VITAMIN A DEFICIENCY

dietary lack of vitamin A resulting in various disorders that most commonly involve the eye and the epithelial tissues (the skin and the mucous membranes lining the internal body surfaces). In humans, one of the earliest signs of vitamin A deficiency is night blindness (nyctalopia), the visual failure to adapt promptly from light to darkness and to see in the dark. This aspect of vision is normally dependent on visual purple, or rhodopsin, a protein found in the eye that maintains its photosensitivity only in the presence of vitamin A. If the deficiency is severe and persists, especially in malnourished infants and children, a condition known as xerophthalmia may develop. In xerophthalmia, the eyes are sensitive to light, the secretion of lubricating tears is stopped, and the eyelids become swollen and sticky with pus. The mucous surfaces of the eye may become eroded in spots, allowing infection to set in, thus leading to ulceration and other destructive changes of the cornea (the transparent outer covering of the eye) and other eye structures. This condition will eventually result in blindness. Early signs of vitamin A deficiency may also be reflected in changes in the mucous membranes of the mouth, throat, and respiratory and genito-urinary passages. These lining membranes become atrophied and dry and lose their cilia, the tiny hairlike projections that normally help in clearing away foreign particles. The defective mucous surfaces have weakened resistance to bacterial invasion, and their susceptibility to various infections increases. If insufficient intake of vitamin A is prolonged, the skin may become dry and rough, with the appearance of plugs of horny material about the hair follicles (follicular hyperkeratosis). Vitamin A deficiency may also result in defective bone and teeth formation and in poor general growth. In experimental rats and in cattle, reproduction may either fail totally or result in few live births. Vitamin A is found in all animal livers (where it is normally stored) and in milk products containing milk fat. In addition, many yellow and green leafy vegetables contain carotenes, chemically related substances that are converted to vitamin A in the body. Except in the later stages, when cellular damage in the cornea and associated deeper structures is too extensive, xerophthalmia can be effectively treated with vitamin A. It is usually most effective when supported by a well-balanced diet rich in protein. Although xerophthalmia is seldom encountered in countries where dairy products are readily available, it has been a serious problem in certain parts of the world. Xerophthalmia is common among poor preschool children in Indonesia, Bangladesh, India, and the Philippines. It also occurs in some parts of Africa. The global incidence has been estimated at some 500,000 new cases per year, half of which lead to blindness.