Among the micronutrients, vitamin A plays a central role in normal immune function. In a comprehensive review of the literature, Scrimshaw et al. Although conventionally it has been thought that the main clinical manifestations of vitamin A deficiency involve the eye for example, night blindness and xerophthalmiait is now well established that widespread immune alterations, anemia, and increased infectious disease essay writer funny and mortality occur during vitamin A deficiency.
The use of vitamin A supplements to enhance immunity has been demonstrated in the recent seriesRichard Semba, The Johns Hopkins University, Department of Ophthalmology, Ocular Immunology Service, Baltimore, MD 21205of clinical trials involving children in developing countries Beaton et al. This chapter will review the wriger importance of vitamin A to immune function in adult men and women. The most commonly recognized clinical manifestations of vitamin A deficiency involve the eye, and a complete description of the clinical staging is funny 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 writwr risk writdr 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. 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 essay writer funny 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. Vitamin A deficiency is one of the most common micronutrient deficiencies in eriter 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 theworld 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 writed United States prior to improvements in diet, fortification essay writer funny 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 essag 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 Liver disease, such as hepatitis, may impair the capacity of the liver to store and release vitamin 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. 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 essay writer funny 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 deprivation experiments have wrkter conducted among adults in order to determine the minimal requirements of vitamin Wriiter, and these experiments illustrate the capacity of the liver to store vitamin 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.
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 thembetween the ages of 20 and 30 years Hume and Krebs, 1949.essay on community service essay writing help assignment essay writing services us freelance essay writer do my assignment how to get a discount for write my paper