Vitamin A and Immune Function
Richard D. Semba1
Vitamin A is an essential micronutrient for immunity, cellular differentiation, growth, reproduction, maintenance of epithelial surfaces, and vision. Vitamin A is found as preformed vitamin A in foods such as liver, cod-liver oil, butter, eggs, and dairy products and as provitamin A carotenoids in foods such as spinach, carrots, and orange fruits and vegetables. Among the micronutrients, vitamin A plays a central role in normal immune function. In a comprehensive review of the literature, Scrimshaw et al. (1968) concluded ''no nutritional deficiency is more consistently synergistic with infectious disease than that of vitamin A'' (p. 94). Although conventionally it has been thought that the main clinical manifestations of vitamin A deficiency involve the eye (for example, night blindness and xerophthalmia), it is now well established that widespread immune alterations, anemia, and increased infectious disease morbidity and mortality occur during vitamin A deficiency. The use of vitamin A supplements to enhance immunity has been demonstrated in the recent series
of clinical trials involving children in developing countries (Beaton et al., 1993). This chapter will review the potential importance of vitamin A to immune function in adult men and women.
Clinical Manifestations of Vitamin A Deficiency
The most commonly recognized clinical manifestations of vitamin A deficiency involve the eye, and a complete description of the clinical staging is described in Sommer (1982). The occurrence of ocular signs and symptoms of mild and advanced vitamin A deficiency have relevance to immune status as their presence signals a high risk of vitamin A-related immune abnormalities in the individual. Night blindness is one of the earliest symptoms of mild vitamin A deficiency. Typically the vision is normal during the day, but the ability to distinguish objects under less well-illuminated conditions—at dusk or during the night—is impaired. Vitamin A is involved in the generation of rhodopsin, the visual pigment in rod photoreceptors, which allows the retina to detect light in dark-adapted conditions. Other conditions besides vitamin A deficiency can cause night blindness, including advanced glaucoma, pupillary abnormalities involving a small pupil, retinitis pigmentosa, and certain rare retinal disorders involving rod function of the retina. Mild vitamin A deficiency is also characterized by keratinizing, squamous metaplasia of the conjunctiva. A well-defined, raised area of conjunctival metaplasia can sometimes be recognized, typically on the temporal and/or nasal bulbar conjunctiva, and this lesion, known as a Bitot's spot, is considered pathognomonic for vitamin A deficiency.
Advanced vitamin A deficiency is characterized by corneal xerosis, in which the clear, shiny corneal epithelium is replaced by areas of keratinized epithelium, giving the cornea a dull, grayish-white appearance. Cornea ulcers may occur in advanced vitamin A deficiency, and typically these ulcers are small, round or oval, full-thickness ulcers that may allow the aqueous humor to drain from the anterior chamber of the eye. The most advanced eye lesion of vitamin A deficiency is keratomalacia, a condition in which the cornea undergoes widespread ulceration and necrosis, with or without concomitant bacterial or fungal superinfection. Usually under these circumstances, the affected individual will become blind in the affected eye(s) and is at greatly increased risk of immunodeficiency and death.
Epidemiology of Vitamin A Deficiency
Vitamin A deficiency is one of the most common micronutrient deficiencies in the world, affecting an estimated 125 million individuals (Sommer and West, 1996). The groups at highest risk for the development of vitamin A deficiency are infants, preschool children, pregnant women, and lactating women. Vitamin A deficiency is the most common in developing countries, affecting areas of the
world such as Southeast and southern Asia, sub-Saharan Africa, areas of the South Pacific, and parts of South and Central America (Sommer and West, 1996). Vitamin A deficiency was once common in Europe and the United States prior to improvements in diet, fortification of foods with vitamin A, and general advances in public health. Pregnant women are at higher risk of developing vitamin A deficiency because of increased demands for vitamin A by the growing fetus. Breastfeeding provides the vitamin A for infants before weaning, and lactating women are at risk of vitamin A deficiency during this period.
Several factors may contribute to the development of vitamin A deficiency, including inadequate intake of vitamin A-containing foods, malabsorption due to infections in the gut, liver disease, the acute phase response and abnormal urinary losses of vitamin A, zinc deficiency, concomitant protein energy malnutrition, and increased utilization of vitamin A during infections. Diarrheal disease and intestinal parasites may interfere with the absorption of vitamin A. Liver disease, such as hepatitis, may impair the capacity of the liver to store and release vitamin A. Zinc deficiency has been shown to impede the release of retinol-binding protein and vitamin A from the liver (Smith, 1980). During infections, circulating vitamin A levels usually decrease because of the acute phase response, possible increased utilization of vitamin A by peripheral tissues, and abnormal urinary losses of vitamin A (Alvarez et al., 1995).
The Recommended Dietary Allowance (RDA [NRC, 1989]) of vitamin A for adult men and women in the United States is 800 µg RE (retinol equivalents)/d. For lactating women, the RDA is 1,200 to 1,300 µg RE/d. In general, healthy adult men and women who receive the RDA of vitamin A would be at low risk of developing vitamin A deficiency. This is primarily due to the fact that the liver, which stores about 90 percent of the body's vitamin A, has a capacity to store vitamin A for a prolonged period. In healthy children, the liver can store enough vitamin A to last for a few months, whereas in healthy adults, it seems that the adult liver can store enough vitamin A to last for several months to a year or more.
Vitamin A Deficiency in Adults
Vitamin A deprivation experiments have been conducted among adults in order to determine the minimal requirements of vitamin A, and these experiments illustrate the capacity of the liver to store vitamin A. Impaired dark adaptation was noted in five subjects after various periods from 16 to 124 days of vitamin A and carotenoid deprivation (Booher et al., 1939). Wagner (1940) deprived 10 subjects of dietary vitamin A and carotenoids for 6 months and noted signs of vitamin A deficiency. Brenner and Roberts (1943) noted no signs of deficiency in three subjects who were deprived of vitamin A and carotenoids for over 7 months.
During World War II, a vitamin A deprivation experiment was conducted by the Medical Research Council on 20 men and 3 women, most of them
between the ages of 20 and 30 years (Hume and Krebs, 1949). These individuals were conscientious objectors to military service who volunteered for human experimentation at a research institute in Sheffield, England. The plan was to give 16 adults a diet devoid of vitamin A and carotene until signs of deficiency appeared and then determine the dose of vitamin A or carotene that was necessary to return their levels to normal. The original experiment was expected to last 6 to 8 months; however, the experiment continued for over 18 months in several of the volunteers because no signs of deficiency developed. After 8 months, the plasma vitamin A levels began to decline in most subjects. Only 3 men became deficient, as evident by a gradual drop in plasma vitamin A levels that was accompanied by impaired dark adaptation. After 16 to 20 months of deprivation, 2 previously healthy volunteers developed tuberculosis.
Xerophthalmia and keratomalacia have been infrequently reported in adult men and women, usually under conditions of extreme dietary deprivation or in association with chronic infections and wasting. Night blindness and keratomalacia were reported among adults in a prison in Kampala, Uganda, where the diet was monotonous and contained little vitamin A (Mitchell, 1933; Owen, 1933). Vitamin A deficiency has been reported in concentration camps (Salus, 1957). Scattered case reports exist of keratomalacia in adults (Pillat, 1929), and these cases occurred mostly among adults with liver disease, malnutrition, or severe diarrheal disease. Alcoholic liver disease can increase the risk of vitamin A deficiency. Low vitamin A levels consistent with deficiency have been described in adults with HIV infection (Karter et al., 1995; Semba et al., 1993a, 1994). There is little evidence that noninfectious stress such as heat or cold will affect vitamin A status. It should be noted that vitamin A deficiency was once a major problem in the nineteenth century for American sailors involved in long voyages and was widely reported in both Union and Confederate armies during the Civil War.
Pregnancy and lactation represent a period of high risk in women for the development of vitamin A deficiency. Night blindness is not uncommon among pregnant women in developing countries, and a recent case control study from Nepal suggests that pregnant women with night blindness have increased infectious disease morbidity (Christian et al., 1996). In sub-Saharan Africa, HIV-infected pregnant women with vitamin A deficiency have higher risk of passing HIV infection to their infants, and their infants have lower birthweight and higher infant mortality (Semba et al., 1994, 1995). Vitamin A deficiency has been reported in pregnant women of lower socioeconomic class in the United States (Duitsman et al., 1995). The requirements for vitamin A may increase in pregnant women because of needs for vitamin A by the growing fetus. During lactation, the daily requirement for vitamin A is higher because of the transfer of circulating vitamin A into breastmilk.
Vitamin A Deficiency As An Immunodeficiency Disorder
Epidemiologic studies, clinical trials, and experimental studies in animal models have firmly established that vitamin A deficiency is a nutritionally acquired immunodeficiency disorder that is characterized by widespread immune alterations and increased infectious disease morbidity and mortality (Semba, 1994). In humans, most of the understanding of vitamin A-related immunodeficiency comes from studies involving children rather than adults. Autopsy studies in children have shown that vitamin A deficiency is associated with atrophy of the thymus, lymph nodes, and spleen (Blackfan and Wolbach, 1933; Sweet and K'ang, 1955). Vitamin A deficiency is often associated with protein-energy malnutrition (PEM), making it difficult to attribute these pathologic alterations in the immune system to vitamin A deficiency alone. However, under well-controlled conditions, animal models have shown a similar association between vitamin A deficiency and atrophy of the thymus, lymph nodes, and spleen (Ahmed et al., 1990; Krishnan et al., 1974).
Vitamin A deficiency is associated with widespread alterations in mucosal surfaces, including that of the eye (Natadisastra et al., 1987), oropharynx, respiratory tract, gastrointestinal tract, and genitourinary tract (Brown et al., 1979; Sweet and K'ang, 1955). Pathologic changes in the conjunctiva associated with night blindness include keratinizing metaplasia of the conjunctiva and loss of goblet cells and mucus (Natadisastra et al., 1987). In the respiratory tract, vitamin A deficiency is associated with loss of ciliated respiratory epithelium, goblet cells, and mucus (Goldblatt and Benischek 1927; Wolbach and Howe, 1925). Squamous metaplasia has been reported to occur in the tracheal epithelium of vitamin A-deficient animals (McDowell et al., 1984). Loss of microvilli from enterocytes and loss of goblet cells and mucus have also been observed in animals deficient in vitamin A (Rojanapo et al., 1980). These pathologic alterations in mucosal surfaces constitute a violation in the first line of defense of the immune system and may explain the more severe morbidity from respiratory and diarrheal disease in children with mild vitamin A deficiency.
T-cell subset alterations have been reported in vitamin A deficiency. In preschool children in rural Indonesia, clinical and subclinical vitamin A deficiency is associated with T-cell subset alterations such as lower circulating percentage of CD4 cells and lower CD4/CD8 ratio (Semba et al., 1993b). A study involving HIV-seronegative injection drug users from inner city Baltimore showed that adults with vitamin A levels consistent with deficiency (serum vitamin A < 1.05 μmol/liter) had significantly lower CD4 counts than those without deficiency (Semba et al., 1993a). Studies in HIV-infected adults have shown a fairly consistent association between low vitamin A levels and low CD4 counts (Phuapradit et al., 1996; Semba et al., 1993a, b). Other T-cell subset alterations include lower circulating natural killer (NK) cells in vitamin A-deficient animals, and treatment with retinoic acid is associated with increases in
circulating NK cells (Zhao and Ross, 1995). In children with AIDS, high-dose vitamin A supplementation increases circulating CD4 T-cells and NK cells (Hussey et al., 1996). Vitamin A appears to play a key role in lymphopoiesis, which may explain the increases in total lymphocyte counts, CD4 T-cells, and NK cells associated with vitamin A supplementation or improvement of vitamin A status.
A hallmark of vitamin A deficiency is depressed antibody responses to T-cell-dependent antigens, such as tetanus toxoid; viral and parasite antigens; and T-cell independent type 1 antigens; such as meningococcal polysaccharides (Ross, 1992; Semba, 1994). The mechanisms by which vitamin A deficiency impairs antibody responses include alterations in interleukin (IL)-4 and interferon-gamma production (Cantorna et al., 1996) and IL-2 receptor expression (Sidell et al., 1993). In addition, the growth and activation of T- and B-cells are dependent on vitamin A and its metabolites (Buck et al., 1990; Garbe et al., 1992; Wang et al., 1993). Vitamin A-deficient animals that are experimentally infected with different pathogens generally have impaired immune responses and increased morbidity and mortality. Children with clinical or subclinical vitamin A deficiency have depressed IgG responses to tetanus toxoid compared with children supplemented with vitamin A (Semba et al., 1992). There are few data regarding antibody responses in vitamin A-deficient adults, probably because deficiency is generally uncommon, except in those with chronic infections such as HIV infection.
Considerations for Adult Men and Women
In healthy adult men and women, it is reasonable to expect that a diet that meets or exceeds the RDA of vitamin A of 800 μg RE/d will be sufficient to avoid a deficiency state. Such requirements could be met through a diet that includes butter, cheese, egg yolks, whole milk, vitamin A-fortified skim milk, and liver. Foods such as carrots and spinach contain β-carotene and other provitamin A carotenoids that are converted to vitamin A. The absorption of provitamin A carotenoids is improved if accompanied by dietary fat, such as cooking oil. Recent data suggest that the bioavailability of carotenoids in vegetables and fruits for conversion to vitamin A is much lower than previously estimated (de Pee et al., 1995), and currently further studies are being undertaken to address this issue. Individuals with prolonged infections such as diarrhea or malaria may have reduced their hepatic reserves of vitamin A during the period of infection, and it would be reasonable to encourage increased consumption of vitamin A-containing foods during infection and convalescence. If periods of dietary deprivation are anticipated, it would be reasonable to expect that a daily supplement that contains 800 to 1,500 μg RE (2,666 to 5,000 IU) could minimize the risk of vitamin A deficiency and associated immunodeficiency. Because vitamin A is an immune enhancer, another issue is whether supplementation beyond the daily requirement for vitamin A will
increase immunity. There are few data to suggest that megadoses of vitamin A will confer additional protection against infections, and supplementation above levels of 3,000 μg RE (10,000 IU)/d in healthy adults may increase the risk of vitamin A toxicity or of birth defects in women of childbearing age.
Vitamin A deficiency is a nutritionally acquired immunodeficiency disorder that primarily affects infants, preschool children, pregnant women, and lactating women. Healthy adult men and women of military age represent the lowest risk group for the development of vitamin A deficiency. However, under certain conditions such as chronic infection or prolonged dietary deprivation, the risk of vitamin A deficiency and associated immune abnormalities may be significant. Under such circumstances, daily supplementation with the RDA for vitamin A would be expected to minimize such risk. Individuals who meet the RDA of vitamin A per day would be likely to avoid the risk of vitamin A deficiency under normal circumstances. Because of the capacity of the adult liver to store several months or more of vitamin A, adults are generally buffered against developing vitamin A deficiency.
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ROBERT NESHEIM: Are there questions or discussion of this paper? I guess you had a question, Dr. Chandra?
RANJIT CHANDRA: The first question is what was your perception of the two studies related to mortality? The second question or comment is that I think the nutrient that probably has not a very widespread effect or is not as clear as for instance, zinc—perhaps the discrimination would come from antibody responses to those agents which I think have a more flexible response.
RICHARD SEMBA: I don't think I understand your second question, quite as well, but of the eight clinical trials I showed, all showed reductions in mortality except one.
The one study in India showed a 6 percent reduction in mortality. I cannot explain that study, but I can explain the Sudan study. The Sudan study showed actually a negative effect, but it is quite puzzling because those investigators also showed that vitamin A supplementation had no impact on vitamin A deficiency. In other words, in children who had night blindness or xerophthalmia, it did not reduce that at all, which makes me a little bit concerned. I am quite puzzled over that.
So, one would expect that [a decrease in night blindness] at least as a minimum, and those investigators concluded that maybe they weren't giving enough vitamin A.
I think if I understand your second question, I don't think anybody has looked at effects of vitamin A supplementation in humans on antigens that are less immunogenic, such as Dresser (1968) did in mice with, I think, bovine serum albumin. That hasn't been done. It would be very interesting to see.
We are doing a study in adults with conjugated pneumococcal vaccine in Baltimore using vitamin A and zinc in a factorial design to see whether that will increase their immune responses.
RANJIT CHANDRA: Have you looked at the secretory immune system?
RICHARD SEMBA: No.
ROBERT NESHEIM: Thank you very much. We appreciate your comments on vitamin A, and I think it has been very helpful to us.