and diarrhea, but not from pneumonia, in children living in developing countries.²¹ A double-blind trial of vitamin A supplementation in Tanzanian children with pneumonia confirmed its lack of effectiveness for this condition.²² In general, parents in the developed world should not give vitamin A supplements to children unless there is a reason to believe vitamin A deficiency is likely, such as the presence of a condition causing malabsorption (e.g., celiac disease). However, the American Academy of Pediatrics recommends that all children with measles be given short-term supplementation with high-dose vitamin A in cases of hospitalization, malnutrition, and other special circumstances determined by a doctor.²³

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.²⁴ 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.²⁵ To what extent immune-boosting combinations of antioxidants actually reduce the risk of infection remains unknown.

Vitamin A plays an important role in wound healing,²⁶ and one animal study suggests that vitamin A deficiency might contribute to poor recovery after surgery.²⁷ Vitamin A may be particularly beneficial to post-surgical patients who are using corticosteroid medications. These medications typically slow wound healing, and a number of animal studies have found that both topical and oral vitamin A reverse this effect; however, vitamin A does not change healing time in animals not given corticosteroids.^28,29,30 Similar results have been reported for topical vitamin A in some human cases, and these researchers suggest a topical preparation containing 200,000 IU of vitamin A per ounce for improved surgical wound healing in patients using corticosteroids after surgery.²⁹ Topical vitamin A may also reduce scarring in patients taking corticosteroids.³²

Vitamin A deficiency appears to be very common in people with HIV infection. Low blood levels of vitamin A are associated with greater disease severity³² and increased transmission of the virus from a pregnant mother to her infant.³³ However, in preliminary ³⁴ and double-blind^35,36 trials, supplementation with vitamin A failed to reduce the overall mother-to-child transmission of HIV. HIV-positive women who took 5,000 IU per day of vitamin A (as retinyl palmitate) and 50,000 IU per day of beta-carotene during the third trimester (13 weeks) of pregnancy, plus an additional single amount of 200,000 IU of vitamin A at delivery, had the same rate of transmission of HIV to their infants as those who did not take the supplement. However, lower rates of illness have been observed in the children of HIV-positive mothers when the children were supplemented with 50,000–200,000 IU of vitamin A every two to three months.³⁷

Little research has explored whether vitamin A supplements are helpful at halting disease progression. HIV-positive children given two consecutive oral supplements of vitamin A (200,000 IU in a gelcap) in the two days following influenza vaccinations had a modest but significant decrease in viral load.³⁸ In one trial, giving people an extremely high (300,000 IU) amount of vitamin A one time only did not improve short-term measures of immunity in women with HIV.³⁹

Preliminary research shows that supplemental vitamin A improves the likelihood that the measles vaccine will provide protection.⁴⁰ Vitamin A has, since the 1920s, been the subject of much research into the prevention and treatment of childhood exanthems, particularly measles.⁴¹ This nutrient has a critical role in proper immune function, and there is evidence that supplementation with vitamin A reduces the incidence and severity of, and deaths from, childhood measles.^42,43 The World Health Organization (WHO) has therefore recommended that children with signs of deficiency receive supplementation with vitamin A. The recommended amounts are 100,000 IU for children younger than one year and 200,000 IU for children older than one year, immediately upon diagnosis, and repeated once the next day and once in one to four weeks.⁴⁴ A controlled trial of African children given vitamin A supplementation according to the WHO’s recommendations found that severity of measles and its long-term consequences were reduced by 82% on day eight, 61% in week six, and 85% six months after the onset.⁴⁵

Another controlled trial found that giving approximately 200,000 IU of vitamin A once during measles illness was not adequate to provide any benefit in African children whose vitamin A status was unknown.⁴⁶ In a controlled prevention study, Indian children treated with 2,500 mcg (8,333 IU) of vitamin A weekly had fewer measles complications and less than half of the rate of death as compared with children receiving placebo;⁴⁷ but in another study, Indian children receiving 200,000 IU of vitamin A every six months did not have a different rate of total infectious illness nor rate of death as compared with children receiving placebo.⁴⁸

An analysis of 20 controlled trials concluded that vitamin A supplementation reduced deaths from measles respiratory infection by 70%.⁴⁹ While vitamin A deficiency is widespread in developing countries, it has also been reported in the United States and has been linked with more severe cases of measles.⁵⁰ The American Academy of Pediatrics has recommended supplementation with vitamin A for children between the ages of six months and two years who are hospitalized with measles and its complications. The recommended amount is a single administration of 100,000 IU for children aged 6 to 12 months and 200,000 IU for children older than 1 year, followed by a second administration 24 hours later and a third after four weeks in children who are likely to have vitamin A deficiency.⁵¹

One trial showed that low levels of vitamin A are more prevalent in children with measles than in similar children without measles, with levels rising back to normal several days after the onset of the infection. This observation led the authors of the study to conclude that vitamin A deficiency is a consequence of infection with the measles virus and to recommend supplementation with vitamin A during measles infection even when prior deficiency is not suspected.⁵² Vitamin A stores have also been shown to be depleted during chicken pox infection,⁵³ and some preliminary data supports its use in treatment of chicken pox. In a controlled trial, in which children without vitamin A deficiency were given either 200,000 IU of vitamin A or placebo one time during chicken pox, the children given vitamin A had shorter duration of illness and fewer severe complications. The researchers then treated the patients’ siblings with vitamin A before chicken pox became evident, and they had an even shorter length of illness.⁵⁴

Measles appears to increase the body’s need for vitamin A.^55,56 Studies in developing countries have shown that measles infection is more frequent and severe in people with low vitamin A blood levels,^57,58 and preliminary research suggests this may also be true in the developed world.^59,60,61 Repeatedly in controlled trials, preventive supplementation with vitamin A, at oral doses of up to 400,000 IU per day, reduced the risk of death in children with measles living in developing countries.^57,63,64 Whether vitamin A supplementation would help people with measles in developed countries, where deficiency is uncommon, is less clear.⁵⁷ However, the American Academy of Pediatrics recommends that all children with measles be given a short course of high-dose vitamin A. Two controlled studies of urban South African⁶⁶ and Japanese⁶⁷ children hospitalized with severe measles showed that supplementation with 100,000 to 400,000 IU of vitamin A resulted in faster recoveries, fewer complications, and fewer pneumonia-related deaths. An older study in England found one ounce per day of cod liver oil (containing about 40,000 IU of vitamin A, plus vitamin D and omega-3 fatty acids) reduced measles-related deaths in children hospitalized with severe cases of the disease.⁶⁸ Such large doses of vitamin A should only be taken under a doctor’s supervision.

Vitamin A is needed to heal the linings (called mucous membranes) of the stomach and intestines. In one controlled trial, vitamin A supplementation facilitated healing in a small group of people with stomach ulcer.⁶⁷ The amount used in that study (150,000 IU per day) can be toxic and may also cause birth defects. Such a high dose should not be taken by a pregnant woman, by a woman who could become pregnant, or by anyone else without careful supervision from a doctor. Objective evidence of ulcer healing from taking vitamin A has been reported by the same research group.⁶⁸ The effect of lower amounts of vitamin A has not been studied in people with peptic ulcer.

In one study, six people with diet-treated celiac disease had abnormal dark-adaptation tests (indicative of “night blindness”), even though some were taking a multivitamin that contained vitamin A. Some of these people showed an improvement in dark adaptation after receiving larger amounts of vitamin A, either orally or by injection.⁶⁹ People with celiac disease should discuss the possibility of vitamin A deficiency with a healthcare practitioner before taking vitamin A supplements.

Zinc and vitamin A, nutrients that aid in healing, are commonly used to help people with peptic ulcers. For example, the ulcers of people taking 50 mg of zinc three times per day healed three times faster than those of people who took placebo.⁷⁰ Since some types of gastritis can progress to peptic ulcer, it is possible that taking it may be useful. Nevertheless, the research does not yet show that zinc specifically helps people with gastritis. The amount of zinc used in this study is very high compared with what most people take (15–40 mg per day). Even at these lower levels, it is necessary to take 1–3 mg of copper per day to avoid a zinc-induced copper deficiency.

Vitamin A is needed for the growth and repair of cells that line both the small and large intestine.⁷¹ At least two case reports describe people with Crohn’s disease who have responded to vitamin A supplementation.^72,71 However, in one trial, vitamin A supplementation failed to maintain remission of the disease.⁷⁴ Therefore, although some doctors recommend 50,000 IU per day for adults with Crohn’s disease, this approach remains unproven. An amount this high should never be taken without qualified guidance, nor should it be given to a woman who is or could become pregnant.

It is known vitamin A supplements support immune function and prevent infections. This is true, however, only under some circumstances. Vitamin A supplementation can also increase the risk of infections, according to the findings of a double-blind trial.⁷⁴ In a study of African children between six months and five years old, a 44% reduction in the risk of severe diarrhea was seen in those children given four 100,000–200,000 IU supplements of vitamin A (the lower amount for those less than a year old) during an eight-month period. On further investigation, the researchers discovered that the reduction in diarrhea occurred only in children who were very malnourished. For children who were not starving, vitamin A supplementation actually increased the risk of diarrhea compared with the placebo group. The vitamin A-supplemented children also had a 67% increased risk of coughing and rapid breathing, and signs of further lung infection, although this problem did not appear in children infected with the AIDS virus. These findings should be of concern to American parents, whose children are not usually infected with HIV or severely malnourished. Such relatively healthy children fared poorly in the African trial in terms of both the risk of diarrhea and the risk of continued lung problems. Vitamin A provided no benefit to the well-nourished kids. Therefore, it makes sense not to give vitamin A supplements to children unless there is a special reason to do so, such as the presence of a condition causing malabsorption (e.g., celiac disease).

In a study of women with menorrhagia who took 25,000 IU of vitamin A twice per day for 15 days, 93% showed significant improvement and 58% had a complete normalization of menstrual blood loss.⁷⁵ However, women who are or could become pregnant should not supplement with more than 10,000 IU (3,000 mcg) per day of vitamin A.

Taking vitamin A and iron together has been reported to help overcome iron deficiency more effectively than iron supplements alone.⁷⁶ Although the optimal amount of vitamin A needed to help people with iron deficiency has yet to be established, some doctors recommend 10,000 IU per day.

Very high amounts of vitamin A—100,000 IU per day or more—have reduced symptoms of PMS,^77,78 but such an amount can cause serious side effects with long-term use. Women who are or who could become pregnant should not supplement with more than 10,000 IU (3,000 mcg) per day of vitamin A. Other people should not take more than 25,000 IU per day without the supervision of their doctor. As yet, no trials have explored the effects of these safer amounts of vitamin A in women suffering from PMS.

Some doctors recommend vitamin E (taken orally, topically, or vaginally) for certain types of vaginitis. Vitamin E as a suppository in the vagina or vitamin E oil can be used once or twice per day for 3 to 14 days to soothe the mucous membranes of the vagina and vulva. Some doctors recommend vaginal administration of vitamin A to improve the integrity of the vaginal tissue and to enhance the function of local immune cells. Vitamin A can be administered vaginally by inserting a vitamin A capsule or using a prepared vitamin A suppository. Vitamin A used this way can be irritating to local tissue, so it should not be used more than once per day for up to seven consecutive days.

Women with a low intake of vitamin A have an increased risk of cervical dysplasia.⁷⁹ However, there is little research on the use of vitamin A as a treatment for cervical dysplasia.

Vitamin A plays a central role in wound healing,⁸⁰ but the effect of supplemental vitamin A in people who have suffered a minor injury and are not vitamin A-deficient remains unclear. Vitamin A supplements have been shown to improve healing in animal studies,⁸⁰ and may be especially useful in a topical ointment for skin injuries in people taking corticosteroid medications.⁸² Although there are no studies in humans, some doctors recommend 25,000 IU of vitamin A per day, beginning two weeks prior to surgery and continuing for four weeks after surgery.

Large quantities of vitamin A—such as 300,000 IU per day for females and 400,000–500,000 IU per day for males—have been used successfully to treat severe acne.⁸² However, unlike the long-lasting benefits of the synthetic prescription version of vitamin A (isotretinoin as Accutane), the acne typically returns several months after natural vitamin A is discontinued. In addition, the large amounts of vitamin A needed to control acne can be toxic and should be used only under careful medical supervision.

In a study of women with menorrhagia who took 25,000 IU of vitamin A twice per day for 15 days, 93% showed significant improvement and 58% had a complete normalization of menstrual blood loss.⁸³ However, women who are or could become pregnant should not supplement with more than 10,000 IU (3,000 mcg) per day of vitamin A.

Taking vitamin A and iron together has been reported to help overcome iron deficiency more effectively than iron supplements alone.⁸⁴ Although the optimal amount of vitamin A needed to help people with iron deficiency has yet to be established, some doctors recommend 10,000 IU per day.

Very high amounts of vitamin A—100,000 IU per day or more—have reduced symptoms of PMS,^85,86 but such an amount can cause serious side effects with long-term use. Women who are or who could become pregnant should not supplement with more than 10,000 IU (3,000 mcg) per day of vitamin A. Other people should not take more than 25,000 IU per day without the supervision of their doctor. As yet, no trials have explored the effects of these safer amounts of vitamin A in women suffering from PMS.

Vitamin A plays a central role in wound healing,⁸⁷ but the effect of supplemental vitamin A in people who have suffered a minor injury and are not vitamin A-deficient remains unclear. Vitamin A supplements have been shown to improve healing in animal studies,⁸⁷ and may be especially useful in a topical ointment for skin injuries in people taking corticosteroid medications.⁸⁹ Although there are no studies in humans, some doctors recommend 25,000 IU of vitamin A per day, beginning two weeks prior to surgery and continuing for four weeks after surgery.

People with hypothyroidism have been shown to have an impaired ability to convert beta-carotene to vitamin A.^89,90 For this reason, some doctors suggest taking supplemental vitamin A (approximately 5,000–10,000 IU per day) if they are not consuming adequate amounts in their diet.

Vitamin A deficiency increases the risk of many infections. Although much of the promising research with vitamin A supplements and infections has focused on measles,⁹¹ vitamin A is also thought to be helpful in other infections. Some doctors recommend that people with urinary tract infections take vitamin A. A typical amount recommended to correct a deficiency is 10,000 to 25,000 IU per day.

Night blindness may be an early sign of vitamin A deficiency.^92,93 Such a deficiency may result from diets low in animal foods (the main source of vitamin A), such as eggs, dairy products, organ meats, and some fish. Low intake of fruits and vegetables containing beta-carotene, which the body converts into vitamin A, may also contribute to a vitamin A deficiency. Doctors often recommend 10,000 to 25,000 IU of vitamin A per day to correct a deficiency. Beta-carotene is less effective at correcting vitamin A deficiency than is vitamin A itself, because it is not absorbed as well and is only slowly converted by the body into vitamin A.

Vitamin A deficiency has been reported in people with chronic conjunctivitis.⁹⁴ It is unknown whether vitamin A supplementation can prevent conjunctivitis or help people who already have the condition.

Deficiencies of iron, vitamin B12, and folic acid are the most common nutritional causes of anemia.⁹⁵ Although rare, severe deficiencies of several other vitamins and minerals, including vitamin A,^96,97vitamin B2,⁹⁸vitamin B6,^99,100vitamin C,¹⁰¹ and copper,^102,103 can also cause anemia by various mechanisms. Rare genetic disorders can cause anemias that may improve with large amounts of supplements such as vitamin B1.^104,105

Beta-carotene is the most widely used supplement in the treatment of leukoplakia. In a clinical trial of betel nut chewers with leukoplakia, supplementation with 150,000 IU of beta-carotene twice per week for six months significantly increased the remission rate compared with placebo (14.8% vs. 3.0%).¹⁰⁶ The effectiveness of beta-carotene for treating leukoplakia was also confirmed in a double-blind trial that used 100,000 IU per day for six months.¹⁰⁷ In one trial, supplementation with 33, 333 IU of beta-carotene per day, alone or combined with 50 IU of vitamin E, was reported not to reduce the incidence of leukoplakia.¹⁰⁸ These results have also been observed in smaller trials.^109,110

Drug therapy with a synthetic, prescription form of vitamin A (known as Accutane®, isotretinoin, and 13-cis retinoic acid) has been reported to be more effective than treatment with 50,000 IU per day of beta-carotene.¹¹¹ However, because of the potential toxicity of the vitamin A-like drug, it may be preferable to treat leukoplakia with beta-carotene, which is much safer.

Before the research on beta-carotene was published, vitamin A was used to treat leukoplakia.¹¹² One group of researchers reported that vitamin A (28,500 IU per day) was more effective than beta-carotene in treating people with leukoplakia.¹¹³ Another trial found that the combination of 150,000 IU per week of beta-carotene plus 100,000 IU per week of vitamin A led to a significant increase in remission time compared to beta carotene alone in betel nut chewers.¹⁰⁶ Women who are or who could become pregnant should not take 100,000 IU of vitamin A per week without medical supervision.

Night blindness may be an early sign of vitamin A deficiency.^114,115 Such a deficiency may result from diets low in animal foods (the main source of vitamin A), such as eggs, dairy products, organ meats, and some fish. Low intake of fruits and vegetables containing beta-carotene, which the body converts into vitamin A, may also contribute to a vitamin A deficiency. Doctors often recommend 10,000 to 25,000 IU of vitamin A per day to correct a deficiency. Beta-carotene is less effective at correcting vitamin A deficiency than is vitamin A itself, because it is not absorbed as well and is only slowly converted by the body into vitamin A.