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 cheap essay help 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. 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. Hellp and keratomalacia have been infrequently reported in adult hellp and women, usually under conditions of extreme dietary deprivation or in association with chronic infections and wasting. Vitamin A deficiency has cheap essay help reported in concentration camps Salus, 1957. Scattered case reports exist of keratomalacia in adults Pillat, 1929and these cases occurred mostly among adults with liver disease, malnutrition, or severe diarrheal disease.

Alcoholic liver disease can hceap the risk of vitamin A deficiency. Low cheap essay help A levels consistent with deficiency have been described in adults with HIV infection Karter et al. 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 cheap essay help involved in long voyages and was widely reported in both Union and Confederate armies wssay 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. In sub-Saharan Africa, HIV-infected pregnant women with vitamin A deficiency have higher risk of passing HIV infection to their cheap essay help, and their infants have lower birthweight and higher infant mortality Semba et al.

Vitamin A deficiency has been reported in pregnant women of lower socioeconomic class in the United States Duitsman et al. The requirements for vitamin Cheap essay help 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.

Epidemiologic studies, clinical trials, and experimental studies in animal models have firmly established that vitamin A deficiency is a nutritionally cheap essay help immunodeficiency disorder that is characterized by widespread immune alterations hep increased infectious disease morbidity and mortality Semba, 1994. In humans, most of the understanding of vitamin A-related immunodeficiency comes from cheap essay help involving children rather than adults.

Vitamin Essaj deficiency is often associated with protein-energy malnutrition PEMmaking 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 chep al. Vitamin A deficiency is associated with widespread alterations in mucosal surfaces, including that of the eye Natadisastra et al.

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. Squamous metaplasia has been reported to occur in the tracheal epithelium of vitamin A-deficient animals McDowell et al. 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. 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. Studies in HIV-infected adults have shown a fairly consistent association between low vitamin A levels and low CD4 counts Phuapradit et al. 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 incirculating 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. 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 cheap essay help of vitamin A status. The mechanisms by which vitamin A cheap essay help impairs antibody responses include alterations in interleukin IL -4 and interferon-gamma production Cantorna et al. In addition, the growth and activation of T- and B-cells are dependent on vitamin A and its metabolites Buck et al.

Vitamin A-deficient animals that are experimentally infected with different pathogens generally have impaired immune jelp and increased morbidity and mortality. Children with clinical or subclinical vitamin A deficiency have depressed IgG responses to tetanus toxoid compared with children essya with vitamin A Semba et al.

There are esszy 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. Such requirements could be met through a diet that includes butter, cheese, egg yolks, whole milk, vitamin A-fortified skim milk, and liver. The absorption of provitamin A carotenoids is improved if accompanied by dietary fat, cheap essay help 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. Individuals with prolonged infections such as diarrhea or fheap 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. Because vitamin A is an immune enhancer, another issue is whether supplementation beyond the daily requirement for vitamin A willincrease immunity.

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.

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