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.

Preliminary research shows that supplemental vitamin A improves the likelihood that the measles vaccine will provide protection.¹² Vitamin A has, since the 1920s, been the subject of much research into the prevention and treatment of childhood exanthems, particularly measles.¹³ 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.^14,15 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.¹⁶ 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.¹⁷

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.¹⁸ 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;¹⁹ 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.²⁰

An analysis of 20 controlled trials concluded that vitamin A supplementation reduced deaths from measles respiratory infection by 70%.²¹ 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.²² 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.²³

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.²⁴ Vitamin A stores have also been shown to be depleted during chicken pox infection,²⁵ 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.²⁶

Measles appears to increase the body’s need for vitamin A.^27,28 Studies in developing countries have shown that measles infection is more frequent and severe in people with low vitamin A blood levels,^29,30 and preliminary research suggests this may also be true in the developed world.^31,32,33 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.^29,35,36 Whether vitamin A supplementation would help people with measles in developed countries, where deficiency is uncommon, is less clear.²⁹ 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 African³⁸ and Japanese³⁹ 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.⁴⁰ Such large doses of vitamin A should only be taken under a doctor’s supervision.

Night blindness may be an early sign of vitamin A deficiency.^39,40 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.

Deficiencies of iron, vitamin B12, and folic acid are the most common nutritional causes of anemia.⁴¹ Although rare, severe deficiencies of several other vitamins and minerals, including vitamin A,^42,43vitamin B2,⁴⁴vitamin B6,^45,46vitamin C,⁴⁷ and copper,^48,49 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.^50,51

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%).⁵² 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.⁵³ 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.⁵⁴ These results have also been observed in smaller trials.^55,56

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.⁵⁷ 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.⁵⁸ One group of researchers reported that vitamin A (28,500 IU per day) was more effective than beta-carotene in treating people with leukoplakia.⁵⁹ 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.⁵² 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.^60,61 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.