Insufficiency of vitamin A
Last reviewed: 23.04.2024
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Vitamin A deficiency is a systemic disease that affects the cells and organs of the entire body. The resulting changes in the structure of the epithelium are called "keratonizing metaplasia." Keratonizing metaplasia of the epithelium of the respiratory and urinary tract and associated changes in the epithelium of the digestive tract develop at a relatively early period of the disease, even before the appearance of clinically pronounced changes from the conjunctiva of the eyes, but in most cases they proceed secretly. Typical clinical manifestations of vitamin A deficiency include reduced dark adaptation and twilight vision (hemostalopia), slower bone growth, hyperkeratosis, dryness of the cornea (xerophthalmia). As a result of these processes, the risk of developing inflammatory diseases of the skin and mucous membranes (dermatitis, rhinitis, bronchitis, etc.) increases. The physiological consequences of vitamin A deficiency, such as a violation of adaptation to darkness or an abnormal differentiation of the conjunctival epithelium (revealed by cytological examination of the conjunctival smear), usually begin to develop at a serum retinol concentration below 1 μmol / L. Obvious xerophthalmia usually appears at concentrations below 0.7 μmol / L and takes a more severe form at values less than 0.35 μmol / L (severe insufficiency and depletion of liver stores). The risk of disruption of iron utilization and lethal outcome gradually increases as the concentration of vitamin A in the blood decreases.
Taking drugs that reduce the concentration of cholesterol in the blood, reduces the absorption of vitamin A in the intestine.
There are 2 forms of vitamin A deficiency in the body - primary (alimentary), associated with insufficient intake of retinol or carotenoids with food, and secondary, which occurs when the metabolism of vitamin A is disturbed, more often with diseases of the gastrointestinal tract, liver, pancreas.
Doses of vitamin A more than 100,000 IU / day in adults and 18,500 IU / day in children, obtained for many months, can cause a toxic effect, manifested by a decrease in appetite, hair loss, insomnia, nausea, vomiting, diarrhea, yellow-red pigmentation skin and mucous membranes, pain in the bones, headache, enlargement of the liver. Symptoms of intoxication occur when the vitamin A content in the serum exceeds the ability of the vitamin A-binding protein to bind it. Normally, the molar ratio of vitamin-A / vitamin A-binding protein is 0.8-1, with intoxication it decreases. With the development of acute hypervitaminosis A, retinol esters can account for more than 30% of the total amount of vitamin A (62.82 μmol / L).
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